Microbial remineralization of the dissolved organic carbon(DOC)derived from the benthic producers of Gulf of Mannar(GoM) reef ecosystem

Abstract

Coral reefs are one of the most productive ecosystems with primary production rates compared to the tropical rain forest. Carbon dioxide is fixed as organic matter through photosynthesis and contributes to the temporary or permanent storage of carbon in the ocean. This is achieved by the sedimentation of particulate organic matter (POM) and transport of dissolved organic matter (DOM) to deeper environments through the movement of water masses. Almost, 10-15% of the gross organic production by reef benthic producers is released into the external environment and act as a main source of DOC in the reef environment. DOC represents one of the largest reservoirs of CO2 in the ocean equivalent to the atmospheric CO2 and acts as a source of carbon and energy for many organisms that depend on organic carbon for nutritional sources. DOC released by macroalgae enters into the food web through the microbial loop and plays a vital role in trophic energy transfer, however, not all DOC enters the microbial loop where some amount remains resistant to microbial action representing the recalcitrant DOC. Our results show that the organic matter released by different macroalgae accelerated the growth of bacterioplankton, and differed in their effect on bacterial community composition. All the macroalgae studied in this experiment released a significant amount of DOC into the reef with T. ornata the maximum DOC and H. tuna the least DOC with 72.1 μM C/g/h (dry weight) and 1.9 μM C/g/h (dry weight) respectively. The amount of DOC released by different macroalgae remained recalcitrant to microbial action by the 100th day ranged from 9.2% to 32.2% among different macroalgae. Green macroalgae possessed a weak shift in bacterial community composition whereas the brown macroalgae altered the bacterial community composition. The brown macroalgae contributed to a slight decrease in the dominant bacterial groups thereby enhancing the dormant bacterial groups. However, in this study, the existing bacterial population was able to utilize maximum DOC resulting in a weak shift in the bacterial composition among the green macroalgae giving no chance for the dormant bacterial communities. The type of macroalgae and the composition of

DOC they produce are considered critical in determining the effect of DOC on the bacterial community. These comprehensive data provide an important basis for understanding the influence of DOC over coral-associated bacterial communities and the dynamics of carbon cycling in algal dominated reefs.