Seasonal Variation in Surface Water Temperature Influences Bacterioplankton Community Structure and Diversity in Sundarbans Mangrove Ecoregion

Ghosh, Anwesha (2015) Seasonal Variation in Surface Water Temperature Influences Bacterioplankton Community Structure and Diversity in Sundarbans Mangrove Ecoregion. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS dissertation of Anwesha Ghosh) Anwesha_Ghosh_13IP003.pdf - Submitted Version Restricted to Repository staff only Download (2MB)

Abstract

Anthropogenic activities that annually release gigatonnes of carbon into the Earth’s atmosphere have resulted in the change of environmental conditions on Earth including rise in seawater surface water temperature (SWT). Coastal ecosystems, including mangroves are especially vulnerable to rise in SWT. These changes are likely to affect bacterioplankton community structure and function in mangrove environments. The present study was conducted to understand the change in bacterioplankton community structure and diversity with seasonal variation in SWT. The study was undertaken across two predefined sampling stations of Sundarbans Biological Observatory Time Series (SBOTS) designated as Station 1 (Stn1) and Station 3 (Stn3) spanning over the pre-monsoon (June-July), monsoon (August-September) and post-monsoon (October) of 2014. Environmental parameters including SWT were measured in situ at the time of sampling. A clone library and sequencing approach using 16S rRNA was undertaken to understand the significance of SWT in shaping bacterioplankton communities across both stations. Based on molecular phylogeny of 200 clones sequenced, Proteobacteria were found to be the most dominant phylum with predominance of Gammaproteobacteria, Alphaproteobacteria across both stations. Besides Proteobacteria, sequences were found to be affiliated other bacterial phyla such as Bacteroidetes, Planctomycetes, Cyanobacteria, Actinobacteria, Sphingobacteria and Firmicutes as well as the domain Archaea. In total, 140 operational taxonomic units (OTUs) were encountered based on the analysis of 200 clones sequenced as part of this study. Diversity of OTUs was found to be highest in the month of June (35 OTUs) and lowest in the months of September and October (28 OTUs for each month). Marinobacter and Hyphomonas-like 16S rRNA signatures (members belonging to Proteobacteria) were found in both stations from July to August irrespective of the changes in SWT. Overwhelming abundance of Muricauda-like 16S rRNA sequences (a member of the phylum Bacteroidetes) were encountered exclusively in the month of July in Stn1 when there was an increase in SWT. On the other hand, Arcobacter-like 16S rRNA sequences (a member of Proteobacteria) were encountered exclusively in Stn3 during the months of August and September when recorded SWT was lesser compared to July. A fall in Fusibacter-like 16S rRNA sequences (belonging to Firmicutes) was encountered in clone libraries during the postmonsoon (October) when surface water temperature started to decrease gradually. Incidentally, 27 OTUs belonging to Proteobacteria were always present in the study area irrespective of seasonal variation in SWT. To understand the influence of seasonal variation in SWT on bacterioplankton community structure, principal component analysis (PCA) was performed. The PCA plots highlighted a strong correlation between SWT and bacterial diversity. A substantial portion of the variance in bacterioplankton diversity was also accounted for by other environmental factors such as dissolved nitrate concentration and salinity. Changes in surface water temperature can affect environmental parameters such as dissolved oxygen (DO), pH and salinity, which can ultimately influence bacterioplankton community structure and their functions including aquatic nutrient cycling in Sundarbans ecoregion.

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