# Optical properties of a benthic diatom, Pleurosigma strigosum, from the subtidal sediment of the bay of Brest

Ingrid Kernalléguen, Anna Isaia, Aude Leynaert, Kevin Heggarty, Johann Lavaud

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Ingrid Kernalléguen, Anna Isaia, Aude Leynaert, Kevin Heggarty, Johann Lavaud. Optical properties of a benthic diatom, Pleurosigma strigosum, from the subtidal sediment of the bay of Brest. Silicamics 4, Oct 2024, Moulin Mer, France. [hal-04862769](https://hal.science/hal-04862769v1)

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Distributed under a Creative Commons Attribution 4.0 International License
# OPTICAL PROPERTIES OF A BENTHIC DIATOM,
## PLEUROSIGMA STRIGOSUM
### FROM THE SUBTIDAL SEDIMENT OF THE BAY OF BREST

INGRID KERNALLEGUEN ¹ (ingrid.kernalleguen@univ-brest.fr)
ANNA ISAIA ¹, AUDE LEYNAERT ¹, KEVIN HEGGARTY ² & JOHANN LAVAUD ¹

1 LABORATOIRE DES SCIENCES DE L'ENVIRONNEMENT MARIN (LEMAR) UMR 6935, PLOUZANÉ, FRANCE

2 DÉPARTEMENT D'OPTIQUE DE L'IMT ATLANTIQUE, PLOUZANÉ, FRANCE

## INTRODUCTION

Diatoms are **single-celled microalgae** found in aquatic environments worldwide. They produce intricately structured silica cell walls, called **frustules**, that appear to manipulate optical radiation through diffraction, refraction, scattering and wave-guiding, potentially contributing to their remarkable photosynthetic efficiency.

To explore the mechanisms by which *Pleurosigma strigosum* frustules **modulate light**, we examined the correlation between the frustule's optical response and its interaction with light in living, individual cells in their natural aquatic environment.

1. WHAT ARE THE OPTICAL PROPERTIES OF PLEUROSIGMA?

2. WHICH PART OF THE FRUSTULE IS RESPONSIBLE FOR THESE PROPERTIES—ITS NANOSTRUCTURE OR ITS OVERALL LENS-LIKE SHAPE?

3. WHAT IS THE PURPOSE OF THOSE OPTICAL PROPERTIES?

<figure><img src="image_2.png" /></figure>

*Pleurosigma strigosum* is a **motile benthic diatom** found in **subtidal and intertidal** environments.

Living cells were collected from the Bay of Brest and cultivated at the LEMAR-IUEM laboratory.

The frustule has a pennate shape and appears sigmoid when viewed

from the valve side

<figure><img src="image_3.png" /></figure>

## OPTICAL PROPERTIES

<figure><img src="image_4.png" /><figcaption>Living cell of *Coscinodiscus wailesii*</figcaption></figure>

The optical properties of diatoms were first studied in **centric diatoms** (e.g., *Coscinodiscus*) due to their larger size. Most studies have focused on the frustule, as it is more accessible and stable. (Goessling et al., 2019)

However, the optical properties change significantly depending on the **refractive index** of the surrounding medium, **making it difficult for studies on the frustule alone to accurately describe the properties of living cells.**

<figure><img src="image_5.png" /><figcaption>SEM images of *Coscinodiscus* frustule (round) against a *Pleurosigma* frustule (pennate); *Pleurosigma* pores on the top valve,</figcaption></figure>

**Frustules:** The thin silica structure is perforated with pores (areolae) ranging from 200 nm to 600 nm in size, depending on the species, which are regularly distributed across its surface. This structure is thought to interact with light similarly to a photonic crystal, contributing to the iridescent behavior and variations in reflectivity observed with clean frustules.

Diatom frustules, due to their silica composition, are thought to provide physical protection against UV radiation, as silica absorbs part of the UV spectrum. (De Tommasi et al., 2021)

<figure><img src="image_6.png" /></figure>

Living cells: The absorption spectrum is determined by **pigments** (Chl a, Chl c, fucoxanthin, diadinoxanthin), which give the cells their brown color.

The chloroplasts, whose shape and number vary with species, scatter light. The membrane surrounding the frustule reflects light depending on the angle. (De Tommasi et al., 2024)

## CONTEXT

## MATERIALS AND METHODS

### MEASUREMENTS OF OPTICAL PROPERTIES IN LIVING CELLS

To study the light transmitted by a living cell, images of a single cell immersed in water were captured at a **hundred different positions along the z-axis** (a).

These images were taken using a **home-made automated inverted microscope** (b).

