G Model revisited: Seasonal changes in the kinetics of sulphate-reducing activity in the salterns of Ribandar, Goa, India

Abstract

To evaluate the contribution of Sulfate-Reducing Bacteria (SRB) to the carbon flux from marine salterns, sulfate reducing activity (SRA) was measured for a year (Sep 2000-Aug 2001). This covered the pre-monsoon period (Feb-May), the monsoon (June-Sept) and the post-monsoon (Oct-Jan) in the salterns of Ribandar, Goa, India. Sulphate reduction was examined to gain insights into the decomposition characteristics (kinetics) of organic carbon; this connection is based on the theory of stoichiometric coupling between anaerobic carbon degradation and sulphate reduction to sulphide. SRA was measured using radiotracer techniques. Spatially, recalcitrance of the substrates, as depicted by the reaction rate coefficient (k), increased with depth, except during the monsoons. The values ranged from 0.0061 during the pre monsoon to 0.06 month-1 during the post-monsoon at 5-10 cm depth. The recalcitrance was highest at the surface during the monsoons at a k value of 0.014 month-1. The anaerobic degradation of labile organic matter followed first order kinetics or G model during non-monsoon seasons when the substrate was limiting due to closure of the system to the addition of extraneous source of organic matter. However, the integrated recalcitrance or the reaction rate coefficient was much higher during the post monsoon at 0.642 month-1 compared to 0.053 month-1 during the pre-monsoon season. Interestingly, the reaction rate followed zero-order kinetics when the substrate was non-limiting during monsoon due to exposure to external inputs.