A newly identified microbial process may be stealing nutrients from phytoplankton, the microorganisms at the base of the ocean food web (Environ. Sci. Technol. Lett. 2018, DOI: 10.1021/acs.estlett.8b00330). The process could change researchers’ understanding of nutrient cycling in the ocean, which influences the growth of these microscopic marine plants and bacteria and their effect on climate.
Like plants on land, phytoplankton use carbon dioxide and sunlight to build their biomass. The total amount of this photosynthesis in the ocean, which scientists call primary productivity, is generally limited by how much nitrogen, fixed in the form of ammonium or nitrate, is dissolved in the seawater. In low-oxygen environments, microbes like bacteria and archaea use these sources of nitrogen as an oxidant to help run their metabolism. These so-called nitrogen-loss processes produce N2 gas, which most phytoplankton can’t use. So the loss processes limit the growth of phytoplankton and thus the amount of carbon dioxide they can remove from the atmosphere, influencing climate over geological timescales.
Until now, scientists have known that microbes remove fixed nitrogen from the ocean in two main ways: They either reduce nitrate to N2 via a process called denitrification, or they reduce nitrite to oxidize ammonium, which produces N2 in a process called anaerobic oxidation of ammonium, or anammox. In this study, Francisco J. Cervantes of the San Luis Potosí Institute of Scientific Research and Technology and his colleagues show that another anammox-type of process may be removing fixed nitrogen in ocean sediments. In this process the anaerobic oxidation of ammonium is spurred on by the reduction of natural organic matter rather than nitrite.
The researchers’ discovery was prompted by studying nitrogen loss in coastal sediments at two sites in northwest Mexico, one on the Baja California Peninsula and the other in the state of Sinaloa. They detected about 40% more anaerobic oxidation of ammonium in the sediments than could be explained by the conventional anammox process. Cervantes knew that some soil microbes use humic material—organic matter that is usually highly resistant to microbial degradation—as an oxidant to run their metabolism. This recalcitrant humic matter also accumulates in ocean sediments. He calculated that the redox potential of a process fueled by reduction of organic matter was sufficient to drive ammonium oxidation.
“It was thermodynamically feasible,” he says. “I was curious—could that happen in nature?”
To test the hunch, he and his colleagues took sediment samples from the Baja site and incubated them with simulated seawater that contained ammonium labeled with 15N as a tracer. They soon detected labeled N2, and after about a month of incubation determined that 0.4 µg of labeled N2 was produced per gram of sediment per day. Meanwhile, spectroscopic analysis showed that organic matter in the sediments was chemically reduced. The team did not detect nitrate or nitrite during the incubation, suggesting no denitrification or conventional anammox was occurring. When the team added more organic matter to the sediments in a follow-up experiment, the rate of labeled N2 production more than tripled, suggesting that the carbon-containing material was the limiting resource for the microbes.
Other oxidants in sediments, including ferric iron and sulfate, could also contribute to ammonium oxidation, according to an earlier study by the group (Biodegradation 2018, DOI: 10.1007/s10532-018-9839-8).
Andrew Babbin, a marine biogeochemist at Massachusetts Institute of Technology, calls the proposed pathway “unique and noteworthy” and notes that the pool of organic carbon in the ocean that could drive the process is plentiful. He and the authors say that the finding must now be validated by in situ studies in ocean sediments. These studies could also help assess the pathway’s importance compared to other nitrogen loss pathways. If confirmed, he says the new process may change our understanding of what controls fixed nitrogen loss in different parts of the ocean and predictions of how this loss may be affected by climate change.