Abstract
Sediment denitrification rates seem to be lower in tropical environments than in temperate environments. Using the isotope pairing technique, we measured actual denitrification rates in the sediment of tropical coastal lagoons. To explain the low denitrification rates observed at all study sites (<5 μmol N2 m-2 h-1), we also evaluated potential oxygen (O2) consumption, potential nitrification, potential denitrification, potential anammox, and estimated dissimilatory nitrate (NO3-) reduction to ammonium (NH4+; DNRA) in the sediment. 15NO3- and 15NH4+ conversion was measured in oxic and anoxic slurries from the sediment surface. Sediment potential O2 consumption was used as a proxy for overall mineralization activity. Actual denitrification rates and different potential nitrogen (N) oxidation and reduction processes were significantly correlated with potential O2 consumption. The contribution of potential nitrification to total O2 consumption decreased from contributing 9% at sites with the lowest sediment mineralization rates to less than 0.1% at sites with the highest rates. NO3- reduction switched completely from potential denitrification to estimated DNRA. Ammonium oxidation and nitrite (NO2-) reduction by potential anammox contributed up to 3% in sediments with the lowest sediment mineralization rates. The majority of these patterns could be explained by variations in the microbial environments from stable and largely oxic conditions at low sediment mineralization sites to more variable conditions and the prevalences of anaerobic microorganisms at high sediment mineralization sites. Furthermore, the presence of algal and microbial mats on the sediment had a significant effect on all studied processes. We propose a theoretical model based on low and high sediment mineralization rates to explain the growth, activity, and distribution of microorganisms carrying out denitrification and DNRA in sediments that can explain the dominance or coexistence of DNRA and denitrification processes. The results presented here show that the potential activity of anaerobic nitrate-reducing organisms is not dependent on the availability of environmental NO3-.
Highlights
Denitrification rates have been predominantly measured in temperate regions with ranges varying by orders of magnitude, whereas tropical environments have been underrepresented [1,2]
The extent to which surplus inorganic N in sediments is removed or recycled in the sediment or is released into the water column as NH4+ or NO3- depends on the magnitude and coupling of nitrification, denitrification, anammox, and dissimilatory nitrate reduction to ammonium (DNRA), as well as the NO3- assimilation driven by microorganisms
As O2 concentrations in the water column close to the sediment were near air saturation levels in all studied ecosystems and O2 was consistently available near the sediment surface at all sampling locations, potential O2 consumption rates were used as a proxy for the degree of organic matter turnover, as used in previous studies [39]
Summary
Denitrification rates have been predominantly measured in temperate regions with ranges varying by orders of magnitude, whereas tropical environments have been underrepresented [1,2]. The few studies performed to date in aquatic tropical environments have reported relatively low or absent denitrification rates in sediments obtained from coastal lagoons [3, 4], mangroves [5], floodplain lakes from Pantanal [6] and Amazon [7,8,9,10] regions, and streams [2] In most of these environments, the nitrate (NO3-) concentration in the water column was relatively low.Oxygen (O2) in the sediment is a determining factor in nitrogen (N) transformation [11]. The extent to which surplus inorganic N in sediments is removed or recycled in the sediment or is released into the water column as NH4+ or NO3- depends on the magnitude and coupling of nitrification, denitrification, anammox, and dissimilatory nitrate reduction to ammonium (DNRA), as well as the NO3- assimilation driven by microorganisms
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