For each image (corresponding to a specific z-position), a region of interest within the micrograph was selected (c).

The light intensity for each z position was measured (d), and the results were combined into a 'z-pile graph' to visualize the path of light through the cell and below (e).

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>Z-pile graphic of a living cell of *Pleurosigma strigosum* in water</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>d) Light intensity profiles at different depth below the cell</figcaption></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

*"Z PILE GRAPHICS" SHOW A CONCENTRATION OF LIGHT UNDER THE CELL, SIMILAR TO FOCUSING BY A LENS.*

## LIGHT TRANSMISSION

The path of transmitted light **differs between the "clean" frustule** (organic matter removed by boiling in bleach, rinsing and air drying) **and the living cell**:

* In water, a clean frustule has a weak refractive index contrast, causing light to **scatter due to edge effects**.

* Living cell focuses light in a **triangular pattern**, similar to the way a glass lens concentrates light.

The light is focused between **25 to 100 µm below the cell**, equivalent to the height of 1 to 4 cells, depending on the cell.

### HOW DOES THE CELL FOCUS LIGHT?

**A refractive** lens (with a homogeneous refractive index and smooth surface) can be modeled with a ray simulator and then compared to the living cell.

**A diffractive** lens, caused by a nano pattern of varying thickness, focuses different wavelengths in a unique way (experiments still ongoing). We are comparing the focusing behavior of different wavelengths on the same living cell.

<figure><img src="image_15.png" /><figcaption>Biconvexe Lens in a ray simulator
$n_{\text{ext}}$ = 1,33 $n_{\text{cell}}$= 1,428 $\lambda$= 540 nm</figcaption></figure>

## COMPARISON WITH A BICONVEXE LENS

Using a **ray simulator**, we can model and compare a **refractive homogeneous lens** with a defined refractive index to a living cell under different conditions (such as medium refractive index, cell curvature, and incident wavelength).

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>Z-pile graphic of a living cell of *Pleurosigma strigosum*</figcaption></figure>

## RESULTS

Recent studies indicate that frustules can bring many diverse optical properties to diatom such as wave-guiding (Goessling et al., 2020), filtering UV (Ellegaard et al., 2016), etc. ...

However, these studies have almost exclusively centered on dead clean diatom frustules.

In contrast, when covered with organic material and in water, these optical properties can be very different.

> We have studied the optical properties of living diatoms in water and observed a strong focusing effect (similar to a refractive cylindrical lens) a few tens of microns below the cell.

A simple ray tracing simulation of structure with the same size and shape using a contrast refractive index in water produce very similar focusing performance, and this without the frustule nano-structuration.

We conclude that the **pennate shape of the diatom *Pleurosigma strigosum*** is sufficient to explain this external focusing effect.

We hypothesise that focusing light approximately 50 µm below a cell may contribute to the distribution of light inside the biofilm.

<figure><img src="image_18.png" /></figure>

## REFERENCES:

De Tommasi, E., Rea, I., Ferrara, M. A., De Stefano, L., De Stefano, M., Al-Handal, A. Y., Stamenković, M., & Wulff, A. (2021). Underwater light manipulation by the benthic diatom ctenophora pulchella: From par efficient collection to uvr screening. *Nanomaterials*, 11(11). https://doi.org/10.3390/nano11112855

De Tommasi, E., Rea, I., Ferrara, M. A., De Stefano, L., De Stefano, M., Al-Handal, A. Y., Stamenković, M., & Wulff, A. (2024). Multiple-pathways light modulation in *Pleurosigma strigosum* bi-raphid diatom. *Scientific Reports*, 14(1), 1–18. https://doi.org/10.1038/s41598-024-56206-y

Ellegaard, M., Lenau, T., Lundholm, N., Maibohm, C., Friis, S. M. M., Rottwitt, K., & Su, Y. (2016). The fascinating diatom frustule—can it play a role for attenuation of UV radiation? In *Journal of Applied Phycology* (Vol. 28, Issue 6, pp. 3295–3306). Springer Netherlands. https://doi.org/10.1007/s10811-016-0893-5

Goessling, J. W., Su, Y., Maibohm, C., Ellegaard, M., & Kühl, M. (2019). Differences in the optical properties of valve and girdle band in a centric diatom. *Interface Focus*, 9(1). https://doi.org/10.1098/rsfs.2018.0031

Goessling, J. W., Wardley, W. P., & Lopez-Garcia, M. (2020). Highly Reproducible, Bio-Based Slab Photonic Crystals Grown by Diatoms. *Advanced Science*, 7(10). https://doi.org/10.1002/advs.201903726