The Ocean's Cleaners

The seabed is surprisingly clean considering the amount of waste, both natural and man-made, that gets dumped in the oceans. So how does the ocean do it?
10 March 2011


As any good diver will tell you, the sea bed is always pretty clean. You won't see any fish skeletons or rotting seaweed. It is surprisingly clean considering the amount of waste, natural and man-made, that gets washed into the oceans from every river on the planet. Think what a suburb looks like when waste removal does not take place for a few days and the smell generated by the decaying dirt.

Major surface ocean currentsHow does the ocean do it?

We can imagine the world's seas as the heart, blood, kidney and liver of our planet, and in a way the lungs as well.

The sea currents that pump vast quantities of water from the equator to the poles and then back again can be thought of as the circulatory system of the world. The water movement dilutes and distributes any pollutants, transporting nutrients from the polar waters to the equatorial seas.

In 2010, the world's first, comprehensive census of marine life was published. It showed that we know substantially less about life in the sea than we thought. For example, in the 1950's marine biologists predicted that there were about 100,000 microbes per litre of sea water. The 2010 census has shown that the number is greater than a billion. The total weight of all the microbes in the ocean is estimated as equivalent to 280 billion elephants. That's 40 elephants for every human on the planet.

These microbes act as the kidneys, liver and lungs of the planet by converting all the waste into useable molecules, which are then precipitated to the ocean floor, which in turn cleans and purifies the water. Although not all microbes help to clean up the ocean, they are all essential to maintaining the sustainable food web that marine life inhabit.

The microbes found in this web stretch from so small you would need an electron microscope to see them, such as viruses, all the way up to complex animals, such as copepods, which you can see with just a good pair of glasses.

The micro-organisms can all be grouped into three distinct categories, 'The Decomposers', 'The Producers' and 'The Consumers'. Many of them have dual functions.

The Decomposers


We think of them as carriers of disease, but that only accounts for a small number of the millions of types that exist. Viruses, and their role in the food web, are little understood, but of the three categories, they best fit in as a 'decomposer'. They remove the sick, the weak and the old of the oceans' micro-organisms thus making this huge mass available to bacteria and archaea for conversion. We also know that they are critical to life on earth because of the way that they can change the genes of micro-organisms helping them to adapt to environmental changes.


Once again we think of them as bearing disease but in actual fact ocean bacteria are hugely responsible for the well being of our planet. They can convert atmospheric nitrogen to useable nitrogen molecules, essential for all life on Earth. They also help other marine creatures process carbon into sugars and oxygen, and covert essential minerals into useful molecules. But perhaps most importantly, they break down dead organisms converting bones, flesh and excrement back into the molecules that can be used by other organisms for life to exist.


Grand Prismatic SpringBefore 1994, this group of single cell animals were thought of as 'freaky bacteria' because they were found in boiling geysers, or in the freezing water at the bottom of ocean trenches, or in acid ponds. DNA analyses showed that these creatures were a completely different life form and are now recognised as such. They carry out similar functions as bacteria but in extreme environments. There is growing evidence that Archaea may be the second most abundant life form on Earth.

The Producers


For the sea to remain clean and healthy, carbon dioxide needs to be removed and oxygen introduced. Phytoplankton, mainly made up of ocean algae and different types of bacteria, perform this function, acting as the lungs of the ocean. They not only remove the CO2 using sunlight, but produce oxygen and sugar molecules as by products. It is thought that over 50% of the Earth's oxygen is produced by the phytoplankton.

There is another 'producer' we should mention, the zooxanthellae. This special algae is associated with coral reefs. It supplies the coral polyps with sugar and oxygen, which promotes the healthy growth and repair of the reef. The symbiotic relationship creates an ecosystem, the coral reefs, in areas that would otherwise be deserted.

The Consumers

The final step in keeping the sea clean is carried out by the 'consumer' microbes. They complete the circle by eating the 'decomposers' and 'producers', making the energy and food from these bugs available for the bigger animals.

One of these consumers is known as the Red Tide, so called because of the large blooms of it that can occur along coastlines across the world. The algae that make up the Red Tide, have a reddish-brown colour, and when a bloom occurs, the water is discoloured and appears red. It cleans up the water by eating bacteria and the waste left by other plankton species. In turn, it becomes food for the filter feeders, mussels, perlemoen and oysters. In some parts of the United States, the Red Tide blooms can cause disruption to the fishing and tourism, as the algae are poisonous, although this is not true of all the species.

Two 'consumers' we often hear about are krill (Norwegian for whale food) and copepods. These shrimp like animals eat the phytoplankton and in turn become food for whales, squid, seals, dolphins, and fish. Without their intervention the sea would look like a green swamp. They live in a vertical column of water descending to around 200m during the day and ascending at night to feast on the phytoplankton.

Evidence shows that it was microbes that converted the hostile, poisonous earth that was created a few billion years ago into the rather comfortable home we now know it to be. They may also be the salvation of the planet from global warming.


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