Betaproteobacteria growth and nitrification rates during long-term natural dissolved organic matter decomposition experiments

As a first attempt to relate net nitrification rates with bacterial community structure in the coastal embayment of the Ria de Vigo (NW Spain), a set of 6 long-term (lasting 53 to 74 d) dissolved organic matter decomposition experiments were conducted in the dark and at a constant temperature of 15 degrees C with surface seawater collected in January, February, April, and June 2008. Net nitrification rates were estimated from nitrate concentration changes. Evolution of bacterial community composition was followed using catalyzed reporter deposition-fluorescent in situ hybridization (CARD-FISH) and specific probes for 6 relevant bacterial groups, including Beta-and Gammaproteobacteria. The growth rates of Betaproteobacteria showed a significant linear positive relationship with nitrification rates that explained 82% of the observed variability, which strongly suggests that the main nitrifying microorganisms during the incubations belonged to the Beta subclass of Proteobacteria. A positive relationship was found between net nitrification rates and both ammonium (r(2) = 0.92) and phosphate (r(2) = 0.97) concentrations, which suggests a tight link among the nitrogen and phosphorus cycles, likely as a result of the role of phosphate as the bacterial growth-limiting nutrient.