The role of phytoplankton

As we learnt in school, the oceans cover about three-fourths of the earth's crust. Everywhere, on the surface of the oceans, live zillions of tiny creatures, called phytoplankton. There are about 5000 species of these creatures, which include diatoms, cyanobacteria, dinoflagellates, green algae and seaweeds. In the ocean, they do the same job as plants on earth. That is, they convert carbon dioxide to sugar by photosynthesis, and provide food for the other creatures of the sea.

Phytoplankton have many beneficial effects ob the earth's atmosphere. Most importantly, they take up carbon dioxide from the atmosphere and convert it to food. They support the growth of fish and other marine populations. A group of phytoplankton called coccolithophorids release large amounts of dimethyl sulphide (DMS) into the air. DMS reacts with oxygen to form sulphate ions. These ions act as nuclei for condensation of water vapour, and thus cause clouds to form. As clouds reflect excess sunlight, they help cool the earth.

Here's a glimpse* into their fascinating world:

How chemistry can help phytoplankton

Phytoplankton face many problems getting nutrients other than carbon dioxide and water. Seawater is poor in nitrogen, phosphorus and iron, which are abundant in soil. Now we add fertilisers to soil when it becomes poor in nutrients that crops need. How about putting fertiliser in the sea?

This idea was first suggested by the oceanographer John Martin (1935-1993), who discovered that oceans were poor in iron. In 1991, when the volcano Mt. Pinatubo erupted, it deposited 40,000 tonnes of iron dust into the sea. A major consequence was that there was a huge bloom of phytoplankton, and a measurable decrease in atmospheric CO₂.

A few scientists are testing whether adding artificial fertiliser in the sea will make a difference. In one experiment, Australian scientists sprayed several tonnes of urea into a large are of the ocean. In another experiment, iron dust was added an area of the Atlantic Ocean off Argentina.

In both cases, a great 'bloom' was seen over the ocean surface, as phytoplankton multiplied in large numbers. Fertilising the sea may have other benefits too. An increase in phytoplankton also leads to an increase in the number of small fish that eat them, and the bigger fish that eat those small fish. Phytoplankton can also be harvested by humans as a rich source of protein. And the increase in coccolithophores would increase cloud cover and cool the earth!

Risks and rewards

As with every scientific experiment, there are a few risks. One is that fertilisers may lead to harmful 'algal blooms', similar to those seen in rivers. These blooms deplete oxygen from water very fast, depriving other organisms. And as some phytoplankton release toxic substances, these may lead to poisoning of the sea. And finally, fertilising would alter the ecological balance in the ocean. This is because many species evolved to do without too much iron or nitrogen in the environment.

Yet, the benefits of stopping climate change may be quite large. Adding nitrogen to the sea can reduce global warming due to atmospheric CO₂ by about a quarter, while iron could reduce a sixth. Phosphorus could potentially reduce it by half! But these measures are not enough. The best solution is still to not put any CO₂ into the atmosphere at all, which we can achieve by reducing our carbon footprint.

*Video titled "AmericanCritiquer - January 18, 2010 - "Half of the world's oxygen is produced by these organisms,"..." sourced from www.YouTube.com