Scattering properties of microalgae: the effect of cell size and cell wall

Enhancing Microalgae Content in Biocomposites through a Mechanical Grinding Method

Microalgae-based biocomposites are gaining traction as ecofriendly and cost-effective alternatives to conventional petroleum-based plastics. However, achieving a homogeneous dispersion of microalgae within a biocomposite matrix remains a challenge. In this study, we investigated the effect of the size of dried microalgae (Chlorella sp.) on the quality of biocomposites. Ball milling, a mechanical grinding process, was used to control the size of the pretreated dried microalgae. Our results demonstrate that the microalgae size strongly depends on the total weight of the stainless-steel balls, rather than the number of balls used in the milling process. Poly(ethylene-vinyl acetate) (EVA), with functional groups resembling those of Chlorella sp., was incorporated into the ball-milled microalgae to produce homogeneous biocomposites. Smaller Chlorella sp. particles improved the ratio of microalgae and the mechanical properties of the biocomposites. Dried Chlorella sp. particles up to 161.43 μm, which were 72.84% smaller than the untreated microalgae, were obtained after 6 h of ball milling using 3/8-inch balls. This enabled the production of biocomposites with 60 wt.% microalgae and 61.02% of the tensile strength of pure EVA, comparable to traditional polymers. Our findings suggest that controlling the microalgae size through ball milling can improve the quality of microalgae-based biocomposites.

Optical Camera Communication as an Enabling Technology for Microalgae Cultivation

Optical Camera Communication (OCC) systems have a potential application in microalgae production plants. In this work, a proof-of-concept prototype consisting of an artificial lighting photobioreactor is proposed. This reactor optimises the culture's photosynthetic efficiency while transmitting on-off keying signals to a rolling-shutter camera. Upon reception, both signal decoding and biomass concentration sensing are performed simultaneously using image processing techniques. Moreover, the communication channel's theoretical modelling, the data rate system's performance, and the plant distribution requirements and restrictions for a production-scale facility are detailed. A case study is conducted to classify three different node arrangements in a real facility, considering node visibility, channel capacity, and space exploitation. Finally, several experiments comprising radiance evaluation and Signal-to-Noise Ratio (SNR) computation are performed at different angles of view in both indoor and outdoor environments. It is observed that the Lambertian-like emission patterns are affected by increasing concentrations, reducing the effective emission angles. Furthermore, significant differences in the SNR, up to 20 dB, perceived along the illuminated surface (centre versus border), gradually reduce as light is affected by greater dispersion. The experimental analysis in terms of scattering and selective wavelength attenuation for green (Arthrospira platensis) and brown (Rhodosorus marinus) microalgae species determines that the selected strain must be considered in the development of this system.

Classification of marine microalgae using low-resolution Mueller matrix images and convolutional neural network

In this paper, we used a convolutional neural network to study the classification of marine microalgae by using low-resolution Mueller matrix images. Mueller matrix images of 12 species of algae from 5 families were measured by a Mueller matrix microscopy with an LED light source at 514 nm wavelength. The data sets of seven resolution levels were generated by the bicubic interpolation algorithm. We conducted two groups of classification experiments; one group classified the algae into 12 classes according to species category, and the other group classified the algae into 5 classes according to family category. In each group of classification experiments, we compared the classification results of the Mueller matrix images with those of the first element (M11) images. The classification accuracy of Mueller matrix images declines gently with the decrease of image resolution, while the accuracy of M11 images declines sharply. The classification accuracy of Mueller matrix images is higher than that of M11 images at each resolution level. At the lowest resolution level, the accuracy of 12-class classification and 5-class classification of full Mueller matrix images is 29.89% and 35.83% higher than those of M11 images, respectively. In addition, we also found that the polarization information of different species had different contributions to the classification. These results show that the polarization information can greatly improve the classification accuracy of low-resolution microalgal images.

Analysis of Intracellular Metabolites from Microorganisms: Quenching and Extraction Protocols

Sample preparation is one of the most important steps in metabolome analysis. The challenges of determining microbial metabolome have been well discussed within the research community and many improvements have already been achieved in last decade. The analysis of intracellular metabolites is particularly challenging. Environmental perturbations may considerably affect microbial metabolism, which results in intracellular metabolites being rapidly degraded or metabolized by enzymatic reactions. Therefore, quenching or the complete stop of cell metabolism is a pre-requisite for accurate intracellular metabolite analysis. After quenching, metabolites need to be extracted from the intracellular compartment. The choice of the most suitable metabolite extraction method/s is another crucial step. The literature indicates that specific classes of metabolites are better extracted by different extraction protocols. In this review, we discuss the technical aspects and advancements of quenching and extraction of intracellular metabolite analysis from microbial cells.

