Significance of metabolically active bacterioplankton in the frontal regions of the Northeastern Arabian Sea

Abstract

The Northeastern Arabian Sea (NEAS) experiences convective mixing during winter monsoon, which facilitates nutrient enrichment in the surface layers. This region is also known to harbour several sea surface temperature (SST) fronts and filaments, hotspots of primary production and bacterioplankton activity, which play an important role in the microbial loop. Observations on bacterioplankton with an emphasis on their physiological state, i.e., metabolically active (HNA; high nucleic acid content) bacteria and metabolically inactive (LNA; low nucleic acid content) bacteria were carried out in the NEAS region during early and peak winter monsoon (EWM and PWM). HNA bacteria were dominant in frontal zones coupled with high bacterial production (BP) indicating their significant role, whereas, LNA bacteria were abundant in non-frontal regions irrespective of the seasons. The differentiation in bacterial metabolic types points out organic matter enrichment in the fronts. During PWM, the nutrient concentration increased resulting in a further increase in HNA bacteria and BP in frontal regions. The transparent exopolysaccharides (TEP) were also higher in frontal zones, and inversely related to total bacterial abundance indicating a fast turnover of the organic matter. The age of the front and background condition prior to the formation of the front influences the relative contribution by bacterioplankton food web dynamics and these attributes would probably also help predict bacterial activities.