Influence of Salinity on Bacterioplankton Communities in a Tropical Coastal Brackish Water Lagoon- Chilika

Behera, Swarnaprava (2015) Influence of Salinity on Bacterioplankton Communities in a Tropical Coastal Brackish Water Lagoon- Chilika. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Chilika is the largest coastal brackish water lagoon of Asia located along the East coast of India (Bay of Bengal). It was designated as the first Ramsar site in India during 1981 because of its rich aquatic biodiversity and socio-economic importance. Hydrological conditions in this lagoon are dynamic, receives both freshwater and coastal seawater inputs, resulting in salinity variation as a result of mixing. The present study was undertaken to investigate the composition of bacterioplankton communities in regions of the lagoon which receive relatively higher freshwater input and limited mixing with seawater results in relatively lower salinity. Surface water was collected once a month between June and August 2014 from two stations (CNS18 and CNS19) in the lagoon. Additionally, sampling during same period was also conducted in another station (CNS14) which has higher salinity due to its close proximity to coastal Bay of Bengal. At the time of sampling, in situ physico-chemical parameters including salinity were also measured. Using an uncultured approach based on bacterial 16S rRNA PCR, clone library and sequencing, community composition of bacterioplankton from three stations were assessed. In total, 179 bacterial 16S 7rRNA clones were sequenced as part of this study and these were broadly grouped into 116 operational taxonomic units (OTUs). Based on molecular phylogeny, sequences showing phylogenetic affiliation with seven major bacterial phyla namely, Proteobacteria, Planctomycetes, Bacteroidetes, Firmicutes, Actinobacteria, Cyanobacteria and Verrucomicrobia were encountered in this study. Alphaproteobacteria and Planctomycetes-like sequences were found in all the months sampled across three stations. Many of the sequences showed appearance disappearance pattern in link with salinity. Cyanobacteria-like 16S rRNA sequences were exclusively encountered in CNS14 and CNS18 during June and July but were absent in August when salinity was much less due to increased freshwater intrusion as a result of precipitation. Similarly, Betaproteobacteria-like 16S rRNA sequences were found exclusive to CNS18 and CNS19 during July and August when the salinity was zero but never encountered in any of studied months for CNS14. The presence of Firmicutes-like 16S rRNA sequences in stations CNS18 and CNS19 indicated that organic matter input from nearby landmass through drainage and freshwater flow sustained the bacterioplankton population in Chilika lagoon. Interestingly, Epsilonproteobacteria-like 16S rRNA sequences were encountered only in CNS19 station during June when salinity was relatively higher. Erythrobacter-like 16S rRNA sequences were encountered from June-August indicating that some of the bacterioplankton cells persisted in study area irrespective of observed changes in salinity. Principal Component Analysis (PCA) clearly showed that salinity plays an important role in shaping bacterioplankton community structure across studied stations in Chilika lagoon which can ultimately influence their functional significance in this ecosystem.

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