Sandy coastal sediments important to limiting greenhouse gases

Anthropogenic involvement, through fertilization for example, has resulted in many water bodies being enriched in nitrate nitrogen. The nitrate is eventually transported to the sea where it is degraded by bacterial communities in shallow coastal sediments. Depending on species composition, the end product of degradation can be nitrogen gas, a naturally occurring component of the atmosphere, or nitrous oxide, a potent greenhouse gas that is also harmful to the ozone layer.

By measuring the abundance of genes involved in degrading nitrous oxide in relation to those producing it, it is possible to estimate whether a community is producing or degrading nitrous oxide. This process, known as denitrification, has many steps, and the potential of a community to conduct the entire chain of reactions may either depend on few bacterial species single-handedly being able to perform all steps, or on a combination of species doing so together in tandem.

Affected variation

Different bacteria thrive in different environments and habitats. The coastal seafloor naturally consists of many different micro-environments, which benefits the microbial diversity and improves the sediment’s ability to perform various kinds of ecosystem services – including degrading nitrous oxide.

Human behavior tends to affect all kind of natural variation. Eutrophication, terrestrial run-off and other factors threaten to homogenize coastal sediments, which could lead to lesser species variation and thus poorer ability to perform ecosystem services. Whether this could also lead to increased nitrous oxide production, as an effect of fewer nitrous oxide degrading microbes in the community, is unclear and has been investigated in a new study from the Department of Forest Mycology and Plant Pathology at the Swedish University of Agricultural Sciences.

Comparing coastal sediments

– We compared denitrification in experimental seafloors with four degrees of habitat variation, says Lea Wittorf, main author of the study. We took samples from sediments that were sandy, silty, covered with Ruppia maritima, and with cyanobacterial mats. We arranged these into artificial seafloors consisting of one, two, three or all four habitats, and subsequently measured biodiversity, denitrification and the prevalence of nitrate- or nitrous oxide degrading genes.

As expected, species variation in the artificial lake floors increased the more habitat types they contained, but Lea and her colleagues did not observe any difference in neither actual denitrification nor genetic potential for denitrification. The nitrous oxide degrading bacteria appear to be diversified enough to perform the ecosystem service even when only coming from one type of habitat. However, since species variation and diversity is affected, the loss of habitats could make the seafloor more susceptible to further stresses in the future.

Sandy and silty sediments important

On the other hand, a significant difference was observed in nitrous oxide degrading potential between habitats. Silty and sandy habitats contained more nitrous oxide degrading bacterial species. The measured denitrification was also higher, and so was the estimated ability to degrade nitrous oxide, based on gene prevalence.

– Our hypothesis was that a diversity of seafloor habitats lead to greater species variation, and thus greater potential for degradation of nitrous oxide, Lea Wittorf summarizes. Our study does not show that homogenized habitats increase the risk of nitrous oxide production, but we do see that sandy and silty sediments has the best potential for nitrous oxide degradation. Thus, we still believe that a loss of habitats in coastal floors may lead to increased nitrous oxide production and contribute to the greenhouse effect.

Written by Mårten Lind.