# Biosealight: A Low-Cost, Compact, Autonomous, Low-Energy, High-Sensitivity Sensor for Detecting Marine Bioluminescence

Jeanne Maingot-Lépée, Laurent de Knyff, Amandine Caillat, Jacques Benoit, Fabien Soulier, Florence Azaïs, Karim Mahiouz, Didier Louber, Carl Gojak, Jean-Jacques Fourmond, et al.

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Jeanne Maingot-Lépée, Laurent de Knyff, Amandine Caillat, Jacques Benoit, Fabien Soulier, et al. Biosealight: A Low-Cost, Compact, Autonomous, Low-Energy, High-Sensitivity Sensor for Detecting Marine Bioluminescence. OCEANS 2025, Jun 2025, Brest, France. pp.1-10, 10.1109/OCEANS58557.2025.11104671. hal-05304252

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Biosealight: A low-cost, compact, autonomous,
low-energy, high-sensitivity sensor for detecting
marine bioluminescence

1st Jeanne Maingot-Lépée

Aix Marseille Univ, Université de Toulon

CNRS, IRD, MIO

Marseille, France

jeanne.maingot-lepee@mio.osupytheas.fr

2nd Laurent de Knyff

LIRMM, University of Montpellier

CNRS

Montpellier, France

laurent.deknyff@lirmm.fr

3rd Amandine Caillat

Division Technique de l'INSU

UAR CNRS 855

La Seyne sur Mer, France

amandine.caillat@cnrs.fr

4th Jacques Benoit

LIRMM, University of Montpellier

CNRS

Montpellier, France

jacques.benoit@lirmm.fr

5th Fabien Soulier

LIRMM, University of Montpellier

CNRS

Montpellier, France

fabien.soulier@umontpellier.fr

6th Florence Azais

LIRMM, University of Montpellier

CNRS

Montpellier, France

florence.azais@lirmm.fr

7th Karim Mahiouz

Division Technique de l'INSU

UAR CNRS 855

La Seyne sur Mer, France

karim.mahiouz@cnrs.fr

8th Didier Louber

Division Technique de l'INSU

UAR CNRS 855

La Seyne sur Mer, France

didier.louber@cnrs.fr

9th Carl Gojak

Division Technique de l'INSU

UAR CNRS 855

La Seyne sur Mer, France

cari.gojak@cnrs.fr

10th Jean-Jacques Fourmond

Division Technique de l'INSU

UAR CNRS 855

La Seyne sur Mer, France

jean-jacques.fourmond@cnrs.fr

11th Sylvain Bonhommeau

IFREMER DOI

Le Port, France

sylvain.bonhommeau@ifremer.fr

12th Serge Bernard

LIRMM, University of Montpellier

CNRS

Montpellier, France

serge.bernard@cnrs.fr

13th Séverine Martini

Aix Marseille Univ, Université de Toulon

CNRS, IRD, MIO

Marseille, France

severine.martini@mio.osupytheas.fr

Abstract—Bioluminescence, the light emitted naturally by ma-
rine organisms, is an important way of communication between
organisms in the mesopelagic zone. Nearly 75% of marine
organisms, from the surface to the deep sea, and across a wide
taxonomic diversity, use this capability for communication with
diverse ecological goals. Bioluminescence detection thus offers
an indirect way of tracking the presence of organisms (ranging
from zooplankton, dinoflagellates to fishes), their distribution
and diel migrations. Such detection can lead, for example, to
a better understanding of auto- and heterotrophic plankton
communities, vertical migrations of organisms and consequently
of a better quantification of the active carbon export in the
mesopelagic ocean. However, current technologies still limit large
deployments, and high acquisition frequency observations of in
situ bioluminescence.

This publication aims to describe a new low cost, compact,
autonomous, and multi-sensors instrument developed to measure
in situ bioluminescence: Biosealight.

The sensor's acquisition system is built around a high-
sensitivity photon detector that detects photons and converts
their number into proportional electrical signals. The photon
detection system is integrated with additional sensors that record
environmental, including pressure (rated up to 600 bar), tem-
perature, and various system voltages. These measurements are
managed by a motherboard featuring an STM32 microcontroller.

This motherboard interfaces with a dedicated daughter board,
which hosts voltage regulators, the pressure sensor, a high-
voltage power supply (>50 V) necessary for the photon detector,
amplification electronics, and data storage capabilities consisting
of a 1 GB flash memory module with an optional µSD card.
Biosealight detects very low light at wavelengths around 450 nm.
The acquisition frequency for the bioluminescence sensor is 500
Hz. Other sensors (pressure, temperature) have an acquisition
frequency of 38 Hz. High-capacity battery packs power the entire
system. The sensor is designed to be easily deployable on a wide
range of vectors (such as underwater gliders, CTDs, buoys, trawls
...). The sensor being easy to deploy will enable to collect a wide
range of bioluminescent data with high spatiotemporal resolution,
while recording environmental variables related to the organisms.
Various stages of specification research and test protocol under
controlled conditions have led to the characterization of the
sensor performance in capturing subtle bioluminescent events. It
involved calibration procedures designed to verify and optimize
key performance parameters such as sensor sensitivity, sampling
frequency, and measurable intensity ranges. Dedicated tests were
implemented to assess environmental robustness, along with
marinization integration protocols, ensuring the sensor main-
tains operational integrity under challenging marine conditions,
including long-term exposure to high pressure cycles and varying
temperature.
By proposing another way to measure the bioluminescence in situ, and being easy to deploy, this instrument will increase the capacity to observe and characterize bioluminescence of organisms and improve our understanding of its ecological role.

