Methanogenesis: Seasonal changes in human impacted regions of Ashtamudi estuary (Kerala, South India)

Environmental variables as well as methanogenic abundance and activity were analysed in selected human impacted regions of Ashtamudi estuary. Sediment samples were collected during summer and monsoon of 2013. Each was analysed for environmental variables such as temperature, pH, electrical conductivity, sulphate, total kjeldahl nitrogen, organic carbon, organic matter and redox potential. Abundance and methanogenic potential of two distinct groups of methanogenic archaea (i.e. aceticlastic and methylotrophic methanogens) were quantified by incubating the sediment samples in basal media, added with acetate or methanol as substrate. Most of the environmental variables showed significant differences spatially and temporally. Among the environmental variables, temperature, pH, electrical conductivity and salinity were higher during summer, while total kjeldahl nitrogen, sulphate and organic carbon were higher during monsoon. Abundance of both aceticlastic and methylotrophic methanogens showed significant variations both spatially and temporally. Aceticlastic methanogens were abundant during monsoon, with a maximum value of 810 ± 13 CFU g−1, and methylotrophic methanogens were abundant during summer, with a maximum value of 1770 ± 30 CFU g−1. Results of methanogenic potential of sediment samples showed a range of 0.01 ± 0.00 to 12.03 ± 0.35 mol m−3. Among the two substrates, methanol favoured the abundance of methylotrophic methanogens, while acetate induced the methanogenic activity. Methanogenic activity was higher during monsoon than summer that can be attributed to the favourable sedimentary conditions (like reduced redox potential and increased substrate availability). Aceticlastic methanogens were abundant at bottom layers and methylotrophic methanogens in top layers of the sediments. The results of canonical correspondence analysis revealed the existence of linear relationship between methanogenic archaea and environmental variables among the sampling stations. Higher values of salinity, electrical conductivity and total kjeldahl nitrogen favoured the distribution and abundance of aceticlastic methanogens. However, the abundance of methylotrophic methanogens was favoured by highly reduced conditions and high pH values.