Coupled nitrification–denitrification in autotrophic and heterotrophic estuarine sediments: On the influence of benthic microalgae

Field data obtained from 18 European estuaries using the isotope pairing technique were analyzed for trends in relationship between activity of benthic microalgae and coupled nitrification–denitrification. Kruskal–Wallis tests and analyses of covariance performed on the field dataset showed strong statistical evidence for the hypothesis that sediments colonized by microalgae whose activity exceeds community respiration display lower rates of coupled nitrification–denitrification than do heterotrophic sediments. In fully heterotrophic sediments, 90% of the measurements fell within the range 0–92 mmol N m22 h21 with a median of 20.3 mmol N m22 h21. In highly autotrophic sediments, 90% of the measurements fell within the range 0–34 mmol N m22 h21, and the median was 4.2 mmol N m22 h21. The hypothesis was tested experimentally using 15N and microsensor (NO ) techniques in prepared 3 microcosms with and without algal activity. The results of the experimental studies were consistent with the hypothesis derived from the field data analysis. For the 15N study, coupled nitrification–denitrification in alga-colonized sediments was between 4 and 51% of the activity in sediments without algae activity, depending on the N load. For the microsensor study, there was no indication of net NO production in alga-colonized sediments before 3 addition of NH . In contrast, NO accumulated in the oxic zone of a similar alga-free sediment. The experiments 1 2 4 3 furthermore showed that compared to heterotrophic sediment, the presence of active microalgae might reduce the population of nitrifying bacteria capable of having an active metabolism. These bacterial populations could display diurnal variations in activity correlated with the diurnal variations in O2 penetration depth, however. The results showed that induction of nitrogen limitation of the nitrifying bacteria population is a major controlling mechanism of coupled nitrification–denitrification in alga-colonized sediments. Benthic microalgae might be important primary producers in shallow estuaries, and it has been estimated that these organisms can be responsible for more than 50% of estuarine primary production (Underwood and Kromkamp 1999). Benthic microalgae are concentrated at the sediment–water interface, and this allows them to significantly affect the chemical microenvironment in the upper few millimeters of the sediment. Via their assimilatory N uptake, they can reduce nutrient concentrations in the sediment porewater (Lorenzen et al. 1998) while, via photosynthesis and respiration, they can cause large diurnal fluctuations in both O2 and dissolved inorganic carbon concentrations in the superficial sediment strata, affecting both the pH and the redox potential in this zone (Revsbech et al. 1988). In addition, the algae might excrete part of their photosynthetic products (Smith and Underwood 2000), thus providing the heterotrophic bacteria community with an additional carbon source (Middelburg et al. 2000). Nitrification and denitrification are bacterially mediated processes central to the N cycle. Nitrification is the aerobic oxidation of NH to NO and NO , whereas 1 2 2 4 2 3 denitrification represents the anoxic reduction of NO or 3 1 Corresponding author ([email protected]).