Carbon flow mediated by brackish-water algal mats in Indian Sunderbans: A case study for environment-friendly and sustainable source of biodiesel

High nutrient influxes through regional river systems strongly affect the algal populations in the heavily populated Sunderbans brackish-water ecozone. Twelve brackish-water sites in the Indian Sunderbans were surveyed for growth and carbon (C) sequestration performances of algal / cyanobacterial taxa dominating the mat-forming communities at different seasons. The dominant species at the twelve sites included seven genera (Spirogyra, Rhizoclonium, Ulva, Cladophora, Pithophora, Chaetomorpha) belonging to Chlorophyta, three genera (Polysiphonia, Gracilaria, Catenella) belonging to Rhodophyta and Lyngbya majuscula from cyanobacteria. Environmental variables and biomass productivity parameters monitored at each site and analysed by statistical techniques indicated that nutrient availability, particularly dissolved P concentration and N:P ratio in the water column, as well as with salinity, mainly affected biomass yield and C sequestration of mat-forming genera and net OC input into water column. However, OC contents of the underlying muck proved to be very stable, with small increments following each seasonal bloom of algal mats. High biomass yields (34-3107 g/m²) of the dominant mat components accumulated large stocks of OC, very little of which were found to reach the pedologic pool (muck). Thus the algal mats actually form a large but transient pool of harvestable OC which would ultimately be consumed and emitted back to the atmosphere, but might be alternatively utilized as feedstock for dietary supplements or biofuel production. Availability of important dietary fatty acids in Spirogyra punctulata, Gracilaria sp., Polysiphonia mollis, Rhizoclonium riparium, R. tortuosum, Pithophora oedogonia and Ulva lactuca was considered as suitability of these taxa for nutraceutical applications. Fatty acid compositions of L. majuscula, Catenella repens, R. tortuosum and Cladophora crystallina were estimated to be applicable for producing biodiesel usable in sub-tropical climates.