Microbial ecology of sea ice at a coastal Antarctic site:community composition, biomass and temporal change
The coastal sea ice in the vicinity of Davis Station, Antarctica (68* 35' S, 77* 58' E), supported a diverse microbial community which varied in composition and biomass in response to increasing insolation and temperature during the austral summer. To understand more fully the fate of photosynthetically fixed carbon in sea ice, we examined the dynamics of community composition, biomass and production in autotrophs, heterotrophic protozoa and bacteria. The microbial community inhabiting the bottom few centimeters of land fast ice differed markedly from the interior communities in taxonomic composition and biomass and in the timing and fate of production. Total microbial biomass integrated throughout the ice depth declined during the season from a mean of 1150 mg C m-2 on 17 November to 628 mg C m-2 by 22 December. This largely reflected a decrease in the biomass of the bottom ice community which was dominated by the diatom Entomoneis spp. In contrast, the biomass of the interior ice community increased during summer and was dominated by autotrophic forms <20 um in length with a small dinoflagellate, Gymnodinium sp., becoming particularly abundant. Heterotrophic protozoa, composed of mainly nanoflagellate, euglenoid and dinoflagellate taxa, contributed between 16 and 19% of the total integrated microbial biomass in the interior ice and between 1 and 11% in the bottom ice. The biomass of heterotrophic protozoa increased throughout the ice depth during summer and estimated taxon-specific net growth rates ranged between 0.168 d-1 for a heterotrophic euglenoid and 0.05 d-1 for the heterotrophic nanoflagellate population over a 23 d period. Bacterial biomass varied by several orders of magnitude between ice depths mainly due to the occurrence of an abundant population of large epiphytic bacteria attached to Entomoneis spp. in the bottom ice. However, bacterial biomass contributed a similar proportion of between 4 and 16% of the total microbial biomass in both interior and bottom ice. The biomass of unattached bacteria increased throughout the ice depth during summer and exhibited an estimated net growth rate of 0.05 d-1. These data are used to quantify autotrophic production in bottom and interior communities, to estimate the flux of carbon to heterotrophs and to illustrate the complexity of the trophic interactions in coastal sea ice.