In this month’s Geosciences Column, Celso Gomes highlights a recent result published in EGU’s Biogeosciences on the role of a particular single-celled microorganism in the production of nitrous oxide in the ocean.
About 30% of atmospheric nitrous oxide (N2O), a greenhouse gas, is a product of the ocean. Therefore, by studying the mechanisms behind the oceanic production of this gas, scientists can understand how the concentration of atmospheric N2O might change in the future.
Traditional theories saw ammonia-oxidising bacteria as having a central role in N2O production in the oceans, but recent findings are pushing a different prokaryote to the spotlight – ammonia-oxidising archaea. The study, led by C. R. Löscher of the University of Kiel in Germany, helps reinforce this hypothesis, by both analysing N2O production in the ocean and providing pioneer data on the production of this gas in a pure ammonia-oxidising-archaea culture.
Two oceans
The first part of the study relied on vertical profiles of two oceans with wildly different oxygenation levels – the eastern tropical North Atlantic and the eastern tropical South Pacific. While in the former oxygen concentrations were above 40 μmol/l, in the latter oxygen levels were 20 times lower than this value between depths of around 75 and 600 metres, creating an oxygen minimum zone.
At the depths where data was collected, minimum N2O concentration was similar in both cases but maximum N2O levels in the North Atlantic were 10 times lower than in the South Pacific. It is likely that the lower oxygen levels of the South Pacific can explain this significant difference. This idea is backed up by the observation in most samples (from both oceans) that N2O accumulation is inversely correlated with oxygen levels.
A key gene
Still, these observations alone do not single out ammonia-oxidising archaea as the likely N2O producers, so the researchers called on more robust tools. Using marker genes for N2O production in both bacteria and archaea, they found a strong correlation between the number of copies of archaeal amoA, a key gene for nitrification (the biological oxidation of ammonia into nitrite), and N2O concentration in areas with low oxygen levels of the North Atlantic (see figure below).
Number of archaeal amoA copies and N2O concentration at varying O2 levels. (Retrieved from Löscher et al.)
For the South Pacific data, the correlation was not as clear-cut: while there was a similar correlation at particular depths, this was not a general trend. Such observations suggest other N2O producing pathways are in place as well.
Nevertheless, the number of amoA gene copies for archaea was multiple orders of magnitude larger than bacterial amoA, which reinforces the notion that nitrification by ammonia-oxidising archaea plays a leading part in N2O production in the eastern tropical North Atlantic, and also at certain depths of the eastern tropical South Pacific.
Producing nitrous oxide in the lab
Despite the indicators archaea are likely to be making a sizeable contribution to oceanic N2O levels, direct evidence to back such claims was still lacking. The authors then set to assess pure-culture N2O production by the one marine ammonia-oxidizing archaea that has been successfully cultured – N. maritimus – at O2 concentrations comparable to the decreasing gradient observed in the eastern tropical North Atlantic.
N2O concentration over time in N. maritimus pure cultures with initial O2 concentrations of 112 μmol/l (red); 223 μmol/l (purple); and 287 μmol/l (blue). (Retrieved from Loescher et al.)
This experiment yielded striking results, with N2O concentration increasing in sharp contrast with O2 levels, indicating N2O production by the organism is oxygen sensitive and has much higher yields in low-oxygen waters. Not only that, but N2O levels in cultures of N. maritimus were several times higher than N2O levels of ammonia-oxidising bacteria cultures, which backs up the idea archaea may be the main N2O producers in low-oxygen areas of the ocean.
Future increase of atmospheric N2O
Previous studies have showed that low oxygen zones in the oceans are expected to expand as the world warms. The results of the Biogeosciences study show that N2O production by archaea would, therefore, be enhanced under climate change, which could seriously impact the atmospheric budget of N2O. Considering the strong greenhouse effect of N2O, this is certainly an area that deserves further study in years to come.
By Celso Gomes, freelance science writer
