Denitrification in hypersaline and coastal environments

Abstract

As the association of denitrification with global warming and nitrogen removal from ecosystems has gained attention in recent decades, numerous studies have examined denitrification rates and the distribution of denitrifiers across different environments. In this minireview, reported studies focused on coastal saline environments, including estuaries, mangroves, and hypersaline ecosystems, have been analysed to identify the relationship between denitrification and saline gradients. The analyses of the literature and databases stated the direct effect of salinity on the distribution patterns of denitrifiers. However, few works do not support this hypothesis thus making this topic controversial. The specific mechanisms by which salinity influences denitrifier distribution are not fully understood. Nevertheless, several physical and chemical environmental parameters, in addition to salinity, have been shown to play a role in structuring the denitrifying microbial communities. The prevalence of nirS or nirK denitrifiers in ecosystems is a subject of debate in this work. In general terms, in mesohaline environments, the predominant nitrite reductase is NirS type and, NirK is found predominantly in hypersaline environments. Moreover, the approaches used by different researchers are quite different, resulting in a huge amount of unrelated information, making it difficult to establish comparative analysis. The main techniques used to analyse the distribution of denitrifying populations along salt gradients have been also discussed.

Keywords: coastal ecosystem; denitrification; denitrifiers distribution; halophilic microorganisms; nitrite reductase; saline ecosystem.

Figure 1.

Summary of reactions and enzymes involved in the denitrification process. The complete reduction of nitrate to dinitrogen is driven by metalloenzymes nitrate reductase (NarGH: membrane-bound nitrate reductase; NapAB: periplasmic nitrate reductase), nitrite reductase (NirK: copper-containing nitrite reductases; NirS: cytochrome-cd1-dependent nitrite reductases), nitric oxide reductase (qNor: quinol dependent nitric oxide reductase; cNor: short-chain respiratory nitric oxide reductase; Cu_ANor: copper-containing nitric oxide reductase), and nitrous oxide reductase (NosZ).

Figure 2.

Bar charts comparing the number of publications available in the Web of Science database on denitrification over the last 20 years in hypersaline, mangroves, and estuarine ecosystems. (A) Number of publications by year about denitrification in mangroves. The search equation utilized to obtain the data was: estuar* AND denitrification (Topic). (B) Number of publications by year about denitrification in estuaries. The search equation utilized to obtain the data was: mangrove* AND denitrification (Topic). (C) Number of publications by year in hypersaline habitats. The search equation utilized to obtain the data was: hypersalin* AND denitrification (Topic).

Figure 3.

Overview of the NiR-type distribution along aquatic environments regarding salinity based on the analysis of the literature done in this review. The dotted lines indicate the environments where the type of NiR is less abundant. Arrows indicate the higher abundance of that NiR type.