Bacterial activity in sea ice and open water of the Weddell Sea, Antarctica: A microautoradiographic study

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Abstract
Metabolic activity of bacteria was investigated in open water, newly forming sea ice, and successive stages of pack ice in the Weddell Sea. Microautoradiography, using [3H]leucine as substrate, was compared with incorporation rates of [3H]leucine into proteins. Relation of [3H]leucine incorporation to the biomass of active bacteria provides information about changes of specific metabolic activity of cells. During a phytoplankton bloom in an ice-free, stratified water column, total numbers of bacteria in the euphotic zone averaged 2.3 × 105 ml−1, but only about 13% showed activity via leucine uptake. Growth rate of the active bacteria was estimated as 0.3–0.4 days−1. Total cell concentration of bacteria in 400 m depth was 6.6 × 104 ml−1. Nearly 50% of these cells were active, although biomass production and specific growth rate were only about one-tenth that of the surface populations. When sea ice was forming in high concentrations of phytoplankton, bacterial biomass in the newly formed ice was 49.1 ng C ml−1, exceeding that in open water by about one order of magnitude. Attachment of large bacteria to algal cells seems to cause their enrichment in the new ice, since specific bacterial activity was reduced during ice formation, and enrichment of bacteria was not observed when ice formed at low algal concentration. During growth of pack ice, biomass of bacteria increased within the brine channel system. Specific activity was still reduced at these later stages of ice development, and percentages of active cells were as low as 3–5%. In old, thick pack ice, bacterial activity was high and about 30% of cells were active. However, biomass-specific activity of bacteria remained significantly lower than that in open water. It is concluded that bacterial assemblages different to those of open water developed within the ice and were dominated by bacteria with lower average metabolic activity than those of ice-free water.

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