***Index Terms***—Biosealight, Bioluminescence, Sensor, Photomultiplier, Marinization

## I. INTRODUCTION

The vastness of the ocean makes it one of the least explored frontiers on Earth, with many biological processes still poorly understood due to technological limitations in data collection. Among these, bioluminescence — the natural production and emission of light by marine organisms — plays a crucial ecological role [1]. Below a few hundred meter depth and even at the surface at night, the light disappear. In this dim-lit environment, in the water column, more than 75% of marine species are known to be capable of bioluminescence [2]. Many species of phytoplankton and zooplankton (including dinoflagellates [3], copepods, or gelatinous organisms [4]) are capable of emitting bioluminescence after being mechanically stimulated. Bioluminescence provides a noninvasive biological proxy that provides information on the spatiotemporal distribution of marine organisms [5], or predator-prey interactions. It has been linked to key oceanographic and ecological processes, such as vertical diel migration [6] — the largest synchronized animal movement on Earth — and the biological carbon pump, which plays a fundamental role in global carbon sequestration [7], [8]. Observing bioluminescence at fine spatial and temporal scales could thus provide new perspectives on ecosystem dynamics and oceanic carbon fluxes.

Bioluminescence can be measured as the bioluminescence potential (mechanically stimulated light), or spontaneous emission but despite its significance, it remains difficult to measure. The light emission is fairly low ($10^7$ to $10^{12}$ photons.s$^{-1}$ depending on organisms), flashes are fast (mainly between 100 and 1000 ms) [9], and deployments require deep sea technology (resistant to high pressure, corrosion). Existing sensors commercially available are limited to the GlowTracka (Chealsea Technologies) and the Underwater Bioluminescence Assessment Tool (UBAT, SeaBird) [10]. Other custom developments have been published but often within a project limited in time and without accessibility to the scientific community, including quantitative (measuring light intensity dynamics) [11]–[13] and qualitative tools (cameras tacking in situ images) [14], [15].

To address these limitations, Biosealight is a compact (small size and weight), low-cost instrument (less than €1000 for all components) adapted for deployment down to 1000 m depth that integrates a high-sensitivity photomultiplier capable of detecting extremely low-light signals with a high sampling rate (500 Hz), appropriate for capturing short bioluminescent flashes, while remaining within the bandwidth limits of the analog amplification stage (~250 Hz). It also includes environmental sensors (pressure, temperature) to contextualize bioluminescent signals with oceanographic conditions. Biosealight allows deployment across various platforms, including autonomous underwater vehicles (AUVs), gliders, CTDs, or moorings.

Controlled experiments with artificial light and natural bioluminescent sources allow to establish detection thresholds, spectral sensitivity, and optimal sampling frequencies. Field deep-sea deployments at 600 m, assess the sensor's robustness under high pressure, temperature, high light intensity before deployment and salinity conditions as well as an adapted data acquisition and storage for several days.

By advancing the detection and interpretation of bioluminescent signals, the Biosealight aims to bridge the technological gap in oceanographic instrumentation, paving the way for large-scale, high-resolution bioluminescence monitoring. Biosealight is designed to be a high-performance, yet compact, low-cost, autonomous and low-energy sensor, that can be easily deployed. Increasing data acquisition of bioluminescence will contribute to a better understanding of the ecological and biogeochemical roles of bioluminescence in oceanic ecosystems, its link with diversity of organisms, and physical oceanographic structure.

The objectives of this article are to describe the conceptual development and mechanical design of a compact, low-cost, and highly sensitive bioluminescence sensor intended for in situ applications; to validate the sensor's performance through a series of laboratory and environmental tests; and to discuss the current limitations, deployment constraints, and potential improvements of the prototype, while outlining future directions for its integration into ecological studies and broad-scale ocean observation strategies.

## II. MATERIAL AND METHODS

### A. Conceptual design of the initial sensor prototype

The initial concept of the Biosealight prototype was developed in direct response to the identified requirements for accurately capturing marine bioluminescence. At the most fundamental level, the sensor operates by detecting and quantifying the low-intensity light spontaneous, or stimulated, emitted by marine organisms. To achieve this primary goal, the conceptual design incorporated several essential elements: a sensitive photon detection system, a suite of auxiliary environmental sensors (e.g., pressure, temperature sensors), an electronic data acquisition and control system, and an integrated memory for onboard data storage. The prototype was intentionally designed as a flexible and modular platform, allowing subsequent adjustments and refinements in response to evolving scientific needs, performance feedback, and practical deployment experiences.

