Denitrification and Biodiversity of Denitrifiers in a High-Mountain Mediterranean Lake

Abstract

Wet deposition of reactive nitrogen (Nr) species is considered a main factor contributing to N inputs, of which nitrate (NO3−) is usually the major component in high-mountain lakes. The microbial group of denitrifiers are largely responsible for reduction of nitrate to molecular dinitrogen (N2) in terrestrial and aquatic ecosystems, but the role of denitrification in removal of contaminant nitrates in high-mountain lakes is not well understood. We have used the oligotrophic, high-altitude La Caldera lake in the Sierra Nevada range (Spain) as a model to study the role of denitrification in nitrate removal. Dissolved inorganic Nr concentration in the water column of la Caldera, mainly nitrate, decreased over the ice-free season which was not associated with growth of microbial plankton or variations in the ultraviolet radiation. Denitrification activity, estimated as nitrous oxide (N2O) production, was measured in the water column and in sediments of the lake, and had maximal values in the month of August. Relative abundance of denitrifying bacteria in sediments was studied by quantitative polymerase chain reaction of the 16S rRNA and the two phylogenetically distinct clades nosZI and nosZII genes encoding nitrous oxide reductases. Diversity of denitrifiers in sediments was assessed using a culture-dependent approach and after the construction of clone libraries employing the nosZI gene as a molecular marker. In addition to genera Polymorphum, Paracoccus, Azospirillum, Pseudomonas, Hyphomicrobium, Thauera, and Methylophaga, which were present in the clone libraries, Arthrobacter, Burkholderia, and Rhizobium were also detected in culture media that were not found in the clone libraries. Analysis of biological activities involved in the C, N, P, and S cycles from sediments revealed that nitrate was not a limiting nutrient in the lake, allowed N2O production and determined denitrifiers’ community structure. All these results indicate that denitrification could be a major biochemical process responsible for the N losses that occur in La Caldera lake.

The DNRA-Denitrification Dichotomy Differentiates Nitrogen Transformation Pathways in Mountain Lake Benthic Habitats

“…The overall high proportion of denitrifying nitrite versus nitrous oxide reductase genes (∼30 nir : nosZ ratio on average) suggests a dominance of partial denitrification, especially in productive habitats dominated by nirS denitrifier communities (i.e., nirS -cluster showed nir : nosZ higher ratios compared to the other clusters, WMW test, p < 0.01). This observation agrees with Castellano-Hinojosa et al (2017) who found high N 2 O/N 2 emissions in a productive, shallow warm Mediterranean mountain lake, as well as Myrstener et al (2016) who demonstrated that addition of nitrate, phosphorus and labile C to sediments from a boreal lake resulted in higher relative N 2 O production compared to addition of nitrate alone. Other studies have shown that higher nir : nosZ1 ratios in the sediments of boreal lakes were associated with hypolimnion N 2 O excess, as well as increased phosphate and nitrate concentrations (Saarenheimo et al, 2015a).…”

“…Links between taxonomic composition and functional potential has been observed in previous works in lakes based on metagenomes or sequencing of functional genes. The occurrence of several proteobacterial families, in particular, Rhodobacteraceae (Alphaproteobacteria), Methylococcaceae (Gammaproteobacteria), and Burkholderiales, Comamonadaceae and Rhodocyclaceae (Betaproteobacteria) has been shown to be strongly associated with denitrification gene presence or abundance (Vila-Costa et al, 2014; Peura et al, 2015; Saarenheimo et al, 2015b; Castellano-Hinojosa et al, 2017; Chen R. et al, 2017). These taxa were also highly abundant in samples within the nirS -denitrifier and mixed functional gene communities, which included nirK -type denitrifiers.…”

Denitrification Temperature Dependence in Remote, Cold, and N‐Poor Lake Sediments

“…However, they were not able to evaluate whether an enhancement of denitrification could be an alternative explanation due to the lack of empirical information. The difficulty of the measuring DEN rates at low N r concentrations may justify why remote systems have been overlooked with a few exceptions (Castellano‐Hinojosa et al, ; McCrackin & Elser, ; Vila‐Costa et al, ). Indeed, it has been only one attempt to estimate DEN E a in these systems (Myrstener et al, ).…”

Housekeeping in the Hydrosphere: Microbial Cooking, Cleaning, and Control under Stress

“…Those processes of reactive nitrogen removal are performed mainly by anaerobic bacteria inhabiting environments with low O 2 concentration, such as subsurface soils and sediments and anaerobic hypolimnion of lakes. Unexpectedly, active and diverse communities of denitrifying bacteria have been found in some well-oxygenated ecosystems, such as high-mountain lakes, where denitrification can explain the seasonal decline in nitrate concentration observed in the water column [ 32 ]. On the whole, aquatic denitrification may be on the rise coincident with increasing ocean deoxygenation, a trend that may greatly reduce productivity in nitrogen-limited waters by disadvantaging nitrogen-needy non-nitrogen fixing phytoplankton and heterotrophic bacteria—with impacts extending all the way up in the food chain [ 33 ].…”