Influence of salinity stress on bacterial community composition and ß-glucosidase activity in a tropical estuary: Elucidation through microcosm experiments.

The influence of changing salinity on community composition and functional activity (Bacterial Production (BP) and ectoenzyme activity) of major bacterial taxa was evaluated using microcosm experiments in a tropical monsoon influenced estuary. Natural bacterial inocula at different salinities, representing marine, brackish, and freshwater, were inter-transferred and elucidated their response with an emphasis on community composition and ß-Glucosidase (BGase) activity. The results revealed a significant decrease in the total bacterial count (TBC) and BP on the translocation of bacterial inocula to different salinity conditions in the case of freshwater bacteria. However, a significant increase in BGase activity coupled with shifts in the studied bacterial groups was evident in the case of marine as well as freshwater bacteria. Quantitative PCR (qPCR) revealed a shift in major bacterial taxa upon translocation to different waters, which was dependent on salinity and the source of inocula. Redundancy and qPCR analyses showed that members belonging to Gammaproteobacteria and Betaproteobacteria were higher, and possibly influenced BGase activity in marine and freshwater, respectively. Translocation of marine inocula to brackish and freshwater resulted in an emergence of Bacteroidetes, Actinobacteria, and Betaproteobacteria, respectively. Whereas, when freshwater inocula were translocated to marine or brackish water, Alphaproteobacteria and Gammaproteobacteria taxa emerged, and this was coupled with increased BGase activity. In contrast, brackish water bacteria showed a strong persistence in bacterial community composition when translocated to different salinities within this estuary. Such phylogenetic persistence or changes suggests species level shifts in specific bacterial taxa, and unravelling the same using different functional gene markers would ascertain their role in organic matter processing and is way forward.

Seasonal variation in ß-glucosidase-producing culturable bacterial diversity in a monsoon-influenced tropical estuary.

The episodic monsoon condition in a tropical estuarine environment inputs high allochthonous materials that are rich in carbohydrates and are mainly broken down by enzymatic hydrolysis thus alter both organic matter and microbial community composition of an estuary. ß-Glucosidases are one of the enzymes mediating the degradation of carbohydrates and are majorly produced by heterotrophic bacteria. The present study elucidated ß-glucosidase-producing culturable bacterial diversity and measured their activity during non-monsoon and monsoon seasons in Zuari estuary. The results revealed that both bacterial abundance and ß-glucosidase activity decreased significantly from non-monsoon to monsoon, whereas phylogenetic diversity increased. Majority of ß-glucosidase producers during non-monsoon belonged to the members of Bacillales (53%), Pseudomonadales (26%), and Vibrionales (11%) which shifted to the members of Enterobacteriales (51%), Bacillales (14%), Alteromonadales (12%), Aeromonadales (9%), Xanthomonadales (7%), Pseudomonadales (5%), and Flavobacteriales (2%) during the monsoon. The shift in bacterial community structure points out the occurrence of different allochthonous forms with carbohydrate-metabolizing ability during the monsoon, and their relevance in ecology and health of this estuary can be elucidated by studying their functional diversity and is a step ahead.

Elucidation of the tidal influence on bacterial populations in a monsoon influenced estuary through simultaneous observations.

The influence of tides on bacterial populations in a monsoon influenced tropical estuary was assessed through fine resolution sampling (1 to 3 h) during spring and neap tides from mouth to the freshwater end at four stations during pre-monsoon, monsoon and post-monsoon seasons. Higher abundance of total bacterial count (TBC) in surface water near the river mouth, compared to the upstream, during pre-monsoon was followed by an opposite scenario during the monsoon When seasonally compared, it was during the post-monsoon season when TBC in surface water was highest, with simultaneous decrease in their count in the river sediment. The total viable bacterial count (TVC) was influenced by the depth-wise stratification of salinity, which varied with tidal fluctuation, usually high and low during the neap and spring tides respectively. The abundance of both the autochthonous Vibrio spp. and allochthonous coliform bacteria was influenced by the concentrations of dissolved nutrients and suspended particulate matter (SPM). It is concluded that depending on the interplay of riverine discharge and tidal amplitude, sediment re-suspension mediated increase in SPM significantly regulates bacteria populations in the estuarine water, urging the need of systematic regular monitoring for better prediction of related hazards, including those associated with the rise in pathogenic Vibrio spp. in the changing climatic scenarios.