### B. Specifications: from design requirements to prototype development

The definition of the Biosealight's specifications was guided by three main sources: a review of scientific literature on bioluminescence, feedback from previous similar oceanographic sensor deployments, and the progressive refinement of the prototype during its development. Together, these inputs shaped the design priorities and technical choices leading to the
first functional version of the instrument and provided the
foundation for the experimental protocols described in the
following sections.

1) *Design requirements from literature and similar sensor deployment experience:* Scientific and technical literature, alongside feedback from previous similar sensor deployments, provided a foundational understanding of the operational and environmental challenges of measuring marine bioluminescence. Bioluminescent emissions in the ocean are typically low in intensity, with photon fluxes ranging from $10^7$ to $10^{12}$ photons per second, depending on species, organism size, and ecological context [9], [16]. The sensor therefore had to be highly sensitive at the lower end of this range, while also avoiding saturation at higher intensities.

Bioluminescent signals are often rapid and transient, par-
ticularly in species such as dinoflagellates, which can emit
light flashes lasting only a few milliseconds [17], [18]. Cap-
turing these brief events requires a sufficiently high sampling
frequency. Most existing bioluminescence sensors have an
acquisition frequency below a few tens of hertz and struggle
to detect well-defined fast flashes. Based on several dataset
of bioluminescence, a rate of several hundred hertz was
considered appropriate, balancing temporal resolution with
power efficiency and data management constraints.

Field experience from prior deployments further highlighted
several limitations [10], [12], [19], [20]. Sensors must be
carefully shielded from artificial light sources. An active pump
into a specific chamber allows sampling a well defined volume
and rate but is also limited by organisms size and their
swimming speed. Instruments also have to be isolated from
electromagnetic interference from nearby instruments (e.g.,
deployment on CTDs), which can introduce noise or false
signals. Likewise, increasing sensor gain to improve sensitivity
must be approached cautiously, as it often comes at the
cost of increased electronic noise. These operational lessons
emphasized the need for an optimized signal-to-noise ratio and
robust shielding within the design.

Environmental robustness also emerged as a major concern. Sensors must reliably operate at depths of at least 1000 m depth, where they are exposed to pressures nearing 100 bar and low temperatures (typically 2-4 °C). This demands a waterproof and pressure-resistant design, as well as thermal and mechanical stability to ensure sensor integrity during deployment and recovery operations [9], [10].

Additionally, prior systems were often bulky and energy-
intensive. These experiences motivated the shift toward a com-
pact, modular sensor architecture, better suited for integration
into a wide range of oceanographic platforms.

2) *Refinement of specifications during prototype development:* As the Biosealight progressed from a conceptual prototype to an operational version, performance specifications were refined to address identified challenges.

Gain optimization was prioritized to ensure a balance
between low-light detection and minimal noise. Laboratory
testing was planned to systematically adjust gain settings and
quantify their effect on sensitivity and background noise.

Temporal resolution for the light sensor was addressed by
targeting a sampling frequency of 500 Hz, based on known
flash durations in bioluminescent organisms. This acquisition
frequency was selected as a compromise between capturing
fast events and maintaining energy efficiency and manageable
data volumes. Experimental validation would confirm whether
this sampling rate was sufficient. Additional environmental
sensors have an acquisition frequency of around 38 Hz, to
limit storage issues.

Further electronic modifications, including tuning the high-
voltage supply and optimizing dynamic range, were introduced
to improve the sensor's stability and ability to detect a wide
range of signal intensities. The power consumption was also
minimized, enabling week-long autonomous deployments. Ef-
forts were made to limit sensor saturation in bright sunlight
which increases energy consumption, thereby reducing battery
life and limiting overall deployment duration. To avoid satura-
tion in bright sunlight, a secondary light sensor (VEML6030)
less sensible was used to switch from atmospheric light to
bioluminescence light measurements.

Autonomy was a key design goal: once activated, the sensor is expected to operate independently without human intervention until recovery. This autonomy requires reliable power management, robust internal memory, and a durable mechanical housing.

The mechanical design was also refined. The sensor had to
withstand deployments down to 1000 m depth while remaining
compact, lightweight, and easy to handle. To meet these
requirements, the mechanical housing was divided into two
distinct parts: the battery pack is fully encapsulated in epoxy
resin to ensure structural integrity and resistance to high
pressure, while the electronics components are enclosed in
an oil-filled chamber with the light sensor in direct contact
with the outside. This configuration avoids optical distortions
or light attenuation that could result from resin encapsulation.

These specifications led to the laboratory and field protocols
described in section II-C and II-D, which were designed to
validate the sensor's ability to meet the performance, environ-
mental, and operational challenges of *in situ* bioluminescence
detection.

C. Characterization protocols

1) *Low-light sensitivity characterization:* The main objective of the low-light sensitivity characterization was to evaluate the sensor's capability to detect very faint bioluminescent signals. To achieve this, the Biosealight underwent rigorous tests using two types of controlled sources: a stable LED and a naturally emitting bioluminescent bacterial culture (*Photobacterium phosphoreum*).

