Diatoms are single-cell microalgae renowned for their uniquely complex and beautifully intricate silica-based cell walls. Diatoms are evolutionarily diverse and extraordinarily successful, and of major ecological importance. Their photosynthesis captures 40% of the total carbon fixed by marine ecosystems, the equivalent to 20-25% of the total carbon fixation by all aquatic and terrestrial ecosystems combined. Of all diatoms, the group of raphid pennates stands out as being the evolutionarily most recent (ca. 30 million years old) but nevertheless the largest in number of species. In contrast with their planktonic ancestors, these species are mostly benthic and capable of directed motility, a unique trait amongst diatoms. This group has been immensely successful in colonizing benthic habitats. Particularly in tidal estuaries, the raphid pennates dominate microbial communities (microphytobenthos, MPB) that form dense and diverse biofilms covering extensive intertidal sedimentary areas. MPB biofilms are highly productive, capable of exceeding the areal and ecosystem-level productivity of the overlying phytoplankton.The capacity for motility has been hypothesized to be a major factor explaining the evolutionary success, diversification, and productivity of benthic diatoms, as it allows for coping with and actively exploiting the resource heterogeneity of the sedimentary habitat. While this unique form of behavioral photoregulation has attracted considerable interest over decades, several major gaps in knowledge still persist. This project addresses these gaps through the use of novel methodological approaches, designed to overcome the limitations of previous studies: (i) combining for the first time microfluidics and microscopy bioimaging of photosynthetic activity, enabling the study of single cell photobehavior and photosynthesis in light microgradients; and (ii) applying a high-throughput chlorophyll fluorescence imaging method, recently developed by the PI and team members, allowing the fast and integrated quantification of photosynthetic and VM responses of multiple samples, including PSII photoinactivation and repair, key processes of photoinhibition. The results of the project are expected to generate an integrated understanding of the adaptive value of phototaxis in diatoms, and of how photobehavior enhances photosynthesis at the cellular level, and ultimately supports the high productivity of estuarine intertidal areas.
