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

Abstract. Phytoplankton and bacterial pigment compositions were determined by high performance liquid chromatography (HPLC) and liquid chromatography- mass spectrometry (LCMS) in two freshwater reservoirs (Tillari Dam and Selaulim Dam), which are located at the foothills of the Western Ghats in India. These reservoirs experience anoxia in the hypolimnion during summer. Water samples were collected from both reservoirs during anoxic periods while one of them (Tillari Reservoir) was also sampled in winter, when convective mixing results in well-oxygenated conditions throughout the water column. During the periods of anoxia (summer), bacteriochlorophyll (BChl) e isomers and isoreneiratene, characteristic of brown sulfur bacteria, were dominant in the anoxic (sulfidic) layer of the Tillari Reservoir under low light intensities. The winter observations showed the dominance of small cells of Chlorophyll-b containing green algae and cyanobacteria, with minor presence of fucoxanthin-containing diatoms and peridinin-containing dinoflagellates. Using total BChl-e concentration observed in June, the standing stock of brown sulfur bacteria carbon in the Tillari Reservoir was computed to be 2.4 gC m−2, which is much higher than the similar estimate for carbon derived from oxygenic photosynthesis (0.82 gC m−2). These results highlight the importance of anoxygenic photosynthetic biomass in tropical freshwater systems. The Selaulim Reservoir also displayed similar characteristics with the presence of BChl-e isomers and isorenieratene in the anoxic hypolimnion during summer. Although sulfidic conditions prevailed in the water column below the thermocline, the occurrence of photoautotrophic bacteria was restricted only to mid-depths (maximal concentration of BChl-e isomers was noted at 0.2 % of the surface incident light). This shows that the vertical distribution of photoautotrophic sulfur bacteria is primarily controlled by light penetration in the water column where the presence of H2S provides a suitable biogeochemical environment for them to flourish.