Vibrio cholerae in waters of the Sunderban mangrove: relationship with biogeochemical parameters and chitin in seston size fractions

Abstract

Wetland dynamics are probably linked to cholera endemicity in South Asia. We focus on links between Vibrio cholerae abundance, chitin content and suspended particle load in size fractions of suspended particulate matter (SPM) along the salinity gradient of Sunderban mangrove waters. SPM decreased downstream, while salinity increased from 0.2 to 4. Particulate organic carbon (90 ± 25 µM) and nitrogen (9.1 ± 3.3 µM) highly correlated with SPM and turbidity, suggesting a significant contribution of fine particles to organic matter. Total chitin ranged 1-2 mg/l and decreased downstream. The distribution among size fractions of SPM, chitin and V. cholerae O1 (the bacterial serogroup mainly associated with cholera epidemics) was similar, with ~98% of the total in the fraction <20 µm. In comparison, the number of V. cholerae O1 attached to zooplankton and microplankton size classes >20 µm was almost negligible, in contrast to usual assumptions. Thus, microdetritus, nanoplankton and fungal cells in size classes <20 µm represent a chitinaceous substrate on which V. cholerae can grow and survive. Total bacteria, cultivable vibrios and V. cholera O1 increased 5-10 times downstream, together with salinity and nitrite concentration. Overall, nitrate and silicate concentrations were relatively constant (>22 µM N and 100 µµM Si). However, nitrite increased ~9 times in the outer sector, reaching ~1.2 µM N, probably as a result of increased abundance of nitrate-reducing vibrios. A characterization of Vibrio habitats that takes account of the presence of nitrate-reducing bacteria could improve the understanding of both mangrove nitrogen cycling and cholera seasonality.