AME 73:51-67 (2014)  -  DOI: https://doi.org/10.3354/ame01704

ABSTRACT: The Northeastern Arabian Sea (NEAS) is a recognized region of intense denitrification, with a 200 to 1200 m anoxic water column. We studied the abundance of bacteria, viral particles and the bacterial community (BC), in addition to various chemical and other biological parameters from the Arabian Sea Time Series (ASTS) station. Water samples from surface, deep chlorophyll maximum (DCM), 250, 500 and 1000 m were collected during spring intermonsoon (SIM), fall intermonsoon (FIM), and northeast monsoon (NEM) seasons and analyzed for various parameters. Bacterial abundance varied seasonally (p ≤ 0.05), with the highest abundance observed during FIM at all sampling depths. Conversely, seasonal variations in viral abundance were minimal, though a significant correlation between viral and bacterial abundance (r = 0.526, p < 0.05, n = 14) was found. Hierarchical clustering of denaturing gradient gel electrophoresis (DGGE) profiles revealed clear patterns of vertical, as well as temporal, partitioning of the BC during all 3 seasons. The BC varied seasonally both in the surface and DCM, whereas in the oxygen minimum zone (OMZ; 250 to 1000 m) it was more or less identical during all 3 seasons in spite of significant seasonal variation in bacterial abundance, pH and dissolved oxygen (DO) levels in the OMZ. Following band-matching, several DGGE bands were excised and sequenced. Phylogenetic analysis of these sequences revealed Alphaproteobacteria, Gammaproteobacteria, and Cyanobacteria as the dominant bacterial groups at the ASTS location. Linkage tree (LINKTREE) and canonical correspondence analysis (CCA) were performed to decipher the effect of environmental factors on the BC. From these analyses, it appears that DO and total organic carbon (TOC) are responsible for vertical separation of the BC between the surface and the OMZ. Our results suggest seasonal variation in the BC occurs in the surface layers, with minimal temporal differences in the OMZ.

KEY WORDS: Arabian Sea · Oxygen minimum zone · Bacterial abundance · Viral abundance · Bacterial community · DGGE · CCA · LINKTREE