Live benthic diatoms from the upper continental slope: extending the limits of marine primary production

Discovery of obligate benthic diatoms living as deep as 191 m substantially extends the known depth range of these primary producers and holds significant implications for oceanic productivity and biogeochemical cycling. Species of the epipsammic, monoraphid genus Cocconeis dominated the >= 35 species of living benthic diatoms identified from the North Carolina continental margin in samples collected at bottom depths from 67 to 191 m. A total of 126 species were identified from prepared samples, more than 90% of which are obligate benthic forms. Mid-day, near bottom, photosynthetically active radiation values recorded at the 191 m site averaged 0.106 mu mol photons m(-2) S-1, representing about 0.028% of surface incident radiation and resulting from a water column attenuation coefficient of 0.0446 m(-1), The presence of active benthic microalgae in these extremely low light conditions suggests the development of special light-harvesting adaptations including elevated levels of the blue-light absorbing accessory pigment, fucoxanthin. Extending the limit of benthic microalgal production to upper slope depths offshore from North Carolina increases the estimated total benthic primary production in that area of the continental margin by about 14%. At present, extrapolating potential increases in benthic microalgal biomass and production resulting from extending the depth limits of viable benthic microalgae to a larger, global, oceanic scale is limited by paucity of data.