Seasonal occurrence of anoxygenic photosynthesis in Tillari and Selaulim reservoirs, Western India

摘要

Phytoplankton and bacterial pigment compositions weredetermined by high performance liquid chromatography (HPLC) and liquidchromatography-mass spectrometry (LC-MS) in two freshwater reservoirs(Tillari Dam and Selaulim Dam), which are located at the foothills of theWestern Ghats in India. These reservoirs experience anoxia in thehypolimnion during summer. Water samples were collected from both reservoirsduring anoxic periods while one of them (Tillari Reservoir) was also sampledin winter, when convective mixing results in well-oxygenated conditionsthroughout the water column. During the period of anoxia (summer),bacteriochlorophyll (BChl) e isomers and isorenieratene, characteristic ofbrown sulfur bacteria, were dominant in the anoxic (sulfidic) layer of theTillari Reservoir under low light intensities. The winter observationsshowed the dominance of small cells of Chlorophyll b-containing green algae andcyanobacteria, with minor presence of fucoxanthin-containing diatoms andperidinin-containing dinoflagellates. Using total BChl e concentrationobserved in June, the standing stock of brown sulfur bacteria carbon in theanoxic compartment of Tillari Reservoir was estimated to be 2.27 gC m?2, which is much higher than the similar estimate for carbon derivedfrom oxygenic photosynthesis (0.82 gC m−2. The Selaulim Reservoir alsodisplayed similar characteristics with the presence of BChl e isomers andisorenieratene in the anoxic hypolimnion during summer. Although sulfidicconditions prevailed in the water column below the thermocline, theoccurrence of photo-autotrophic bacteria was restricted only to mid-depths(maximal concentration of BChl e isomers was detected at 0.2% of thesurface incident light). This shows that the vertical distribution ofphoto-autotrophic sulfur bacteria is primarily controlled by lightpenetration in the water column where the presence of H₂S provides asuitable biogeochemical environment for them to flourish.