Two Multi-Pixel Photon Counter (MPPCs) were evaluated
to characterize their sensitivity under low-light conditions:
the Hamamatsu models S13360-3075CS and S13360-3025CS.
Both detectors exhibit peak sensitivity in the blue-green wave-
length range (detection from 350 to 700nm with an optimal
detection around 450nm), aligning closely with the most com-
mon marine bioluminescence spectra. Additionally, sensitivity
around the red wavelength region was also examined to evaluate potential interference or additional illumination effects, which could be relevant for future applications involving complementary imaging techniques such as camera coupling.

The tests were carried out in a fully darkened laboratory room, specifically arranged to prevent external light interference and minimize electronic noise. Initially, baseline measurements were performed in total darkness, enabling the quantification of inherent sensor noise and defining a minimal photon-detection threshold. Subsequently, the LED source, emitting light in the blue spectral region around 450 nm), was progressively dimmed using neutral density filters (1% and 0.1% transmission) to simulate low-intensity bioluminescent emissions. For biological validation, cultures of bioluminescent bacteria (continuous emission with a maximum at 490 nm) were used to provide a realistic source of weak photon emissions. The liquid medium containing the bacteria, prepared in advance (12 hours before use) to ensure strong light emission, was constantly stirred by a magnetic stirrer.

2) *Distance and angular response testing procedure:* Ac curately characterizing how sensor sensitivity changes with distance is essential, since Biosealight is not designed to record light under a constrained volume and flow rate. Biolumines- cent organisms may emit light at varying distances from the detector in marine environments. To address this, we evaluated the sensor's response at incremental distances from a stable LED light source.

The test setup consisted of fixing the Biosealight in a stationary position and progressively moving the LED source along predefined increments, starting from 8 cm and extending outward to a maximum of 150 cm. At each distance increment (including intermediate points at 12.5, 15, 20, 25, 30, 40, 50, 60, 80, 100, 125 cm), the LED was activated, and the sensor's measurement was recorded. Between each measurement, the LED was briefly turned off, and the sensor was momentarily switched off while adjusting the LED to the next position, preventing saturation of the sensor and ensuring consistency. Finally, measurements were repeated at shorter distances (40 cm and 8 cm) to confirm the reproducibility and stability of the sensor response.

To comprehensively define the sensor's effective field of view, angular response characterization was conducted. Marine bioluminescence can occur from any direction, thus understanding how detection sensitivity varies with angle is crucial for accurately interpreting bioluminescence data collected in situ.

The angular response tests were performed using the same LED source employed previously. Angle measurements were taken at three distances: the LED positioned at 12.5 cm, 60 cm and 125 cm from the sensor. Initially, the LED was positioned directly in front of the sensor (defined as 0°). At each distance, the LED source remained stationary in front of the sensor, while the sensor was repositioned at angular increments of 20° from 0° to +80°, returning to 0°, and then from 0° to -80°, and finally returning again to 0°. Each angular position was maintained for approximately 30 seconds to ensure stable

and reliable data collection.

3) *Sampling frequency validation testing procedure:* To validate the pertinence of the selected light sensor sampling frequency, we carried out an experiment designed to determine the minimum temporal resolution required for accurately capturing bioluminescent flashes. Given that some marine organisms, particularly certain species of dinoflagellates (here *Pyrocystis fusiformis*), produce rapid, transient flashes—among the fastest documented in marine literature—identify the sensor's ability to record such rapid signals was critical [4], [17], [18]. Thus, these organisms provided an ideal test case to ensure the sensor's capability was aligned with ecological needs.

Tests were conducted in a completely dark room, carefully controlled at an ambient temperature of 20 °C to minimize variability and external influences. *Pyrocystis fusiformis* were placed in a matte black measurement container specifically designed to eliminate unwanted reflections or stray light interference. The sensor was placed at a fixed measurement distance of 20 cm from the organisms, ensuring reliable detection without the risk of sensor saturation from intense flashes. The organisms were mechanically stimulated by a reproducible water flow rate (3000 L·h⁻¹) to produce light [21]–[23].

A high acquisition frequency data acquisition test was first performed using a prototype sensor configured at a sampling rate of 5000 Hz. The recorded data represented the most precise temporal resolution, capturing even the fastest bioluminescent events with high fidelity. Subsequently, this high sampling rate dataset was progressively downsampled and filtered (2^nd^ order Butterworth low-pass filter) to simulate lower sampling rates. Through iterative downsampling, we examined signal fidelity at gradually reduced frequencies: minimal mean squared error from the 5000 Hz signal with $MSE = \frac{1}{n} \sum_{i=1}^{n} (y_i - \hat{y}_i)^2$ with n the number of sample, $y_i$ the true value and $\hat{y}_i$ the predicted value.

To quantitatively evaluate the sufficiency of these reduced sampling rates, we compared each downsampled dataset with the original high-resolution (5000 Hz) reference data. The deviations between the downsampled signals and the original reference were carefully analyzed. The sampling frequency was deemed adequate if deviations remained minimal, ensuring that the sensor accurately captured bioluminescent flash events without significant distortion. We still maintained a sampling rate sufficiently higher than the theoretical minimum required, ensuring robust detection of even the fastest bioluminescent signals encountered in marine environments.

However, we ensured that the chosen acquisition frequency was not too high, which would vainly increase data storage requirements and greatly reduce the sensor's autonomy and low-energy objectives.

