Inorganic nitrogen uptake kinetics by estuarine sediment microalgal communities

In this study, the uptake of nitrate and ammonium by benthic microalgal communities dwelling in the sediment/water-column interface in tide flats at Ochlockonnee Bay, FL was investigated. Uptake kinetics experiments were performed from June-2000 to December-2001, in order to characterize the response of these communities to different environmental situations. Some experiments were performed to assess the contribution of different members of these communities, with the help of specific inhibitors of microbial activity and photosynthesis. Other experiments were performed to evaluate the contribution of attached vs. unattached organisms. Another set of experiments investigated the response of these communities to changes in salinity. Finally, some experiments were performed with water-column communities. Uptake was measured by disappearance of substrate, and rates were normalized to chlorophyll a biomass.; The results showed that the inorganic nitrogen uptake exhibited by these communities is affected by changes in the salinity regime, which is a function of river discharge into the estuary. Following large freshwater inputs, uptake was low, variable and associated with microbial activity. For these periods it was not possible to obtain the kinetic parameters uptake capacity (V _max) and half-saturation constant (K_s). When these communities acclimated to the salinity regime, and during periods of higher salinity, they exhibited Michaelis-Menten type of kinetics, and uptake was related to photosynthetic organisms, mainly small diatoms attached to sand grains. Kinetic parameters were variable, and could be related to fluctuations in salinity and photosyntetically available radiation (PAR). The results support the idea that these communities are able to regulate their nitrogen uptake according to environmental conditions, and exploit high nutrient concentrations in the sediment when relieved from physical stress. During periods of high activity, fall and spring, pore-water nutrients may become limiting, showing that benthic microalgae communities have an important role in the processes controlling nitrogen cycling in the sediment.