Ecological relevance of high and low nucleic acid content bacteria in a monsoon influenced tropical estuary: Implications on the β-Glucosidase activity

Heterotrophic bacteria that differ in their nucleic acid content are frequently observed in aquatic environments. However, their relevance and roles in organic matter utilization in monsoon-influenced tropical estuaries have not been addressed. Transparent exopolymer particles (TEP) are one of the carbohydrate rich organic matter forms in aquatic environments, and their breakdown is mostly mediated by a heterotrophic bacterial ectoenzyme, i.e., β – glucosidase (BGase). The present study evaluated the influence of shifts in high nucleic acid content– (HNA) and low nucleic acid content – (LNA) bacteria on the degradation of TEP with emphasis on BGase activity. For this, monthly observations were carried out along the Zuari estuary, India, on the abundance of HNA, LNA bacteria, bacterial production (BP), and BGase activity. The results showed that the contribution of HNA bacteria to total bacterial count was high and positively influenced the BP and BGase activity during the non-monsoon seasons, which represents a well-mixed water column. During this time, HNA also influenced the active utilization of TEP. Whereas, LNA bacteria dominated during the monsoon, wherein the water column is strongly stratified, and showed decoupling with BGase activity, irrespective of the depth. Redundancy and response curve analyses revealed that the interactive effect of monsoon (rainfall) and stratification significantly altered the bacterial activity in this estuary. Besides, the variability in side scatter (SSC), and fluorescence intensity (FL 1) of HNA and LNA bacteria suggest that these two groups play distinct ecological roles in the tropical estuarine systems. Unravelling the links between the different nucleic acid content bacteria and their phylogenetic composition can provide insights into their functional roles in monsoonal estuaries.