Note that while the system samples at 500 Hz, the analog front-end — based on an AD8651 operational amplifier with high gain (~200,000) — limits the effective analog bandwidth to approximately 250 Hz (-3 dB). A 5^th^ order low-pass filter was also implemented on the 500 Hz version of the sensor to prevent aliasing and ensure signal fidelity. This analog band-
width ceiling ensures that signals above 250 Hz are attenuated, and helps define the upper limit of accurately detectable signal frequencies, independent of the digital sampling rate.

## D. Environmental robustness and marinization tests

1) *Mechanical design integration protocol:* The system consists of two main parts: the electronic and power subsystems. For both, an effort has been made to reduce volume and ensure a compact sensor.

The electronic subsystem integrates electronic boards and sensors and is enclosed in a housing produced through 3D printing and filled with oil. A silicone membrane enables the transmission of water pressure to the internal oil. This type of packaging helps reduce mechanical stress on the housing; however, the entire electronic subsystem must withstand high pressure.

The design of a power system that could supply sufficient energy while minimizing bulk, weight, and cost, and ensuring both adequate autonomy and ease of deployment, resulted in a method for encapsulating the battery packs in epoxy resin. The encapsulated battery packs were designed to meet several constraints. The electrical power supply and storage capacity were tailored to the sensor's operational needs. The encapsulation had to guarantee waterproofing and structural stability under high pressure. A Subconn connector was chosen as the electrical interface, while a titanium M5 threaded rod served as the mechanical mounting point. The system was also designed for practical handling and deployment, avoiding the need for oil-filling, and keeping overall volume, weight, and cost to a minimum.

2) *Battery pack qualification:* The objective of the battery pack qualification tests was to verify the integrity and electrical performance of the battery packs intended for use with the Biosealight, specifically by comparing battery conditions before and after multiple deep-sea immersion cycles. For these tests, battery packs encapsulated in resin were attached to a CTD cage, to which was attached a 100 kg ballast to ensure the verticality of the immersions, and deployed at sea. The CTD cage was lowered to a target of 1000 m depth using a winch, with depth accuracy verified in real-time by the CTD instrumentation. Each immersion included a 15 minutes stabilization stage at 1000 m depth before recovery onto the vessel deck.

In total, three immersion cycles were performed. During the initial immersion, four battery packs were deployed. For the two subsequent immersions, the same four battery packs were used again, supplemented by three additional battery packs introduced to assess the influence of repeated immersion cycles on battery pack performance. In addition, two control battery packs, not subjected to immersion, were retained onboard as references to evaluate any impacts of immersion cycles on battery autonomy and overall condition.

3) *Environmental stress testing protocols:*

a) *Hyperbaric chamber protocol:* Hyperbaric chamber test was conducted to evaluate the sensor's performance and integrity under controlled high-pressure conditions, replicating

deep-sea environments. The sensor was placed inside the hyperbaric chamber fully immersed in freshwater at a constant pressure of 100 bar without light. We progressively reached 100 bar by step of 10 bar every 3 minutes approximately to avoid any system damage with abrupt changes. The test continued uninterrupted until the sensor's internal memory reached full capacity, approximately 31 hours later.

b) *Sensor qualification:* A second phase of qualification was carried out to verify the physical integrity and correct operation of the sensor and battery pack attached after several immersion cycles. During this test, the whole system (electronic part and battery pack) has been deployed 40 times at 600 m depth attached to a CTD cage.

## III. RESULTS

### A. Sensor performance characterization results

1) *Low-Light Sensitivity Characterization:* Two Multi-Pixel Photon Counter (MPPCs), Hamamatsu's S13360-3075CS (75CS) and S13360-3025CS (25CS), were evaluated for their sensitivity to low-level light emissions. Both are low-cost (less than two hundred euros each). Comparative tests were performed under controlled laboratory conditions, examining both a fixed LED source and a bioluminescent bacterial source (Fig. 1).

<figure><img src="image_2.png" /><figcaption>Fig. 1. Mean light intensity in function of the distance for the Hamamatsu's S13360-3075CS (75CS) and S13360-3025CS (25CS). Test in a 20 °C dark room with an Erlenmeyer flask containing *Photobacterium phosphoreum* bioluminescent bacteria, constantly agitated (by magnetic stirrer). The bioluminescent source was moved from 8 cm to 150 cm and second measurements at 40 cm and 8 cm were made. At each distance, measurements are taken for approximately 30 seconds. The values shown here are averages over these acquisition times.</figcaption></figure>

The output of the MPPC (the light intensity in Fig. 1) corresponds to the number of photons detected per unit time (averaged over 30 seconds of acquisition). Each pixel in the sensor detects single photons and produces a fixed-amplitude electrical pulse. When multiple photons are detected across different pixels, their pulses are summed, resulting in an output signal that is proportional to the number of photons detected. Thus, the measured signal intensity reflects a photon count rate, making this type of detector especially well-suited for quantifying the very low photon fluxes typical of marine bioluminescence.
The results indicated that the 75CS model provided con-
sistently superior sensitivity across different measurement dis-
tances and light intensities compared to the 25CS, with the
LED source and the bioluminescent source Fig. 1. Conse-
quently, the 75CS was selected as the preferred detector for
integration into the Biosealight due to its enhanced overall
performance, ensuring more reliable detection of faint biolu-
minescent signals both at close and extended ranges. It also
meets the objectives of miniaturization (size: 3.0x3.0 mm) and
low-cost sensor.

