Variation of Bacterial Metabolic Rates and Organic Matter in the Monsoon-Affected Tropical Estuary (Godavari, India)

Abstract

Bacteria play a central role in the oxidation of organic matter. In tropical estuaries, bacterial metabolic rates and carbon modification are regulated by physicochemical changes influenced by the annual monsoon-mediated river runoff. The present study is the first report on the effect of monsoon-influenced river discharge magnitude on bacterial metabolic rates and organic matter variation in the tropical Godavari estuary and adjacent coastal waters. Concentrations, nature, and sources of particulate organic carbon and nitrogen were investigated by elemental analyser-isotopic ratio mass spectrometer, whereas concentrations of dissolved organic carbon and nitrogen were determined by a high-temperature catalytic oxidation in surface waters of Godavari estuary, east coast of India. Bacterial abundance and metabolic rates were measured, and pCO2 levels were calculated for surface water samples. The maxima obtained in bacterial respiration (BR) rate (994 g C l-1 d-1) were associated with maximum concentrations and the terrigenous nature of organic matter with high C:N ratio (~12). The highly depleted 13C POC (-32.9 _+ 1.4‰) associated with Chl-a maxima suggest its contribution from freshwater planktons. The significant positive correlation of bacterial productivity (BP), bacterial growth efficiency (BGE), and BR with particulate organic matter associated with low DOC:POC ratio could imply a heterotrophic preference for an abundant form of organic matter. We, moreover, uncovered that terrigenous organic matter is modified efficiently in the estuarine transect by heterotrophic bacteria under favorable physicochemical conditions like low salinity and high nutrient content. Furthermore, the majority (76%) of BGE values were <8%, and average pCO2 levels were ~14 times greater than the atmospheric levels. Overall, this study reported that the Godavari estuary is net heterotrophic and hence significantly contributes to global carbon cycle biogeochemistry.