Seasonal variation in bacterial community composition and β-glucosidase expression in a tropical monsoon-influenced estuary

Tidal estuaries receive carbohydrate-rich organic matter from various sources, which are mainly broken down by enzymatic processes. β-glucosidase (β-Glu) is one of the ectoenzymes produced by heterotrophic bacteria. An investigation was carried out along the tropical monsoon-influenced Zuari estuary to examine seasonal variability in major bacterial taxa, β-Glu gene abundance, and number of cells expressing β-Glu. The results revealed a higher number of β-Glu expressing cells, and higher β-Glu gene and bacterial abundance at the estuarine mouth compared to the upstream area, and this was coupled with high phytoplankton biomass and low transparent exopolysaccharides. A clear distinction in bacterial assemblages along with β-Glu expression levels was evident during different seasons. Likewise, a spatial pattern in major bacterial taxa was observed along the salinity gradient, with high proportions of Alpha- and Gammaproteobacteria at the saline mouth and Betaproteobacteria at the upstream, freshwater region. During the southwest monsoon, which brings high freshwater influx, Actinobacteria and Firmicutes were abundant in the mid-estuarine region. Bacterial production at this time was mainly fueled by allochthonous riverine sources, with a significant decrease in the number of cells expressing the β-Glu to gene ratio. β-Glu gene abundance was related to diverse groups (Alpha- and Gammaproteobacteria, Actinobacteria, and Firmicutes). The number of cells expressing β-Glu covaried with Gammaproteobacteria abundance and was influenced by suspended load, salinity, chlorophyll a and organic pool. Future studies should assess response of marine, brackish, and freshwater bacteria to different environmental conditions to ascertain their specific role in organic matter processing.