Can spherical eukaryotic microalgae cells be treated as optically homogeneous?

Area of Science:

• Optics and photonics
• Microbiology
• Biophysics

Background:

• Microalgae are crucial photoautotrophs with complex internal structures affecting light interaction.
• Understanding microalgal optical properties is vital for applications in biofuels, biotechnology, and remote sensing.
• Heterogeneous cell structures pose challenges for accurate optical modeling.

Purpose of the Study:

• To determine if microalgae can be simplified as homogeneous spheres for optical modeling.
• To investigate the impact of intracellular compartments and stress-induced changes on spectral radiation characteristics.
• To compare simplified models with heterogeneous cell behavior using experimental data.

Main Methods:

• Utilized the superposition T-matrix method to estimate spectral radiation characteristics.
• Modeled microalgae as homogeneous spheres and coated spheres with effective refractive indices.
• Explored the effects of intracellular lipids and starch accumulation due to nitrogen limitation.
• Investigated the optical properties of cells with strongly refracting cell walls.

Main Results:

• The homogeneous sphere approximation accurately estimated spectral absorption and scattering cross-sections.
• Volume-averaged effective complex refractive index provided good approximations for nitrogen-replete and limited conditions.
• A coated sphere model was more appropriate for cells with strongly refracting cell walls.
• Both simplified models failed to predict resonances observed in heterogeneous cells.

Conclusions:

• Microalgae can be approximated as homogeneous spheres for basic optical property estimation.
• Simplified models are useful but may not capture all spectral radiation nuances, especially scattering.
• Accurate modeling requires considering cell wall properties and internal heterogeneity for specific applications.