Abstract
Few studies have focused on broad scale biogeographic patterns of ammonia oxidizers in coastal systems, yet understanding the processes that govern them is paramount to understanding the mechanisms that drive biodiversity, and ultimately impact ecosystem processes. Here we present a meta-analysis of 16 years of data of ammonia oxidizer abundance, diversity, and activity in New England (NE) salt marshes and 5 years of data from marshes in the Gulf of Mexico (GoM). Potential nitrification rates were more than 80x higher in GoM compared to NE marshes. However, nitrifier abundances varied between regions, with ammonia-oxidizing archaea (AOA) and comammox bacteria significantly greater in GoM, while ammonia-oxidizing bacteria (AOB) were more than 20x higher in NE than GoM. Total bacterial 16S rRNA genes were also significantly greater in GoM marshes. Correlation analyses of rates and abundance suggest that AOA and comammox are more important in GoM marshes, whereas AOB are more important in NE marshes. Furthermore, ratios of nitrifiers to total bacteria in NE were as much as 80x higher than in the GoM, suggesting differences in the relative importance of nitrifiers between these systems. Communities of AOA and AOB were also significantly different between the two regions, based on amoA sequences and DNA fingerprints (terminal restriction fragment length polymorphism). Differences in rates and abundances may be due to differences in salinity, temperature, and N loading between the regions, and suggest significantly different N cycling dynamics in GoM and NE marshes that are likely driven by strong environmental differences between the regions.
Highlights
Nitrification in salt marshes plays a critical role in the fate of nitrogen, yet we lack a full understanding of the distribution and diversity of nitrifiers in geographically distinct marshes, and how these differences may impact ecosystem processes
Rates in Gulf of Mexico (GoM) marshes were more than 50 times higher on average compared to rates in New England (NE) marshes (Fig. 2A)
Ratios of nitrifier groups Ratios of ammonia-oxidizing archaea (AOA) to ammonia-oxidizing bacteria (AOB) were significantly higher in GoM marshes compared to NE marshes (Fig. 3A)
Summary
Nitrification in salt marshes plays a critical role in the fate of nitrogen, yet we lack a full understanding of the distribution and diversity of nitrifiers in geographically distinct marshes, and how these differences may impact ecosystem processes. The recent discovery of a complete nitrifier, comammox,[5] further complicates our understanding of niche partitioning among nitrifying microorganisms. In a recent comparison of nitrification rates among disparate marshes and estuarine systems, Marton et al.[6] reported differences of more than three orders of magnitude in some cases, with rates in Gulf of Mexico (GoM) marshes far surpassing those reported in other marshes. There is evidence that there may be cosmopolitan phylotypes found in similar systems, but there have been few direct comparisons between geographically distant marsh systems ( see[3] for AOB)
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