Benthic Diatoms: What Makes Them Unique

Diatoms are found in almost all aquatic environments as long as light is available for their photosynthesis. In coastal ecosystems, pelagic diatoms are extensively studied, but their benthic counterparts, more discreet and more difficult to access, are often ignored. Yet living at much lower light intensities, they are a key compartment of coastal ecosystems functioning, being able to produce as much biomass as pelagic diatoms that live in the well-lit surface waters. Benthic diatoms differ considerably from pelagic forms on several aspects: First, pelagic and benthic diatoms do not follow the same seasonal dynamics: In the bay of Brest, we showed that benthic diatoms are the first to bloom in the season, providing food and energy to the ecosystem right from the beginning of spring (with 60% of the total biomass until April). The system then moves from a system dominated by benthic biomass in early spring to a system where the pelagic biomass dominates. Regarding biodiversity, most benthic diatoms species are different from those found in the water column, they are mostly pennate forms and are far more diverse. On the physiological side, benthic diatoms are incredibly well adapted to low light intensities. However, the mechanisms which result in this adaptation remain unclear. We search for the role of the nanostructure of the frustule in the capture of light, but no difference between pelagic and benthic diatoms could be highlighted so far. Of particular interest is that benthic diatoms able to migrate into the sediment, are adapted to the high Si(OH)4 concentrations found in their surrounding and evidence a multiphasic Si uptake with very high half saturation constant (KS). Their capacity to reverse benthic Si(OH)4 fluxes at the interface indicate the important role they also play in the coastal silica cycle and particularly in controlling the pelagic diatom production. It remains to examine the coupling between carbon and silica at the water/ sediment interface and to determine which factors control the benthic production. Further studies are also needed to understand which evolution has allowed pennate diatoms to colonize so successfully the benthic habitat.