Enumeration of bacteria from a Trichodesmium spp. bloom of the eastern Arabian Sea: elucidation of their possible role in biogeochemistry

Abstract

The carbon-flux via algal bloom events involves bacteria as an important mediator. The present study, carried out during the spring inter-monsoon month of April 2008 onboard CRV Sagar Manjusha-06 in the Eastern Arabian Sea, addresses the bloom-specific flow of carbon to bacteria via chromophoric dissolved organic matter (CDOM). Eleven stations monitored were located in the coastal, shelf and open-ocean areas off Ratnagiri (16 degrees 59'N, 73 degrees 17'E), Goa (15 degrees 30'N, 73 degrees 48'E) and Bhatkal (13 degrees 58'N, 74 degrees 33'E) coasts.Visible bloom of "saw-dust" color in the Ratnagiri shelf were microscopically examined and the presence of cyanobacteria Trichodesmium erythraeum and T. thieabautii with cell concentrations as high as 3.05 x 10(6) trichomes L(-1) was recorded. Total bacterial counts (TBC) varied between 94.09 x 10(8) cells L(-1) in the bloom to 1.34 x 10(8) cells L(-1) in the non-bloom area. Chromophoric dissolved organic matter (CDOM) concentrations averaged 2.27 +/- 3.02 m(-1) (absorption coefficient 325 nm) in the bloom to 0.28 +/- 0.07 m(-1) in the non-bloom waters respectively. CDOM composition varied from a higher molecular size with lower aromaticity in the bloom to lower molecular size and increased aromaticity in the non-bloom areas respectively. Strong positive relationship of TBC with Chlorophyll a (R(2)=0.65, p less than 0.01) and CDOM concentrations (R (2)= 0.8373, p less than 0.01) in the bloom area indicated hydrolysis and/or uptake of CDOM by bacteria. Absorption by mycosporine-like amino acid palythene (lambda max=360 nm) was recorded in the filtrate of bloom. Morphotypes of Trichodesmium-associated bacteria revealed a higher frequency of Gram-positive rods. The role of bacteria in relation to changing CDOM nature and as a factor in affecting oxygen content of the water column is discussed in context of the Arabian Sea.