Probing suspended particles in seawater, such as microalgae, microplastics and silts, is very important for environmental monitoring and ecological research. We propose a method based on polarized light scattering to differentiate different suspended particles massively and rapidly. The optical path follows a similar design of a commonly used marine instrument, BB9, which records backscattering of non-polarized light at 120°. In addition, polarization elements are added to the incident and scattering path for taking polarization measurements. Experiments with polystyrene microspheres, porous polystyrene microspheres, silicon dioxide microspheres, and different marine microalgae show that by carefully choosing the incident polarization state and analyzing the polarization features of the scattered light at 120°, these particles can be effectively differentiated. Simulations based on the Mie scattering theory and discrete dipole approximation (DDA) have also been conducted for particles of different sizes, shapes and refractive indices, which help to understand the relationship between the polarization features and the physical properties of the particles. The laboratory system may serve as a prove-of-concept prototype of new instrumentations for applications on board or even with submersibles.

Physiological states of marine microalgal cells can influence photosynthesis efficiency, which affects approximately half of global carbon fixation. The detection of the algae physiological profiles is important for marine ecology and economy. In this paper, we propose a polarized light-scattering method to detect sensitive changes in the physiological states of the suspended marine microalgal cells. Our experimental setup is designed to measure the scattered polarization parameters of the cells suspended individually in the seawater. Two species of microalgal cells cultured in the laboratory were measured for several days. Experimental results showed that both species display distinctive changes in their polarized photon scattering features corresponding to changes in their physiological states. The changes are far more prominent than those displayed in unpolarized light scattering. Microscopy observations, simulations for microspheres of different diameters and refractive indices, or different shapes, indicated that the polarization features of the scattered photons are sensitive to the submicrometer microstructures of the cells. This study demonstrates the potential of the polarized light-scattering technique to characterize the physiological states of suspended marine microalgae.

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