Nitrogen fixation in shallow‐water sediments: Spatial distribution and controlling factors

Abstract

Nitrogenase activity (NA) in shallow-water (, 1 m) sediments was investigated at 60 randomly selected sites along a 150 km stretch on the brackish-water Swedish west coast, without targeting any specific type of sediments, such as microbial mats. Benthic nitrogen (N) fixation and diazotrophs (nifH genes) were found at all sites, regardless of the presence of cyanobacterial or microbial mats. The majority of sites showed N fixation rates between 0.03 and 1 mmol N m 22 d 21 . These rates were similar to those of benthic denitrification previously measured in the area. Maximum rates up to 3.4 mmol N m 22 d 21 were measured. A structural equation model was used to investigate direct and indirect effects of biogeochemical and physical factors on NA. Number of nifH genes had the largest direct positive influence on NA, whereas increasing wave exposure had an indirect negative effect on NA through its influence on the diazotrophic abundance. Increased salinity, previously been shown to suppress NA in coastal waters, was found to directly stimulate benthic N fixation, likely by generating favorable conditions for diazotrophic sulfate-reducing bacteria. Our field data confirmed previously observed negative effects of dissolved inorganic nitrogen on NA, which have so far mainly been experimentally studied. Both NA rates and the number of nifH genes correlated positively with pore-water dissolved inorganic phosphorus concentrations. These findings show that the potential for N fixation in illuminated sediments can be considerable, stretching beyond cyanobacterial mats, being controlled by complex interactions between biotic and abiotic factors.

Nitrogen fixation in sediments along a depth transect through the Peruvian oxygen minimum zone

“…Further, our rates were characterized by a similar range of N 2 fixation rates that were previously measured in an organic-rich hypoxic basin in the Baltic Sea (0.08-0.22 mmol N 2 m −2 d −1 , Bertics et al, 2013). In contrast to the above examples, our N 2 fixation rates were 8.5 times lower compared to shallow (< 1 m) soft-bottom sediment off the Swedish coast (Andersson et al, 2014) and 17 times lower than coral reef sediments (Capone, 1988). However, in these environments, phototrophic cyanobacterial mats contributed to benthic N 2 fixation.…”

“…For example, subtidal sediments in Narragansett Bay (Rhode Island) were found to switch from being a net sink in the form of denitrification to being a net source of bioavailable N by N 2 fixation, caused by a decrease of organic matter deposition to the sediments (Fulweiler et al, 2007). Shallow brackish-water sediments off the Swedish coast revealed benthic N 2 fixation along with a diverse diazotrophic community (Andersson et al, 2014). N 2 fixation was positively influenced by a variety of environmental factors, such as salinity and dissolved inorganic Nitrogen, while wave exposure had a negative influence.…”

Can we predict ecosystem functioning using tightly linked functional gene diversity?

“…In the first step, we took the following model that was fitted to data from a previously published survey (Andersson et al, 2014):…”

“…The details of the measurements are described in Alsterberg et al (2017) and Andersson et al (2014). Briefly, four sample of 3 ml homogenized sediment were incubated in gas tight exetainers together with 1.5 ml filtered seawater.…”

Can we predict ecosystem functioning using tightly linked functional gene diversity?

“…The range of nifH copies observed in Andersson et al (2014) [2 ⇥ 10 5 5 ⇥ 10 7 copies g 1 sediment] nearly covered the full range of observed copy numbers in this study [7.2 ⇥ 10 4 7.4 ⇥ 10 6 copies g 1 sediment]. However, as Andersson et al (2014) did not sequence the nifH gene, the nifH copy numbers could not be corrected for the fraction of nifH genes picked up by the primers but not implicated in nitrogen fixation.…”