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Biogeochemistry

Suffocating crabs and a one-way street for carbon

 

Original research article:

Jessen, G.L., A. Lichtschlag, A. Ramette, S. Pantoja, P.E. Rossel, C.J. Schubert, U. Struck, A. Boetius. (2017). Hypoxia causes preservation of labile organic matter and changes seafloor microbial community composition (Black Sea). Science Advances 3(2) e1601897. doi:10.1126/sciadv.1601897.

 

At first glance, the ocean floor looks like a dark, barren, and uniform expanse of mud. In reality, the seafloor is filled with life hiding just out of sight.  Worms and clams living in shallow burrows dot the seafloor, tiny shrimp and protists blend in with the mud, and bacteria too small to see with the naked eye are ubiquitous.  In fact, the seafloor is one of the most biodiverse and productive landscapes on the planet.  But as climate change looms, diminishing oxygen levels threaten these unique and important ecosystems.

Seafloor organisms, such as sea stars, clams, urchins, and worms, play a significant role in eating detritus that falls to the seafloor and releasing carbon dioxide. (Source: https://commons.wikimedia.org/wiki/File:Underwater_mcmurdo_sound.jpg)

Sediment-dwelling organisms are living in an ocean that is quickly running out of the oxygen they need to breathe.  Warm water cannot hold as much oxygen as cold water, which means that as global climate change warms the ocean oxygen is lost from seafloor habitats as well. Worse for these organisms, this warming and oxygen loss is expected to accelerate over the course of this century.  For communities that have made a permanent home in the mud, a cloud of oxygen-deficient water settling over them is a death sentence.

 

While this loss of life at the seafloor has immediate implications for fisheries – for example, mass die-offs from naturally-occurring oxygen loss on the Oregon Coast collapsed the crab fishery there – it can also have big repercussions for how climate change progresses. Normally, carbon moves between the atmosphere and ocean in a cycle. Photosynthetic algae at the ocean’s surface take up carbon dioxide from the atmosphere and carry that captured carbon down to the seafloor when they die and sink.  Seafloor organisms in turn feed on this detritus, breathing in oxygen and exhaling carbon dioxide. The result is a continuous loop: carbon from the atmosphere is taken up at the ocean surface, sinks to the seafloor as detritus, and is released at the seafloor to eventually return to the atmosphere.

 

But what happens to that loop if detritus lands at a seafloor without oxygen, where there are no worms, clams, and crabs to eat it?  Does the detritus still get degraded and its carbon released, or is the detritus and its stored carbon left untouched and eventually buried in the seafloor?

 

Luckily, the earth has a natural laboratory for exploring the effects of low-oxygen conditions on seafloor environments.  The Black Sea, in southeastern Europe, is permanently without oxygen in waters deeper than 150 meters as a result of natural buoyancy differences between shallow and deep waters.  This means that along the coast, where the seafloor is gradually sloping down towards the bottom of the sea, a transition occurs from shallow oxygen-rich seafloor to oxygen-limited and ultimately oxygen-free seafloor in deeper waters.

The transition from oxygen-rich to oxygen-free waters on the Crimean Shelf in the Black Sea. The triangles show the seven sites sampled by the researchers in this study, and the dotted line shows the oxygen level at which many seafloor organisms begin to suffocate. (Modified from Jessen et al. 2017)

A team of researchers took advantage of this natural laboratory in the Black Sea to find out exactly what happens to detritus falling onto areas of seafloor with different levels of oxygen.  Sailing on a research ship off of the Crimean peninsula, the researchers first deployed a remotely operated submersible carrying a video camera and an oxygen sensor.  The video camera allowed them to count the number of visible seafloor organisms living along the oxygen gradient.  Using the submersible’s mechanical arm, the needle-like oxygen sensor was slowly inserted millimeters into the seafloor and returned measurements of oxygen levels in the surface of the mud.  This information is essential for determining how much bacteria and worms living in the seafloor are breathing, which is a strong indicator of how fast they are degrading detritus.

 

Using these oxygen measurements, the researchers identified seven sites spanning the oxygen gradient from which they wanted to take mud to study further.  Lowering a coring device – a cylindrical tube with a cap that is triggered when it reaches the seafloor – the scientists were able to carefully collect samples of the seafloor mud and bring them back to the ship to determine what types of detritus were present and how much of it had been degraded by seafloor organisms.

 

The findings from these sites presented a stark contrast in how much detritus is metabolized by seafloor organisms when oxygen is plentiful versus when oxygen is limited or absent.  First, the number of visible organisms, termed meiofauna – comprised of worms, clams, snails, shrimp, and tiny protists – decreased sharply with oxygen deprivation.  Populations declined rapidly from millions of individuals per square meter at the seafloor sites where oxygen was plentiful, to tens of thousands at low-oxygen sites, to only hundreds per square meter in nearby sites where there was no oxygen in the water.  Additionally, oxygen consumption rates in the uppermost layer of seafloor mud declined by nearly half as soon as oxygen levels began to decrease, indicating that even a tiny drop in oxygen levels can slowly suffocate seafloor meiofauna and drastically decrease the rate at which detritus is consumed.

 

The composition of the detritus in the muds the researcher brought back to shore drove this point home.  Chlorophyll, which is one of the juiciest (that is, energy-rich) components of detritus that rains down to the seafloor, was three times more abundant at seafloor sites that lacked oxygen than at sites with plentiful oxygen.  This indicates that in areas of the seafloor without oxygen, detrital material is left nearly untouched relative to seafloor areas with normal oxygen levels.  The same trend was observed for detrital proteins, another component of detritus that is relatively easy for seafloor organisms and bacteria to break down. Overall, the total amount of detritus left uneaten in areas of the seafloor where oxygen was absent was 50% higher than in areas of the seafloor with plentiful oxygen.

 

This study not only confirms earlier indications that less detritus is consumed at the seafloor when oxygen levels drop, but also pins that decline in detrital degradation on the loss of seafloor meiofauna under suffocating conditions.  Meiofauna are responsible for eating a large fraction of detrital material before bacteria can get to it, and in many cases make it easier for bacteria in the seafloor to consume the remaining detritus by creating oxygen-rich burrows in the mud and grinding up large chunks of food into smaller pieces.  Without sufficient oxygen for these organisms to survive, detritus is left to accumulate and eventually be buried under the seafloor – taking the carbon that once came from the atmosphere with it.

Carbon is transferred from the atmosphere to the seafloor by photosynthetic organisms growing, dying, and sinking as detritus. Under high-oxygen conditions, this detritus is consumed by seafloor organisms and the released carbon dioxide can eventually return to the atmosphere. Under low-oxygen conditions, carbon trapped in detritus is buried in the seafloor and does not return to the atmosphere.

This brings us back to the loop of carbon between the atmosphere and seafloor.  Understanding the effects of low-oxygen conditions on the release of atmospheric carbon at the seafloor is critical since low-oxygen conditions are expected to become more prevalent around the world as the ocean warms over coming decades.  The results of this study enable scientists to predict that under these conditions, atmospheric carbon stored in detritus will be buried in the seafloor rather than be released back to the atmosphere by seafloor organisms consuming that detritus. Thus, while parts of the world may lose significant portions of their seafloor life – with potential consequences for the fishing industry and global biodiversity – this loss of life could actually counteract climate change by turning the atmosphere-ocean carbon loop into a one-way street from atmosphere to seafloor.

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