Evolution of the scattering properties of phytoplankton cells from flow cytometry measurements

After the exponential growth phase, variability in the scattering efficiency of phytoplankton cells over their complete life cycle is not well characterised. Bulk measurements are impacted by senescent cells and detritrus. Thus the analysis of the evolution of the optical properties thanks to their morphological and/or intra-cellular variations remains poorly studied. Using the Cytosense flow cytometer (CytoBuoy b.v., NL), the temporal course of the forward and sideward efficiencies of two phytoplankton species (Thalassiosira pseudonana and Chlamydomonas concordia) were analyzed during a complete life-cycle. These two species differ considerably from a morphological point of view. Over the whole experiment, the forward and sideward efficiencies of Thalassiosira pseudonana were, on average, respectively 2.2 and 1.6 times higher than the efficiencies of Chlamydomonas concordia. Large intra-species variability of the efficiencies were observed over the life cycle of the considered species. It highlights the importance of considering the optical properties of phytoplankton cells as a function of the population growth stage of the considered species. Furthermore, flow cytometry measurements were combined with radiative transfer simulations and biogeochemical and optical measurements. Results showed that the real refractive index of the chloroplast is a key parameter driving the sideward signal and that a simplistic two-layered model (cytoplasm-chloroplast) seems particularly appropriate to represent the phytoplankton cells.

Optical function of the finite-thickness corrugated pellicle of euglenoids

We explore the electromagnetic response of the pellicle of selected species of euglenoids. These microorganisms are bounded by a typical surface pellicle formed by S-shaped overlapping bands that resemble a corrugated film. We investigate the role played by this structure in the protection of the cell against UV radiation. By considering the pellicle as a periodically corrugated film of finite thickness, we applied the C-method to compute the reflectance spectra. The far-field results revealed reflectance peaks with a Q-factor larger than 10³ in the UV region for all the illumination conditions investigated. The resonant behavior responsible for this enhancement has also been illustrated by near-field computations performed by a photonic simulation method. These results confirm that the corrugated pellicle of euglenoids shields the cell from harmful UV radiation and open up new possibilities for the design of highly UV-reflective surfaces.

Scattering of individual particles from cytometry: tests on phytoplankton cultures

This study presents an application of the Cytosense flow cytometer (CytoBuoy b.v., NL) for the analysis of the optical properties of phytoplankton cells. For the first time, the forward, sideward and backward cross sections (σFSC, σSSC and σ_bb ) were derived for two species morphologically different (Chlamydomonas concordia and Thalassiosira pseudonana). The objective of this work is to check the validity of the estimates before any applications in the frame of marine optics studies. Thus, estimates of σFSC and σSSC are tested with radiative transfer computations as no in situ measurements are available. A synthetic database is built considering homogeneous, multi-layered spheres, aggregates and cylinders. Scanning electron micrographs were performed to investigate the cell morphology to simulate particles as close as possible to the real cells. This set of numerical results represents a valuable database for many kinds of applications dealing with marine optics. Comparisons showed that the Cytosense estimates for the cultures are consistent with values predicted by the theory. In average, more than 92% of the Cytosense estimates were encompassed by predicted values. The backscattering cross section and the backscattering efficiency were compared with in situ measurements found in the literature wherever possible. Results showed that σ_bb and Q_bb estimations fall within the range of the referenced values.

Can spherical eukaryotic microalgae cells be treated as optically homogeneous?

This study aims to answer the question of whether spherical unicellular photoautotrophic eukaryotic microalgae cells, consisting of various intracellular compartments with their respective optical properties, can be modeled as homogeneous spheres with some effective complex index of refraction. The spectral radiation characteristics in the photosynthetically active region of a spherical heterogeneous microalgae cell, representative of Chlamydomonas reinhardtii and consisting of spherical compartments corresponding to the cell wall, cytoplasm, chloroplast, nucleus, and mitochondria, were estimated using the superposition T-matrix method. The effects of the presence of intracellular lipids and/or starch accumulation caused by stresses, such as nitrogen limitation, were explored. Predictions by the T-matrix method were qualitatively and quantitatively consistent with experimental measurements for various microalgae species. The volume-equivalent homogeneous sphere approximation with volume-averaged effective complex index of refraction gave accurate estimates of the spectral (i) absorption and (ii) scattering cross sections of the heterogeneous cells under both nitrogen-replete and nitrogen-limited conditions. In addition, the effect of a strongly refracting cell wall, representative of Chlorella vulgaris, was investigated. In this case, for the purpose of predicting their integral radiation characteristics, the microalgae should be represented as a coated sphere with a coating corresponding to the cell wall and a homogeneous core with volume-averaged complex index of refraction for the rest of the cell. However, both homogeneous sphere and coated sphere approximations predicted strong resonances in the scattering phase function and spectral backscattering cross section that were not observed in that of the heterogeneous cells.

