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Biogeochemistry

Methane on the dinner menu

Source: Steinle, L., J. Maltby, T. Treude, A. Kock, H.W. Bange, N. Engberson, J. Zopfi, M.F. Lehmann, and H. Niemann (2016). Effects of low oxygen concentrations on aerobic methane oxidation in seasonally hypoxic coastal waters. Biogeosciences Discuss. doi:10.5194/bg-2016-422.

 

Life, death, and methane at the coast

Coastlines around the world support an enormous amount of life.  Coastal waters receive a lot of sunlight and nutrients compared to the open ocean, providing a favorable habitat for photosynthetic algae to grow. In turn, these organisms support a complex food chain that extends all the way up to fish and humans.

Of course, all life must die; and when it does, it sinks. The detritus that reaches the seafloor is converted by microorganisms to methane, which then bubbles back up through the water column towards the atmosphere. Coastlines in particular are so productive that they produce 75% of the methane coming from the ocean, even though they cover only 15% of the ocean’s area.

 

Shelf Methane Oxidation

Fig. 1. Photosynthetic organisms in coastal waters support a food chain that ultimately sinks and is converted to methane in the benthic food web. Aerobic methanotrophs can then consume this methane in the water column before it reaches the atmosphere.

The fate of this coastal methane – whether it in fact travels all the way up from the seafloor to the atmosphere – is significant for global climate change. Methane that does make it to the atmosphere is 25 times more effective at warming the planet than carbon dioxide.  For scientists trying to predict just how much and how fast the planet will warm in years to come, it is critical to know just how much methane is making it to the atmosphere from the coastal ocean.

 

 

Methane seep

Fig. 2.  Methane bubbling out of the coastal seafloor (Source: USGS/NOAA Okeanos Explorer).

 

How much do hungry microbes breathe?

The biggest unknown for scientists is not how much methane is being produced in the seafloor, but rather how much of it gets eaten in the water column on its way up to the atmosphere.  Microorganisms living in the ocean known as aerobic methanotrophs eat methane in much the same way that humans eat food, breathing in oxygen and exhaling carbon dioxide in the process. Methane that gets consumed by aerobic methanotrophs does not reach the atmosphere, and thus does not contribute to global climate change.

Pinning down a single number for how much methane aerobic methanotrophs consume is tricky, though, since the amount of methane and oxygen in coastal waters changes with the seasons.  In the winter, short days limit life in the coastal ocean so that there is little methane being produced, but plenty of oxygen for aerobic methanotrophs to breathe. In the summer, however, long sunny days enable rapid life and death cycles that fuel methane production in the seafloor.  At the same time, the feeding frenzy of summer draws down oxygen levels in the water column, so that the oxygen that aerobic methanotrophs need to breathe is most scarce at the same time their food source is most abundant.

Which raises the question: are aerobic methanotrophs able to consume most of the methane that is produced in summer despite the low oxygen levels? Or, does the mismatch in timing between methane and oxygen availability result in more methane reaching the atmosphere during the summer?

Seasonal Hypoxia

Fig. 3. In wintertime, oxygen levels throughout the water column are relatively high. Whereas in summertime, oxygen levels can be dramatically lower as a result of many organisms feeding during the productive season.

 

Going to the source

To find how much of the available methane aerobic methanotrophs can consume under high- versus low-oxygen conditions, a team of researchers went to the coastal Eckernfӧrde Bay in the Baltic Sea in both winter and summer.  They collected water samples and measured the amounts of methane and oxygen in the water column.  They also added a trace amount of radioactive methane to the water, which allowed them to track how quickly aerobic methanotrophs present in the water samples were consuming methane under the conditions in the bay in each season.

What they found was that aerobic methanotrophs are surprisingly well adapted to the low-oxygen conditions of summer in the bay.  Measurements of the rates at which aerobic methanotrophs consume methane in the bay across seasons demonstrated that the organisms were actually fastest at eating methane in the summer, when oxygen in the water was too scarce to be measured with traditional instruments. Not only that, but aerobic methanotrophs turned out to be worse at consuming methane when oxygen was plentiful. Even though there was up to five times more methane in the water column in summertime than in wintertime, aerobic methanotrophs were so much more efficient at consuming methane under oxygen-depleted summertime conditions that more methane actually escaped to the atmosphere in the winter.

 

Aerobic methanotrophy oxygen concentrations

Fig. 4. The rates at which aerobic methanotrophs consumed methane in Eckernfӧrde Bay were higher during the summer than in the winter.  The benefit of low oxygen levels to aerobic methanotrophs is especially clear in winter, when oxygen levels are normally high. (Source: Steinle et al. 2016 Biogeosciences doi:10.5194/bg-2016-422)

To confirm this finding, the researchers also took water samples from the bay back to the laboratory.  There, they added varying amounts of oxygen and methane to each water sample, again adding a trace amount of radioactive methane to track the speed at which aerobic methanotrophs consumed methane.

Again, aerobic methanotrophs were fastest at consuming methane in water samples that had the lowest amounts of oxygen added to them. When only trace amounts of oxygen were added, they consumed more than 70% of the added methane; while when amounts of oxygen were added that would be considered normal for wintertime waters in Eckernfӧrde Bay, they consumed only 8% of the added methane.  Moreover, these experiments demonstrated that aerobic methanotrophs seem to care only about the amount of oxygen present and not the amount of methane.  When varying amounts of oxygen and a constant amount of methane were added to the water samples, the rate at which aerobic methanotrophs consumed methane trended strongly with the amount of oxygen in the water. However, when the amount of oxygen added was kept constant and the amount of methane added was varied, the rate at which aerobic methanotrophs consumed methane did not vary across the water samples.

 

Rethinking coastal methane

These results mean that the amount of oxygen present, rather than the amount of methane, controls what proportion of methane produced in the seafloor makes it to the atmosphere.  And, contrary to traditional thinking by scientists, aerobic methanotrophs are more efficient at consuming methane in the oxygen-depleted conditions that characterize coastal waters during summertime than during the oxygen-plentiful conditions of wintertime.  While this research does not answer why this is the case – more research into the workings of aerobic methanotrophs at the cellular level will be needed to determine that – it does have important implications for our understanding of how much methane is being emitted from coastlines.

Whereas the amount of methane reaching the atmosphere from coastal waters has traditionally been thought to be lower in the winter, it is now clear that the opposite is true.  This is important knowledge for scientists working to calculate the amount of methane being delivered to the atmosphere, since many coastal regions around the world can only be sampled during the summertime due to stormy weather during wintertime.

In addition, global climate change is expected to make the summertime decrease in oxygen in coastal waters even more dramatic in many regions. The findings from this study indicate that this will actually enable aerobic methanotrophs to consume methane at even faster rates, further reducing the amount of methane that reaches the atmosphere during summertime.  Thus, methane emissions from the coastal ocean may actually decrease over the coming century as global temperatures continue to increase.

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