Diversity and Environmental Tolerance of Benthic Diatoms Occurring in the Tidal Flat of Seto Inland Sea, Japan

Tidal flats are one of the most productive coastal habitats, where various organisms not only benthic flora and fauna, but also fishes or birds are abundantly found. These organisms are principally supported by the food chain that primarily originated from microphytobenthos, mainly consisted of benthic diatoms; however, our knowledges regarding to these diatoms have been scanty.

In this study, at a well-preserved and natural tidal flat of Seto-Inland Sea, Japan, we investigated occurrence of benthic diatoms throughout years, from February 2018 to August 2019, and found diverse and abundant occurrence of benthic diatoms based on a MPN culture method and chlorophyll a analysis. An average of those cell densities was 1.95×10⁵ cells cm^-3, and that of chlorophyll a was 3.68±2.46 μg cm^-3, which were far denser than the averages of the planktonic populations in this area. Under a scanning electron microscopy, 14 genera consisted of 29 species were observed; among them, Navicula and Nitzschia were densely occurring regardless to seasons and sediment types. A dendrogram based on diatom species and density showed distinct two clusters; those delineated by an event of extensive sedimentation caused by a torrential rain in July 2018. After 20 days of the event, breakouts of Navicula sp. and Amphora sp. were found, and they recorded maximum cell densities of the year, insisting their ability to adapt rapid environmental change.

In intertidal area, environmental parameters drastically change due to tidal cycle. To reveal their unique physiology that enables them adapting such drastic changes, especially acclimating high light exposure during low tide, culture experiment using original diatom strains isolated from the site was conducted with photosynthetic measurement by a pulse amplitude modulated (PAM) fluorometer. All diatom strains showed higher NPQ (non-photochemical quenching) compared with planktonic diatom strain under high light exposure (1,200 μmol photons m^-2 s^-1), indicating benthic diatoms were proficient in dissipating excess light as a manner of organisms on tidal flats. Farther results will be presented in the conference.