Determination of backscattering cross section of individual particles from cytometric measurements: a new methodology

A methodology is developed to derive the backscattering cross section of individual particles as measured with the CytoSense (CytoBuoy b.v., NL). This in situ flow cytometer detects light scatter in forward and sideward directions and fluorescence in various spectral bands for a wide range of particles. First, the weighting functions are determined for the forward and sideward detectors to take into account their instrumental response as a function of the scattering angle. The CytoSense values are converted into forward and sideward scattering cross sections. The CytoSense estimates of uniform polystyrene microspheres from 1 to 90 μm are compared with Mie computations. The mean absolute relative differences ΔE are around 33.7% and 23.9% for forward and sideward scattering, respectively. Then, a theoretical relationship is developed to convert sideward scattering into backscattering cross section, from a synthetic database of 495,900 simulations including homogeneous and multi-layered spheres. The relationship follows a power law with a coefficient of determination of 0.95. To test the methodology, a laboratory experiment is carried out on a suspension of silica beads to compare backscattering cross section as measured by the WET Labs ECO-BB9 and derived from CytoSense. Relative differences are between 35% and 60%. They are of the same order of magnitude as the instrumental variability. Differences can be partly explained by the fact that the two instruments do not measure exactly the same parameter: the cross section of individual particles for the CytoSense and the bulk cross section for the ECO-BB9.

POLVSM (Polarized Volume Scattering Meter) instrument: an innovative device to measure the directional and polarized scattering properties of hydrosols

An innovative instrument dedicated to the multispectral measurements of the directional and polarized scattering properties of the hydrosols, so-called POLVSM, is described. The instrument could be used onboard a ship, as a benchtop instrument, or at laboratory. The originality of the POLVSM concept relies on the use of a double periscopic optical system whose role is (i) to separate the plane containing the light source from the scattering plane containing the sample and the receiver and (ii) to prevent from any specularly reflected light within the sample chamber. As a result, a wide range of scattering angle, namely from 1° to 179°, is covered by the detector. Another originality of the instrument is to measure the Mueller scattering matrix elements, including the degree of polarization. A relevant calibration procedure, which could be of great interest as well for other instruments, is proposed to convert the raw data into physical units. The relative uncertainty in POLVSM data was determined at ± 4.3%. The analysis of measurements of the volume scattering function and degree of polarization performed under controlled conditions for samples dominated either by inorganic hydrosols or phytoplankton monospecific species showed a good consistency with literature, thus confirming the good performance of the POLVSM device. Comparisons of POLVSM data with theoretical calculations showed that Mie theory could reproduce efficiently the measurements of the VSF and degree of polarization for the case of inorganic hydrosols sample, despite the likely non sphericity of these particles as revealed by one of the element of the Mueller matrix. Our results suggested as well that a sophisticated modeling of the heterogeneous internal structure of living cells, or at least, the use of layered sphere models, is needed to correctly predict the directional and polarized effects of phytoplankton on the oceanic radiation. The relevance of performing angularly resolved measurements of the Mueller scattering elements to gain understanding on the mechanisms processes involved in the scattering of light by marine particles, which has important implications for ocean color remote sensing studies, is demonstrated.

Rationales and approaches for studying metabolism in eukaryotic microalgae

The generation of efficient production strains is essential for the use of eukaryotic microalgae for biofuel production. Systems biology approaches including metabolite profiling on promising microalgal strains, will provide a better understanding of their metabolic networks, which is crucial for metabolic engineering efforts. Chlamydomonas reinhardtii represents a suited model system for this purpose. We give an overview to genetically amenable microalgal strains with the potential for biofuel production and provide a critical review of currently used protocols for metabolite profiling on Chlamydomonas. We provide our own experimental data to underpin the validity of the conclusions drawn.

Inherent optical properties of the coccolithophore: Emiliania huxleyi

A realistic nonspherical model for Emiliania huxleyi (EHUX) is built, based on electron micrographs of coccolithophore cells. The Inherent Optical Properties (IOP) of the EHUX are then calculated numerically by using the discrete dipole approximation. The coccolithophore model includes a near-spherical core with the refractive index of 1.04 + m(i)j, and a carbonate shell formed by smaller coccoliths with refractive index of 1.2 + m(i)j, where m(i) = 0 or 0.01 and j(2) = -1. The reported IOP are the Mueller scattering matrix, backscattering probability, and depolarization ratio. Our calculation shows that the Mueller matrices of coccolithophores show different angular dependence from those of coccoliths.

Mueller matrix measurements of algae with different shape and size distributions

The full Mueller matrix was measured to obtain the polarization state of the scattered light for a variety of algae with different shapes, wall compositions, sizes, and refractive indices. The experimental setup was a multiple laser Mueller matrix ellipsometer, by which measurements were performed for scattering angles from 16° to 160° sampled at every second degree for wavelengths of 473 nm and 532 nm. Previously, the polarization of light scattered from microalgae was investigated only for a few species, and the Mueller matrix was found to have little variation between the species. In our work a total of 11 algal species were investigated, representing diatoms, dinoflagellates, coccolithophorids, green algae, and a cryptophyte. The selection of species was made to obtain high variability in shape, size, cell wall, and refractive index. As in previous investigations, very small variations were found between species for most of the Mueller matrix elements, but noticeable variations were found for M(11), (M(12)+M(21))/2 and (M(33)+M(44))/2.