As a source of strange and exotic phenomena, the oceans appear limitless. A while ago we took a look at abyssal plains, which cover more than a third of the Earth’s surface (20 July, p 49). Here’s another one that caught our eye: OMZs, or oxygen-minimum zones.
OMZs are just what you’d expect: bodies of water where oxygen levels are considerably lower than normal – less than 0.5 millilitres of oxygen per litre of sea water, compared with the average of 7. Scientists have found these zones several hundred metres down off the west coasts of North, Central and South America, off the west coast of southern Africa, in the Arabian Sea, the Bay of Bengal and the Black and Baltic Seas. What’s more, global warming could make them more common.
How do they form? When winds and the Earth’s rotation sweep coastal water offshore, cold water rises up from the deep ocean to replace it, bringing extra nutrients to the surface. This increases the number of surface-dwelling organisms. When these organisms die and sink, bacteria in the lower depths decompose them, depriving the water of oxygen. An OMZ will form either where circulation at these depths is poor, or where incoming water is already oxygen-deficient.
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So what makes OMZs interesting? As you’d expect, most marine creatures cannot survive in these zones, and that paves the way for some very unusual life forms. You can divide them into two groups. The first are those that can exist on very little oxygen. These include a large red shrimp called Gnathophausia ingens (see above) that squirts out a luminescent liquid when distressed, and whose large gills can remove 90 per cent of oxygen from the sea water that passes through them.
The other group of OMZ denizens are the bacteria that do not need oxygen at all. They generally use nitrate to oxidise hydrogen sulphide – a by-product of organic decomposition – and thus release energy for growth and reproduction. Typical of this group are large bacteria such as Thioploca, which accumulate on the sea floor in thick mats that can weigh up to 120 grams per square metre. Another is Beggiatoa, also commonly found around hydrothermal vents.
And researchers recently found the world’s biggest bacterium in an OMZ off the coast of Namibia. The cells of Thiomargarita namibiensis can reach three-quarters of a millimetre across.
So where does global warming come into it? We already know that the oxygen content of the oceans is declining. Some researchers believe this is due to warming, which increases the number of oxygen-consuming life forms in the water, depleting it of the gas. This is unlikely to turn the oceans into one giant OMZ, but it could make these zones more common. Which would be good news for sulphur-gobbling bacteria – but bad news for almost everything else.