ABSTRACT: Bacterially produced extracellular enzymes (EEs) 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. EEs 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 EEs and their substrates. However, whether EEs 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 the cold-tolerant Psychrobacter strain 7E, under analogue sea-ice conditions using a standard fluorescence-based activity assay. EEs produced by the psychrophile were active at the most extreme conditions tested, i.e. temperature of -8°C and salt concentration of 120 ppt, with activity enhanced if the EEs concerned were produced under subzero hypersaline conditions. EEs 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.

中文翻译：

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低温高盐度条件下模型冷适应细菌和北极海冰微生物群落的细胞外酶活性

摘要：细菌产生的细胞外酶 (EEs) 在海洋环境中的有机物质循环中发挥着重要作用，将大化合物分解为小到足以跨细胞膜运输的化合物。EE 可能在海冰盐水中发挥特别重要的作用，因为冷冻会将细菌和有机材料浓缩到盐水袋中，导致 EE 与其基质之间的相遇率更高。然而，EE 是否能够在海冰盐水的极端条件下运行，尤其是在冬季，尚不清楚。在这里，我们表征了由嗜冷菌Colwellia psychrerythraea菌株 34H 和耐寒Psychrobacter产生的亮氨酸氨肽酶的 EE 活性 (EEA)菌株 7E，在模拟海冰条件下，使用标准的基于荧光的活性测定。由嗜冷菌产生的 EE 在测试的最极端条件下是有活性的，即 -8°C 的温度和 120 ppt 的盐浓度，如果相关的 EE 在零度以下高盐度条件下产生，则活性会增强。EES通过所产生的嗜冷菌株的冷和盐活性较低。在冻融循环后使用相同的测定法分析高纬度北极海冰盐水、冰下水和海表微层样品时，EEA 在海冰样品中最高，其活性为 - 10°C 和 142 ppt 的盐度。总体而言，这些结果表明，EEA 有助于整个冬季海冰中有机物质的降解，可能在此过程中维持盐水孔隙中的微生物群落，并改变春季融化释放的有机物质的性质。