Sea ice microbial communities. VIII. Bacterial production in annual sea ice of McMurdo Sound, Antarctica

Abstract

Sea ice microalgae may contribute up to 50 % of annual primary production in some regions of the Southern Ocean, but little is known of bacterial production in sea ice. Here we describe seasonal net accumulation (from microscopical direct counts), rate of 'instantaneous' growth (from 'H-thymidine incorporation), and importance of carbon production by bacteria in annual sea ice of McMurdo Sound during the 1982 austral spring and summer bloom of microalgae. Bacterial number and biomass increased less than 10-fold in sea ice over a period of 21/2 mo; yet bacterial cell production rate increased by more than 3 orders of magnitude. Bacterial growth increased throughout the microalgal bloom, but final bacterial biomass was less than 1 % of microalgal biomass. Growth rates calculated from estimates of net accumulation of cells and thymidine incorporation were similar for congelation ice beneath 5 cm of snow and platelet ice beneath 0 to 5 cm of snow. Bacterial production (cell and carbon) lagged behind at first, but later paralleled the rate of primary production in sea ice. Bacterial carbon production was only 9 % of primary production, while maximal rates of growth (μ = 0.02 to 0.2 d -1 ) were comparable to those reported for bacterioplankton of the Southern Ocean. Bacterial biomass and production in sea ice were equivalent to that found in several meters of underlying seawater. Significant correlations were found between bacterial production (cell, biomass and thymidine incorporation per cell) and growth, and microalgal biomass, production, and growth suggesting potential coupling between bacterial growth and microalgal photosynthetic metabolism in sea ice. We propose that the timing and amount of bacterial production in sea ice are dependent upon the growth phase of associated microalgae, the quantity and quality of EOC and compounds in the DOM pool available for bacterial growth, rates of bactivory and herbivory, and retention of the platelet ice layer and bottom layer of congelation ice. Further investigations into the fate of this bacterial production are needed to more fully determine the dynamic aspects of the microbial loop in sea ice.