Inherent optical properties of the coccolithophore: Emiliania huxleyi

Computational modeling of Chlamydomonas reinhardtii cellular radiation properties with synergistic consideration of complex structures and compositions

The radiation characteristics of microalgae are of great significance for the design of photobioreactors and ocean optical remote sensing. Yet the complex structure of microalgae makes it difficult to theoretically predict its radiation characteristics based on traditional Mie theory. In this work, taking Chlamydomonas reinhardtii as an example, a multi-component cell model with a complex structure is proposed, which considers the organelles and shape of microalgae, and the volume change during the production of Chlamydomonas reinhardtii lipids. The theoretical calculation is carried out using the discrete dipole approximation method, and an improved transmission method is used for experimental measurement. The experimental data are compared and analyzed with the multi-component complex structure model, the homogeneous sphere model and the coated sphere model. The results show that the calculation accuracy of the multi-component complex structure model is higher, the error of the scattering cross-section is reduced by more than 8.6% compared with the homogeneous sphere model and coated sphere model, and the absorption cross-section and the scattering phase function are in good agreement with the experimental results. With the increase of lipids, the absorption cross-section and the scattering phase function vary slightly. However, the scattering cross-section has an observed change with increasing wavelength. In addition, the theoretical calculation error can be reduced when the influence of the culture medium is taken into account.

Theoretical and experimental study on the radiative properties of Parachlorella kessleri: considering the effect of internal microstructure

A microalgal cell model with multiple organelles considering both the irregular overall shape and internal microstructure was proposed. The radiative properties of Parachlorella kessleri during the normal phase, starch-rich phase, and lipid-rich phase were calculated by the discrete dipole approximation method in the visible wavelengths. The accuracy of the model is verified with experimental measurements. The results showed that the theoretical calculation of the established microalgal cell model is more accurate than those of the equal volume spheres, such as the homogeneous sphere and the coated sphere, with the errors of the scattering cross-section reduced by more than 10.7%. The calculated scattering phase function of the multi-component model is basically in good agreement with the experimental results. Compared to the normal growth phase, the lipid enrichment during the lipid-rich phase leads to a sharp increase in the scattering cross-section by three to four times, while the absorption cross-section remains stable. Remarkably, in the starch-rich phase, the abundant production of starch results in a reduction of two to three times in the absorption cross-section compared to the normal growth phase, while the scattering cross-section varies little. The results can provide basic data and theoretical support for the design and optimization of photobioreactors.

Meta-analysis reveals responses of coccolithophores and diatoms to warming

A meta-analysis was conducted to explore the effects of warming on the physiological processes of coccolithophores and diatoms by synthesizing a large number of published literatures. A total of 154 studies consisting 301 experiments were synthesized in this study. Under a projected temperature increase of 3-5 °C by IPCC AR6 at the end of this century, our results suggest that the growth and photosynthetic rate of coccolithophores were significantly enhanced by the rising temperature, while the calcification of coccolithophores was only slightly promoted. Warming also had significantly positive effects on the growth but not photosynthesis of diatoms. In comparison, the effect size of warming on the growth rate of coccolithophores was larger than that of diatoms. However, there was no significant effect of warming on either the ratio of particulate inorganic carbon to particulate organic carbon (PIC:POC) of coccolithophores or the ratio of biogenic silica to carbon (BSi:C) of diatoms. Furthermore, the results reveal latitudinal and size-specific patterns of the effect sizes of warming. For diatoms, the effects of warming on growth were more prominent in high latitudes, specifically for the Southern Hemisphere species. In addition, the effect size of warming on the small-sized diatoms was larger than that of the large-sized diatoms. For coccolithophores, the growth of the Southern Hemisphere temperate strains was significantly promoted by warming. Overall, the results based on the meta-analysis indicate that the projected warming of the end of this century will be more favor to the growth of coccolithophores than that of diatoms, thus potentially affect the competitive advantages of coccolithophores over diatoms; while the mid-to high latitude species/strains of both coccolithophores and diatoms will benefit more than their counterparts in the lower latitudes. Therefore, this study offers novel insights into predicting both the inter- and intra-group competitive advantages of diatoms and coccolithophores under the future warming climate change scenario.