2) *Angular and Distance Characterization Outcomes:*

Distance and angular detection capabilities of the selected MPPC (75CS) were characterized using controlled LED within a dark room environment, as explained in section II-C2. Measurements conducted at incremental distances (from 8 cm up to 150 cm) demonstrated acceptable detection range capabilities (Fig. 2).

<figure><img src="image_3.png" /><figcaption>Fig. 2. log(Light Intensity) variation with distance and angle. Test in a 20 °C dark room with a blue (450 nm) LED of constant intensity. The LED was moved from 4 cm to 150 cm from the sensor and at angles ranging from 0° to 80°.</figcaption></figure>

Overall, the sensor demonstrated a wide detection field, as
light was still detected at the farthest tested configuration:
150 cm distance and 80° angle. This broad angular and
distance sensitivity is essential for capturing bioluminescent
flashes originating from a wide range of directions or distance
from the sensor in the water column. At close range, the
sensor also displayed a wide angular sensitivity: it was able
to detect light at up to 80° when positioned 12.5 cm from
the source. This suggests the sensor will be highly responsive
to nearby bioluminescent stimuli, including for example those

caused by physical interactions such as laminar flow over the
sensor during movement. While this enhances the likelihood of
capturing strong signals, it also increases the risk of saturation
when exposed to intense flashes at close distances.

However, the results indicated that the gain settings required further adjustment to improve detection performance at greater distances. Additionally, testing revealed a significant amount of electronic noise generated by the data acquisition system, particularly evident at longer detection distances. To mitigate this, efforts to enhance electronic shielding and refine signal amplification processes were made for optimal sensor performance in future deployments.

3) *Sampling frequency validation outcomes:* Following the protocol described in Section II-C3, measurements of mechanically stimulated *Pyrocystis fusiformis* dinoflagellates were carried out in a fully darkened laboratory. The brief flashes of dinoflagellates represented a relevant scenario to evaluate sampling frequencies capable of capturing rapid bioluminescent events while also minimizing data storage and power consumption.

To assess suitable sampling rates, the original high ac-
quisition frequency (5 kHz) dataset was analyzed using two
downsampling methods. The first method applied a low-pass
filter at various cutoff frequencies corresponding to potential
lower sampling rates (Fig. 3 A). The second method involved
classical downsampling by keeping only one data point every
1/Fcut seconds, directly reducing temporal resolution (Fig. 3
B).

Several criteria were considered when selecting the optimal
sampling frequency. Ideally, a suitable sampling frequency
would preserve the shape and amplitude of rapid flashes,
indicated by a minimal mean squared error compared to
the original 5 kHz signal on the entire timeseries: MSE =
$\frac{1}{n} \sum_{i=1}^{n} (y_i - \hat{y}_i)^2$ with n the number of sample, yᵢ the true
value and $\hat{y}_i$ the predicted value. At the same time, the chosen
acquisition frequency had to significantly reduce the amount
of stored data, as continuous recording at 5 kHz would fully
occupy the sensor memory within approximately three hours,
a limitation incompatible with practical marine deployments.

Both quantitative (MSE values) and qualitative (visual inspection, Fig. 3) analyses clearly showed that sampling rates at 100 Hz (low-pass MSE = 30.51, downsampling MSE = 88.11) and 250 Hz (low-pass MSE = 17.97, downsampling MSE = 47.68) were insufficient to accurately capture rapid flash events due to their high MSE values and significant distortion of the original signal shape. Conversely, frequencies of 1000 Hz (low-pass MSE = 5.33, downsampling MSE = 19.26) and 750 Hz (low-pass MSE = 7.37, downsampling MSE = 22.37) naturally had the lowest MSE values, but the gains in accuracy compared to the 500 Hz frequency (low-pass MSE = 10.79, downsampling MSE = 31.97) were minor and did not justify the substantially increased of power and storage demands.

Specifically, when using the low-pass filtering method,
reducing the cutoff frequency from 1000 Hz (MSE = 5.33)
to 500 Hz (MSE = 10.79) approximately doubled the MSE
value; however, it remained low, indicating a negligible loss
<figure><img src="image_4.png" /><figcaption>Fig. 3. Example of a bioluminescent flash from the dinoflagellates *Pyrocystis fusciformis* recorded into a darkroom, mechanically stimulated and at 5 kHz sampling rate. A) low-pass filter comparison (1000 Hz, 750 Hz, 500 Hz, 250 Hz, 100 Hz) with the original light data from the 5 kHz sensor in scatter in the back. B) Downsampling comparison 1000Hz, 750 Hz, 500 Hz, 250 Hz, 100 Hz) with the original light data from the 5 kHz sensor in scatter in the back. MSE correspond to the entire time series from which this flash is extracted.</figcaption></figure>

of signal fidelity. With the classical downsampling method, the MSE at 500 Hz (MSE = 31.97) was less than twice that observed at 1000 Hz (MSE = 19.26), while simultaneously reducing storage requirements by half.

