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Extracellular enzyme activity under subzero hypersaline conditions by model cold-adapted bacteria and Arctic sea-ice microbial communities

G. M. Showalter*, J. W. Deming

*Corresponding author:

ABSTRACT: Bacterially produced extracellular enzymes (EE) play an important role in the cycling of organic matter in the marine environment, breaking down large compounds to those small enough to be transported across the cell membrane. EE may play an especially important role within the brines of sea ice, as freezing concentrates both bacteria and organic materials into brine pockets, leading to higher encounter rates between EE and their substrates. However, whether EE are able to perform under the extreme conditions of sea-ice brines, particularly during winter, is unknown. Here, we characterized EE activity (EEA) of leucine aminopeptidase produced by the psychrophilic bacterium Colwellia psychrerythraea strain 34H, and cold-tolerant Psychrobacter strain 7E, under analogue sea-ice conditions using a standard fluorescence-based activity assay. EE produced by the psychrophile were active at the most extreme conditions tested, temperature of –8˚C and salt concentration of 120 ppt (‰), with activity enhanced if the EE concerned were produced under subzero hypersaline conditions. EE produced by the Psychrobacter strain were less cold- and salt-active. When high latitude Arctic samples of sea-ice brine, under-ice water, and the sea-surface microlayer were analyzed using the same assay after a freeze-thaw cycle, EEA was highest in the sea-ice samples, with activity at –10˚C and salinity of 142 ppt. Overall these results indicate that EEA can contribute to the degradation of organic material in sea ice through winter, likely sustaining microbial communities in brine pores in the process and altering the nature of organic material released at spring melt.