The influence of particle concentration and bulk characteristics on polarized oceanographic lidar measurements

Oceanographic lidar measurements of the linear depolarization ratio, δ, contain information on the bulk characteristics of marine particles that could improve our ability to study ocean biogeochemistry. However, a scarcity of information on the polarized light-scattering properties of marine particles and the lack of a framework for separating single and multiple scattering effects on δ have hindered the development of polarization-based retrievals of bulk particle properties. To address these knowledge gaps, we made single scattering measurements of δ for several compositionally and morphologically distinct marine particle assemblages. We then used a bio-optical model to explore the influence of multiple scattering and particle characteristics on lidar measurements of δ made during an expedition to sample a mesoscale coccolithophore bloom. Laboratory measurements of linear depolarization revealed a complex dependency on particle shape, size, and composition that were consistent with scattering simulations for idealized nonspherical particles. Model results suggested that the variability in δ measured during the field expedition was driven predominantly by shifts in particle concentration rather than their bulk characteristics. However, model estimates of δ improved when calcite particles were represented by a distinct particle class, highlighting the influence of bulk particle properties on δ. To advance polarized lidar retrievals of bulk particle properties and to constrain the uncertainty in satellite lidar retrievals of particulate backscattering, these results point to the need for future efforts to characterize the variability of particulate depolarization in the ocean and to quantify the sensitivity of operational ocean lidar systems to multiple scattering.

Optical backscattering and linear polarization properties of the colony forming cyanobacterium Microcystis

Light scattering characteristics of the cyanobacterium Microcystis are investigated with numerical models for sphere aggregates. During summer bloom seasons, Microcystis is prevalent in many inland waters across the globe. Monitoring concentrations with remote sensing techniques requires knowledge of the inherent optical properties (IOPs), especially the backscattering properties of Microcystis cells and colonies in natural settings. In situ measurements in waters dominated by Microcystis blooms have previously detected extremely high backscattering ratios, i.e., b_b/b>0.043 at 443 nm [1], the highest to our knowledge in the natural environment. These highb_b/bvalues could hold promise as a diagnostic tool in identifying and monitoring Microcystis using optical approaches. However, it has been unclear how this type of optically 'soft' organic particle can generate such highb_b/bvalues. In this study, the Multiple Sphere T-matrix (MSTM) model is used to calculate the IOPs of model colonies, including b_b/b. Colony sizes in the model ranged from several cells to several hundred and both colony packing density and cell gas vacuole content were varied. Results are compared with model results for equivalent-volume spheres (EVS) and direct in situ measurements. Colony formation was required in the modeling to reproduce the high b_b/bconsistent with in situ measurements. The combination of moderate to very dense colony (packing density >30%) and high gas vacuole content in individual cells (volume percentage >20%) was the most favorable condition leading to rapid increases in b_b/bwith increasing number of cells N_cell of the colony. Significant linear correlations were observed betweenb_b/b and N_cell^1/3 for these colonies, whereb_b/b increased beyond 0.04 once cell number reached about 1000 cells in the case with the most densely packed cells and highest gas vacuole content. Within commonly observed colony sizes (N_cell <10⁶), colonies with high gas vacuole content exhibited b_b/bvalues up to 0.055, consistent with direct measurements from Lake Erie. Polarized scattering was also of interest as a diagnostic tool, particularly with future Earth-orbiting polarimeters being deployed for the NASA Plankton, Aerosols, Cloud, ocean Ecosystem (PACE) mission. The Degree of Linear Polarization (DoLP), expressed by the ratio of two Mueller matrix elements-P₁₂/P₁₁, decreased with increasing colony cell number for Microcystis. Another ratio of two Mueller matrix elementsP₂₂/P₁₁, an index for nonsphericity, also decreased with increasing colony size. In addition to higher relative backscattering, greater colony packing density and larger gas vacuole sizes both led to lower DoLP peak magnitude and lowerP₂₂/P₁₁. An optical opposition feature due to constructive phase interference that was observed previously for cosmic dusts is also present for these modeled colonies, manifested by a narrow intensity peak and negative polarization dip near exact backscattering direction, gradually forming as colony size increases.