Acquisition at 500 Hz offers an optimal compromise, providing adequate resolution to detect rapid bioluminescent flashes, while simultaneously conserving storage space and power. Consequently, 500 Hz was adopted as the standard sampling rate for the light sensor for subsequent experimental protocols and sensor deployments, assuring both a low-energy and high-sensitivity sensor.

## B. Environmental robustness results

1) **Battery Pack:** Based on their material characteristics, two types of resin were chosen to meet the requirements of the battery pack: EPODEX Pro 2 cm and EPODEX Pro Max 10 cm. After the battery pack qualification (part II-D2) of all battery packs, no noticeable physical change or lost in voltage was detected.

Voltage measurements taken after each immersion showed remarkable consistency across all packs. Battery packs 8 and

9 (two D RS Li-SOCl2 batteries) maintained stable voltages at 7.37 V throughout all immersions. Packs 10, 11, 12, 13, and 14 (two D SAFT batteries) consistently recorded voltages of 7.34 V after their immersions. Notably, the three additional packs (12, 13, and 14) introduced in later immersions exhibited stable performance comparable to the repeatedly immersed packs, indicating negligible effects from multiple pressure cycles on battery integrity.

<figure><img src="image_5.png" /><figcaption>Fig. 4. Battery pack qualification in march 2025 on the ANTEDON II. The resin battery packs are attached to a CTD cage winched down to a depth of 1000 m with 100 kg of ballast.</figcaption></figure>

Physically, no noticeable changes or damage were observed on any of the battery packs after the immersion cycles. These results confirmed that the battery packs maintained structural and functional integrity, supporting their reliability for operational use at 1000 m depth.

The battery packs are therefore ready for in situ deployment, and to meet the sensor's autonomy objective.

2) Environmental stress results:

a) **100 bar hyperbaric chamber outcome:** During the initial hyperbaric chamber test described in section II-D3a, the sensor was continuously exposed to a stable pressure of 100 bar, recording data until its internal memory was completely filled (approximately 31 hours).

This test aimed to achieve two key objectives. First, it verified the mechanical and electronic integrity of the sensor after prolonged exposure to high-pressure conditions. Second, it evaluated the performance of integrated environmental sensors (pressure and temperature) that are crucial for future autonomous deployments. Two pressure and temperature sensors were tested: the TE Connectivity MS5837 (operating range: 0 to 30 bar, and -20 to +85 °C) and the KELLER
6LHPD (operating range: 0 to 600 bar, and -40 to +110 °C). Results indicated no detectable physical damage or structural compromise to the sensor housing after the high-pressure test. However, analysis of recorded data revealed the limitations of the MS5837 sensor, which exceeded its specified operating range and ceased functioning correctly around 70 bar. Consequently, the KELLER 6LHPD sensor was selected for future integration due to its demonstrated capability to operate reliably at pressures up to 100 bar and beyond.

No other abnormalities or malfunctions were detected within the sensor's electronic circuits. This outcome confirmed that the oil-filled casing provided effective protection for internal components, preventing both mechanical stress induced by high pressure and water intrusion during prolonged high-pressure exposure. This ensures compact sensor mounting, while remaining resistant to marine conditions and with low-cost components (two hundred euros for the pressure/temperature sensor).

b) Sensor immersion: Biosealight, attached to the already qualified battery pack, was deployed during the BIOLUMOPS (https://anr.fr/Projet-ANR-23-ASTR-0028, access on March 27th 2025) first campaign at sea - 12 to 27th of March 2025, N/O L'Europe (oceanographic fleet) including 42 dives at 600 m depth (Fig. 5).

<figure><img src="image_6.png" /><figcaption>Fig. 5. Sensor qualification on the N/O 'L'Europe' oceanographic fleet in march 2025. The sensor (visible in the blue square) and the resin battery packs are attached to a CTD cage winched down to a depth of 600 m. Sensor faces downwards.</figcaption></figure>

After 40 immersion cycles, spread over several days, there was no physical damage to the sensor, nor did the attached battery pack, which retained its integrity after the dives. The sensor also demonstrated normal operation after immersions, underlining the high resistance of the electronic part. Data analysis is ongoing for ecological interpretation. This successful qualification of the electronics and connected battery pack demonstrates its ability to withstand marine conditions and repeated deployments.

## IV. CONCLUSION AND DISCUSSION

This study presents the development and validation of Biosealight, a compact, low-cost, autonomous, low-energy and high-sensitivity prototype sensor designed to detect marine

bioluminescence in situ. Through a combination of carefully designed electronics, thoughtful mechanical integration, and targeted testing protocols, Biosealight addresses key challenges associated with bioluminescence sensing in marine environments.

