Article: Trimmer, Mark, et al. “Nitrous oxide as a function of oxygen and archaeal gene abundance in the North Pacific.” Nature Communications (2016). DOI: 10.1038. link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5146275/
Nitrous oxide is funny chemical – its common name is laughing gas. We use it at the dentist office when we get cavities drilled. Some other handy uses for nitrous oxide are giving cars that extra nitrous boost in The Fast and the Furious and giving canned whipped cream that airy, delicious texture. Laughing gas is no joke though. Nitrous oxide, or N2O, is also a powerful greenhouse gas; it is 298 times more potent than CO2. Some scientists have called it the biggest threat to the earth’s protective ozone layer.
Up, up, and (reacted) away!
Where does N2O come from, and why is it such a problem? In the troposphere (the part of the atmosphere closest to us), N2O is not exactly harmful. It sticks around for about 100 years and is in at low enough levels that even though we breathe it, it doesn’t give us the numb, funny feeling like in the dentist office. However, when N2O ascends from the troposphere to the stratosphere, it can be become nitric oxide, NO. This NO reacts with ozone (O3) to form NO2 and oxygen (O2). While more O2 sounds great for us, this destruction of ozone lets more UV rays in. Effectively, this causes an increase in UV radiation and heats the surface of the earth, contributing to global warming and sunburns. Oh NO!
Where does N2O come from?
Let’s get back to where N2O comes from. Some N2O is a byproduct of fossil fuel burning and agriculture. The main source of Earth’s N2O is natural though, and 30% of the natural N2O comes from the ocean (Fig 1). Specialized microorganisms such as some Bacteria produce N2O during respiration processes. Typically, N2O is a very minor part of the chemical compounds that are produced by these single celled specialists, but in parts of the ocean where oxygen is low, N2O can be produced in much higher amounts than normal.
Where though do we see low oxygen in the ocean? Oceans often have 4-6 mg/l of oxygen dissolved in the water – that’s how fish can breathe. However, there are regions of the ocean called Oxygen Minimum Zones (OMZs for short), where oxygen falls below 2 mg/l of oxygen (Fig 2). Often the centers of these OMZs are anoxic, i.e. completely devoid of oxygen, and along the edges we see very low oxygen levels. N2O is made in both the fully anoxic center and the low oxygen edges of these OMZs. In the anoxic areas, N2O is likely made by Bacteria breathing nitrate (NO3) instead of oxygen. An intermediate of this process is N2O. However, we do not understand very well how N2O is produced in the very low oxygen edges.
To understand this funny conundrum, scientists from the United Kingdom set sail to a large OMZ in the Eastern Tropical North Pacific. That’s a bit of a mouthful, but basically it’s just a region of the Pacific Ocean to the west of Costa Rica. The seafaring scientists started about 120 km (about 75 miles) west of Costa Rica, and cruised over 550 km (that’s 342 miles!) of ocean, measuring chemical concentrations and collecting water samples from different parts of the OMZ.
The scientists bubbled different kinds of gases through the water samples. After a certain amount of time, the chemistry of the water was measured and compared to the chemistry at the start of the experiment. This gives us some idea of what’s happening in the environment from which these samples came. In this case, the scientists were trying to determine which samples made N2O, and how it was made. Previous studies in an OMZ on the other side of the world indicated that most N2O was due to a process called denitrification, i.e. breathing nitrate, and that Bacteria were involved. They did not find that to be the case here, and denitrification was not a major producer of N2O (Fig 3, Fig 4a))!
The scientists then looked at the DNA of the seawater microorganisms to see if there are microbes who are making N2O. In the incubated samples, the scientists looked for the bacterial gene called ammonium monooxygenase A, which could indicate the bacteria are producing N2O through a process called nitrification. Surprisingly, they did not detect this gene in the Bacteria in their incubations. However, they looked for this same gene in the DNA of single-celled organisms called Archaea, and they found it (Fig 4)!
To understand why this is so exciting to us here at Oceanbites, let’s back up and explain a bit about Archaea in the ocean. Archaea are like Bacteria in that they are single celled microorganisms. The first Archaea studied were from extreme environments like hydrothermal vents (which are extremely hot and under high pressure), and for a long time we thought most Archaea were extremophiles. In 1992, however, it was discovered that Archaea live throughout the ocean (Fig 5). This was a major surprise to microbiologists and oceanographers alike. These microbes are tough to study, though, so these oceanic Archaea remain enigmatic.
The finding of archaeal ammonium oxidizing genes and their correlation to oxygen and nitrous oxide concentrations indicates that these mysterious Archaea, not bacteria, are making our laughing gas in this low oxygen zone. To make this story even more surprising, the chemical reactions the N2O-producing Archaea are using might be novel as well, or put simply – we don’t know exactly how they’re making N2O.
OMZs are major sources of the greenhouse gas N2O. These low oxygen zones are expected to expand in the near future (and already are!) due to an increase in nutrients to the ocean from human activity, which has led to more microorganisms consuming more of the ocean’s oxygen? This makes it crucial that we understand how they are producing this potent greenhouse gas. Through a bunch of clever experiments, scientists discovered that not Bacteria, but actually a whole different type of microbe, the Archaea, make N2O in very low oxygen waters. They are doing it through a mechanism we do not fully understand, making it hard to predict what will happen in the future with changing climate and ocean conditions. There’s still a lot left to discover about microbes, the oceans, and funny greenhouse gases.
Greenhouse gas: a gas that contributes to the greenhouse effect by absorbing infrared radiation, e.g., carbon dioxide and chlorofluorocarbons
Bacteria: a member of a large group of unicellular microorganisms that [generally] have cell walls but lack organelles and an organized nucleus
Archaea: microorganisms that are similar to bacteria in size and simplicity of structure but radically different in molecular organization. They are now believed to constitute an ancient intermediate group between the bacteria and eukaryotes.
Oxygen minimum zone: the zone in which oxygen saturation in seawater in the ocean is at its lowest. This zone occurs at depths of about 200 to 1,000 meters, depending on local circumstances
Stratosphere: the layer of the earth’s atmosphere above the troposphere, extending to about 32 miles (50 km) above the earth’s surface (the lower boundary of the mesosphere)
Respiration: a process in living organisms involving the production of energy, typically with the intake of oxygen and the release of carbon dioxide from the oxidation of complex organic substances
Anoxic: a total depletion in the level of oxygen, an extreme form of hypoxia or “low oxygen”.
Denitrification: a microbially facilitated process of nitrate reduction (performed by a large group of heterotrophic facultative anaerobic bacteria) that may ultimately produce molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products.