Johannes Wilhelm Goessling:
Biophotonics of diatoms – Linking frustule structure to photobiology

Date: 18-06-2017    Supervisor: Michael Kühl

Diatoms are unicellular microalgae present in all aquatic environments on earth. Due to their high photosynthetic productivity and abundance, diatoms are main components of aquatic food webs and among the main contributors of global photosynthetic carbon fixation. A unique feature of diatoms is the encasement of the cell in a silicate frustule compounded of two valves and corresponding girdle bands. Photonic structures in the frustule, i.e. pores and chambers on the micro- to nanoscale, interact with electromagnetic radiation in the visible spectrum of light. It has therefore been proposed that the optical properties of frustules could mediate efficient diatom photosynthesis; however, due to lack of optical data of frustules in water and live cells, such links remained purely speculative. The current thesis investigates the potential implications of frustule biophotonics and photobiology also in living diatom cells.

We could show that the valve of the centric diatom species Coscinodiscus granii guides light in the horizontal plane, and redistributes photosynthetically productive radiation over the entire cell. Optical coupling of chloroplasts to the evanescent field of the valve induced photosynthesis in areas that were not directly illuminated with a laser beam focused onto the valve surface. The C. granii valve also strongly interacted with blue light which was scattered onto the chloroplasts inside the cell. However, this observation was restricted to an acute angle of light incidence in C. granii, while valves of some benthic diatom species showed similar phenomena at more obtuse-angled incidence. We hypothesized that the angle dependency of such phenomenon can be explained by differences in the light climate where these species live, i.e. sunlight might be incident at more acute angles in habitats close to the water surface, while the light climate is more diffusive inside sediments. Scattering of blue light by the frustules of pennate diatom species compensated for strong attenuation of such shorter wavelengths inside muddy sediment. Hence, variation of frustule photonic structures and optical properties between the species could have shaped niche differentiation of diatoms in habitats with different light climates. Data presented in this thesis also suggest that the different components of the frustule have various optical implications upon the living cell, i.e. we observed that photonic structures in girdle bands of the C. granii frustule have different optical properties than valves, such as iridescent coloration as a function of light incidence. We conclude that the different biophotonic properties of frustules contribute to the high photosynthetic flexibility of diatoms to various light conditions. We speculate that the photonic structures of frustules thereby enabled diatoms to inhabit environments with different light climates, and have hence influenced species diversification and evolution of diatoms