The sensor currently measures 9.5 cm x 7.5 cm x 1.7 cm (for the electronic part) and 8 cm x 9 cm x 5 cm (for the battery pack). It integrates a Multi-Pixel Photon Counter (MPPC, Hamamatsu S13360-3075CS) for photon detection, optimized for sensitivity in the blue-green (450-520 nm) spectral range, where most marine bioluminescence occurs. To reduce energy consumption caused by saturation in bright conditions, a secondary, less sensitive light sensor (VEML6030) was added to distinguish between ambient light and bioluminescence measurements. Its acquisition system, built around a microcontroller (STM32) and paired with high-voltage electronics, environmental sensors, and internal memory, enables autonomous operation at a sampling frequency of 500 Hz for the light data. This setup allows the sensor to reliably capture rapid, low-intensity flashes. The pressure and temperature sensor within it (KELLER 6LHPD), covers extreme deployment conditions (0 to 600 bar, and -40 to +110 °C), and operates at a acquisition frequency of around 38 Hz, reducing the amount of data stored. The battery pack, encapsulated in epoxy resin, and the oil-filled electronic compartment together enable robust mechanical performance under high pressure, verified up to 1000 m in hyperbaric testing and sea trials.

<figure><img src="image_7.png" /><figcaption>Fig. 6. Latest version of Biosealight. The connector cable, suitable for deployment at sea, is connected to the resin battery pack.</figcaption></figure>

Laboratory experiments confirmed the sensor's ability to detect bioluminescent emissions across a range of distances and angles, while sampling frequency tests demonstrated that 500 Hz provides an optimal balance between signal fidelity, storage efficiency, and energy consumption. Environmental testing, including repeated deep-sea immersion of the power system, validated the mechanical resilience of the design, confirming its readiness for extended deployments.

These results demonstrate that Biosealight meets its design objectives: it is compact, lightweight, cost-effective (with total hardware cost under €1000), energy-efficient, and capable of autonomous operation. The use of affordable components without compromising sensitivity or robustness significantly
reduces the barrier to widespread deployment of bioluminescence sensing instruments.

Despite these achievements, some limitations remain. One identified limitation relates to the sensor's physical size and its implications for deployment strategies. Currently, the current sensor design is already compact enough for deployment on various underwater platforms such as CTDs, gliders, and other oceanographic vehicles. A further challenge will be decreasing the sensor's size and weight to broaden its applicability, enabling deployments notably on marine animals, for biologging applications. Achieving this would enhance the versatility of the sensor for diverse ecological studies.

The battery pack's autonomy can also emerge as a critical factor influencing operational duration. Energy consumption depends directly on the intensity of the detected signals. A solution could also the integration of the sensor directly into platforms with onboard power sources, such as underwater gliders, which could alleviate battery-life constraints and extend deployment duration.

Further deployments will have to take into account: light attenuation in the water column. The sensor being external, the light penetration depth into the water column will strongly influence the saturation of the sensor. It is estimated that under bright conditions, the minimal depth for data acquisition is around 200 m depth while at night, it is around 30-50 m depth (depending on atmospheric conditions, including moon phase or weather) due to its very high sensitivity.

The high-resolution bioluminescence data produced by the sensor presents new analytical challenges, highlighting the need to develop appropriate databases and specialized processing methods. Effective interpretation of these datasets requires analytical tools capable of classifying bioluminescent flashes based on their spatiotemporal patterns and characteristics. Such classification system is essential for linking bioluminescent signals to organisms and their behaviors. Addressing these challenges will significantly enhance our understanding of key ecological processes, including diel vertical migrations, predator-prey interactions, and communication behaviors, ultimately enriching the ecological insights derived from marine bioluminescence data.

Looking ahead, future sensor development strategies will prioritize significant reductions in size, cost, and energy consumption, alongside improvements in robustness and autonomy. Achieving these goals will enable widespread, affordable and effective bioluminescence monitoring across global oceans, including remote and understudied regions. One advantage of measuring bioluminescence as a functional trait is that it applies across a range of taxonomic groups, across different size scales (from phytoplankton to micronekton). Ultimately, the broader deployment of the Biosealight promises to enhance ecological understanding, leveraging bioluminescence not merely as a phenomenon of biological interest, but as a powerful ecological indicator or proxy within oceanic ecosystems. This instrument associated with other biological end environmental variables of interest will allow obtaining relevant ecological information on the presence and movement of communities, and consequently, carbon transfers.

## ACKNOWLEDGMENT

This project has obtained financial support from the CNRS through its interdisciplinary programs, from the "Mission pour les initiatives transverses et interdisciplinaires" through its exploratory research programs ("Instrumentation Innovante Transverse" and Prime80), from NUMEV 2022 funding, from ANR-ASTRID funding (2024-2026, ANR-23-ASTR-0028)), from the French LEFE-Cyber program (CNRS-INSU 2023-2024). The authors would like to thank the captain and crew of the N/O 'Antedon II' and the N/O 'L'Europe' (Flotte Océanographique Française) for their help during the BIOLU-MOPS cruise.

This work benefited from access to the Observatoire Océanologique de Banyuls-sur-Mer, an EMBRC-France and EMBRC-ERIC Site (EMBRC project 2024).

Some of the plans, schematics, and instrumental realizations within the framework of the project were made with the support of the Technical Division of the Institut National des Sciences de l'Univers (INSU) of the Centre National de la Recherche Scientifique (CNRS).

Authors thank C. Tamburini, C. Guinet and J. Habasque for their support and insightful discussions about the development of Biosealight.

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