How does frontal age influence physiological status of bacteria: a case study from the Northeastern Arabian Sea

Abstract

Sea surface temperature fronts in the Northeastern Arabian Sea (NEAS) facilitate nutrient enhancement in the surface layers, resulting in high primary production. Such high production areas contribute to the accumulation of fresh organic matter rich in carbohydrates, thereby supporting higher heterotrophic metabolism. Transparent exopolysaccharides (TEP) are one of the carbohydrate polymers released as exudates during primary production or decaying process and their breakdown is mediated by a set of ectoenzymes that includes glucosidases and chitinases. Observations were carried out in different aged NEAS fronts to elucidate the influence of frontal age on the abundance of TEP, bacterial nucleic acid content (high nucleic acid content, HNA and low nucleic acid content, LNA), and ectoenzymatic activities. The fronts were classified as younger and older, depending on their first appearance and sampling day. A clear transition from HNA to LNA bacteria, variations in TEP and related ectoenzymatic activities in the fronts was influenced by the frontal age. The younger fronts were dominated by HNA bacteria, alpha-glucosidase, and chitinase activity. In contrast, an acceleration in Beta-glucosidase activity, bacterial production, which was related to TEP concentration, was evident in the older fronts. Further, a significant increase in protist numbers in older fronts was related to effective grazing on HNA bacteria and TEP. The active turnover of TEP by bacteria and protists facilitates strong microbial loop in NEAS frontal regions. Elucidating contribution of such changes in the frontal areas will provide a basis for a better understanding of microbial carbon cycling.