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An Abundant Marine Microbe Makes its Own Oxygen
Growing within the deep waters of the ocean’s twilight zone is one of Earth’s most abundant organisms. They are a group of microscopic, single-celled organisms called the ammonia-oxidizing archaea (or “AOA”). Hundreds of thousands to millions of AOA cells occupy just one liter of deep-sea water, where they make up 40 % of all microbial cells. About ten octillion (1028) AOA cells are in the Earth’s oceans.
In some regions of the world’s oceans, AOA are very abundant at depths where seawater is very low in oxygen. Some environments where high AOA abundance coincides with low-oxygen waters include the Black Sea, the eastern tropical south Pacific, the Sea of Okhotsk, and the Gulf of California. This observation has puzzled scientists, because AOA require oxygen to grow. A new study published in Science earlier this year could help explain how AOA get by in low-oxygen waters.
AOA are named after their method of growth. They convert inorganic carbon dissolved in seawater into biomass (“carbon fixation”). AOA obtain the energy required for carbon fixation by converting ammonia into nitrite using oxygen. Ammonia is “oxidized” in this reaction because it loses electrons that are gained by oxygen in turn. According to Beate Kraft, a marine microbiologist at the University of Southern Denmark who led the research, “there was this enigma that [AOA] actually are frequently found in [low-oxygen] waters, but they theoretically should not be able to survive there because they need oxygen.”
Kraft’s team measured how pure cultures of one AOA species, Nitrosopumilus maritimus, grow at the low-oxygen concentrations found in some regions of the deep ocean. Once the organism used up all of the oxygen, Kraft’s team saw something surprising. Oxygen levels in the culture started to increase.
Kraft and her colleagues think that the organism produces its own oxygen to grow and persist in oxygen-poor waters. The evidence for this is that the organism produced nitrite (the waste product of ammonia oxidation) in tandem with oxygen. The organism produced more nitrite than would have been possible with the oxygen levels in the cultures. So, the organism must have immediately used some of the produced oxygen to oxidize ammonia. Control experiments ruled out abiotic reactions and contamination from air as sources of oxygen and confirmed that oxygen was consumed during ammonia oxidation.
Scientists still do not fully understand how AOA produces oxygen. Kraft and her colleagues proposed that the organism produced oxygen by first converting nitrite into nitric oxide. From four nitric oxide molecules, the organism then creates two nitrous oxide molecules and one oxygen molecule (Figure 2). Finally, the organism converts the nitrous oxide to nitrogen gas (Figure 2). Supporting this idea, Kraft’s AOA cultures produced nitric oxide, nitrous oxide and nitrogen gas, and some bacteria make oxygen from nitric oxide to grow in oxygen-poor environments. However, AOA only definitively have enzymes to convert nitrite to nitric oxide. Kraft chalks up the other reactions in their proposed pathway to yet unidentified enzymes.
The chemistry of this new oxygen-producing pathway could limit where it can feasibly occur. In the new pathway, AOA make one molecule of oxygen from four nitrite molecules. But, AOA require six oxygen molecules to make four nitrite molecules from ammonia. So, the new pathway cannot be a net oxygen source unless surplus nitrite is available beyond what the AOA produce from ammonia (Figure 2).
In the lab cultures, nitrite accumulated from prior ammonia oxidation with initially high oxygen levels. In the ocean, excess nitrite could be harder to find. According to James Hollibaugh, a microbial ecologist and professor emeritus from the University of Georgia, “[AOA oxygen production] has to be supported by nitrite from some source other than the ammonia oxidation itself, and there are not many places in the ocean where you would find those conditions.”
In some oxygen-poor waters, nitrite accumulates in what oceanographers call a “secondary nitrite maximum.” This is an area of the water column at depth where nitrite increases to peak concentrations. According to Kraft, “that is where other nitrite consuming processes . . . are most active, and nitrite is used by a lot of different microbes there.” While AOA are the major source of the nitrite in some locations, nitrite can also accumulate from other processes. “This is additional nitrite that [AOA] could use,” said Kraft.
However, competition with other microbes may also limit where in the ocean AOA produce oxygen (Figure 3). In low-oxygen regions of the ocean, nitrite accumulates to low concentrations, and many other microbes may outcompete AOA for any surplus nitrite. For example, Hollibaugh thinks that denitrifying bacteria (bacteria that can “breathe” nitrite) likely use nitrite faster than AOA and at higher oxygen levels.
Kraft recognizes that interactions with other microbes complicate whether AOA could use the oxygen-production pathway in the environment. Justyna Hampel, a microbial ecologist and biogeochemist at Stockholm University, shares this sentiment. “Findings from culture experiments are exciting, but we should be careful when [extrapolating pure cultures to] the natural environment.”
The next step of the research is to confirm that AOA actually produce oxygen and use nitrite in natural, low-oxygen environments. According to Kraft, “it’s still just speculation. We [now have] a few more research projects [where] we [can] actually look at their role and importance in the environment . . . It’s a new puzzle that we have to work on.”
Link to the original post: Oxygen and nitrogen production by an ammonia-oxidizing archaeonBeate Kraft Nico Jehmlich Morten LarsenLaura A. Bristow Martin Könneke Bo Thamdrupand Donald E. Canfield Science • 6 Jan 2022 • Vol 375, Issue 6576
Featured image: Image by Derek Smith