## 1 Introduction  
Searwater constituents (water molecules, suspended particles, dissolved substances, and air bubbles) impact the propagation of light through absorption and scattering processes. In natural waters, suspended particulate matter is mostly composed of phytoplankton, heterotrophic organisms, viruses, biogenic detritus, and mineral particles. Absorbing and scattering characteristics of water constituents are described by the inherent optical properties (IOP) [1] which do not depend on the radiance distribution but solely on the concentration and chemical composition of dissolved organic matter, and the concentration, size distribution and chemical composition of particulate matter. All IOPs can be defined from the absorption coefficient, \(a\), and the volume scattering function, \(\beta\). For instance, the scattering, \(b\), and backscattering, \(b_{b}\), coefficients are obtained from the integration of \(\beta\) over all scattering angles, and only backward scattering angles, respectively. Owing to the availability of commercial optical backscattering sensors and flow-through attenuation and absorption meters, in situ measurements of bulk IOP have now been routinely performed for more than two decades. While these measurements allow a better description of the IOP variability in natural waters, they can also be used as proxies for the estimation of the bulk particulate matter. For instance, the spectral slope of the particulate beam attenuation coefficient, \(c_{p}\), is tightly linked to the slope of the particle size distribution (PSD), \(\xi\), assuming a Junge-type distribution of PSD [2; 3; 4]. The particulate backscattering ratio \(b_{bp}/b_{p}\) is used to obtain information about the particle composition. Indeed, based on the Lorentz-Mie scattering calculations that assume marine particles as homogeneous spheres, an analytical relationship between \(b_{bp}/b_{p}\), \(\xi\) and \(\vec{n_{r}}\) was generated [5]. This latter equation is used in conjunction with in situ measurements of \(b_{bp}\), \(b_{p}\), and \(c_{p}\) to describe the variability of the physical nature (i.e., refractive index) of the bulk particulate matter in oceanic and coastal environments [6; 7; 8; 9; 10]. In the past, many theoretical and experimental studies, mainly dedicated to phytoplankton, showed that while the absorption, attenuation and total scattering of algal cells are correctly described using the homogeneous sphere model, such model is less appropriate for simulating backscatter. Indeed, the structural heterogeneity and inner complexity of phytoplankton cells (gas vacuoles, chloroplast, silica wall, etc.) explain why the measured backscattering signal is higher than predicted by the Lorentz-Mie theory for homogeneous spheres [11; 12; 13; 14; 15; 16; 17; 18; 19; 20]. The underestimation of \(b_{bp}\) by homogeneous spheres may explain the fact that in situ observations of backscattering are significantly higher than theoretical simulations [21; 22; 23]. In this paper, we examine the impact of particle structural heterogeneity on the bulk backscattering ratio for realistic combinations of optically significant constituents. The purpose of our study is not to provide a new analytical relationship for \(b_{bp}/b_{p}\) as a function of \(\vec{n_{r}}\) and \(\xi\) but rather to assess the sensitivity of \(b_{bp}/b_{p}\) to the modeled structural heterogeneity of phytoplankton cells for some realistic water bodies. Typical phytoplankton bloom and no bloom conditions, as defined in Stramski and Kiefer [24], will first be examined. Then, the last case study will account for the presence of mineral particles, which have a great effect on the scattering properties. Because the bulk scattering (\(b\)) and backscattering (\(b_{b}\)) coefficients of a water body result from additive contributions of all individual constituents that scatter light, we will consider various sub-populations of marine particles, namely organic detritus, minerals, heterotrophic bacteria, viruses, and phytoplankton. [PERSON] et al.