Effect of a Coccolithophore Bloom on the Underwater Light Field and the Albedo of the Water Column

The goal of this work is to study the influence of coccolithophore blooms on the underwater light field and albedo of the water column. A coccolithophore is a single-celled alga with spherical cells surrounded by disk-shaped calcite plates (coccolites), which produce strong light scattering. Because of that, we can observe coccolithophore blooms on satellite ocean color images. We calculated the angular underwater radiance distributions and their integral parameters by the exact numerical method with the input parameters, corresponding to real conditions observed in the Barents Sea and Black Sea. Using the results of the exact calculations, we estimated, for various situations, the accuracy of the approximating formulas applied to the assessment of the water radiance reflectance and the diffuse attenuation coefficients and we make recommendations for their application. As a finding of practical importance, we can note the estimate of the accuracy of the widely used Gordon’s formula for the diffuse attenuation coefficient; this formula results in large errors under strong coccolithophore blooms. We also mention the interesting and important results concerning the features of the asymptotic regime under such conditions.

Polarized lidar and ocean particles: insights from a mesoscale coccolithophore bloom

Oceanographic lidar can provide remote estimates of the vertical distribution of suspended particles in natural waters, potentially revolutionizing our ability to characterize marine ecosystems and properly represent them in models of upper ocean biogeochemistry. However, lidar signals exhibit complex dependencies on water column inherent optical properties (IOPs) and instrument characteristics, which complicate efforts to derive meaningful biogeochemical properties from lidar return signals. In this study, we used a ship-based system to measure the lidar attenuation coefficient (α) and linear depolarization ratio (δ) across a variety of optically and biogeochemically distinct water masses, including turbid coastal waters, clear oligotrophic waters, and calcite rich waters associated with a mesoscale coccolithophore bloom. Sea surface IOPs were measured continuously while underway to characterize the response of α and δ to changes in particle abundance and composition. The magnitude of α was consistent with the diffuse attenuation coefficient (K_d), though the α versus K_d relationship was nonlinear. δ was positively related to the scattering optical depth and the calcite fraction of backscattering. A statistical fit to these data suggests that the polarized scattering properties of calcified particles are distinct and contribute to measurable differences in the lidar depolarization ratio. A better understanding of the polarized scattering properties of coccolithophores and other marine particles will further our ability to interpret polarized oceanographic lidar measurements and may lead to new techniques for measuring the material properties of marine particles remotely.

The Ecology, Biogeochemistry, and Optical Properties of Coccolithophores

Coccolithophores are major contributors to phytoplankton communities and ocean biogeochemistry and are strong modulators of the optical field in the sea. New discoveries are changing paradigms about these calcifiers. A new role for silicon in coccolithophore calcification is coupling carbonate and silicon cycles. Phosphorus and iron play key roles in regulating coccolithophore growth. Comparing molecular phylogenies with coccolith morphometrics is forcing the reconciliation of biological and geological observations. Mixotrophy may be a possible life strategy for deep-dwelling species, which has ramifications for biological pump and alkalinity pump paradigms. Climate, ocean temperatures, and pH appear to be affecting coccolithophores in unexpected ways. Global calcification is approximately 1-3% of primary productivity and affects CO₂ budgets. New measurements of the backscattering cross section of coccolithophores have improved satellite-based algorithms and their application in case I and case II optical waters. Remote sensing has allowed the detection of basin-scale coccolithophore features in the Southern Ocean.

An analytical model for light backscattering by coccoliths and coccospheres of Emiliania huxleyi

We present an analytical model for light backscattering by coccoliths and coccolithophores of the marine calcifying phytoplankter Emiliania huxleyi. The model is based on the separation of the effects of diffraction, refraction, and reflection on scattering, a valid assumption for particle sizes typical of coccoliths and coccolithophores. Our model results match closely with results from an exact scattering code that uses complex particle geometry and our model also mimics well abrupt transitions in scattering magnitude. Finally, we apply our model to predict changes in the spectral backscattering coefficient during an Emiliania huxleyi bloom with results that closely match in situ measurements. Because our model captures the key features that control the light backscattering process, it can be generalized to coccoliths and coccolithophores of different morphologies which can be obtained from size-calibrated electron microphotographs. Matlab codes of this model are provided as supplementary material.