Dimethylsulfoniopropionate (DMSP) Degradation by Marine Bacteria along the Cochin Estuarine System

Dimethylsulfoniopropionate (DMSP) is a key marine organosulfur compound produced by phytoplankton and macroalgae that functions as an osmolyte, antioxidant, and precursor of dimethylsulfide (DMS)—a climate-relevant gas influencing the global radiation balance. Marine bacteria degrade DMSP through demethylation and cleavage pathways, driving the marine sulfur cycle. This study aimed to quantify DMS(P) concentrations and to isolate, identify, and characterize DMSP-degrading bacteria from the Cochin Estuary (CE), Kerala, India.

Surface water and sediment samples were collected from fifteen CE stations across three seasonal regimes (pre-monsoon, monsoon, and post-monsoon) between 2015 and 2018. DMS(P) levels were determined by alkali hydrolysis followed by gas chromatography with headspace sampling. Heterotrophic bacterial abundance was estimated by spread plating on Zobell’s Marine Agar. DMSP-degrading bacteria were isolated on DMSP-enriched minimal medium, and selected isolates were identified by 16S rRNA gene sequencing. PCR amplification was performed to detect DMSP lyase (Ddd) genes, and phylogenetic analyses were conducted using MEGA6.

DMSP concentrations ranged from BDL to 0.15 ng/µL in water and 0.01 to 2.35 ng/µL in sediments, with higher values recorded during the pre-monsoon season. A total of 112 water and 211 sediment bacterial isolates were obtained, with Gram-negative strains dominating (70% in water and 64% in sediment). Sediments harbored higher bacterial counts than water. Four isolates capable of growing on DMSP-enriched medium were identified: Acinetobacter calcoaceticus, Acinetobacter beijerinckii, Bacillus cereus, and Lysinibacillus fusiformis. Amplification of the dddP gene was observed in A. calcoaceticus. Seasonal variations in salinity, temperature, and nutrient levels influenced DMS(P) distribution, with higher concentrations recorded in sediments.

The findings confirm CE sediments as microbial “hotspots,” dominated by γ-Proteobacteria and Firmicutes—groups known for their significant roles in sulfur cycling. Hydrographic seasonality, particularly salinity fluctuations, shaped bacterial diversity and DMSP transformation patterns. The detection of DddP genes suggests active enzymatic cleavage pathways contributing to atmospheric DMS release.

This first baseline study on DMSP degradation in the CE highlights the ecological significance of estuarine sediments in sulfur cycling. The results enhance the understanding of microbial mediation of DMSP catabolism in tropical estuaries and its implications for climate regulation. Further research is warranted to elucidate additional catabolic pathways and environmental controls.