Microscopic marine plants bioengineer their environment to enhance their own growth

A bloom of phytoplankton in the Barents Sea: the milky blue colour strongly suggests it contains coccolithopores. Wikimedia/NASA Earth Observatory

All life affects the environment. All organisms remove material they need, and they all release material that they do not need. During the evolution of life on Earth, the planet has been greatly modified by both non-biological factors and also by life itself.

The most dramatic event was the removal of carbon dioxide from the atmosphere, and its replacement by oxygen. Driven by photosynthesis, it perpetrated for the original life forms – which evolved in strictly anaerobic conditions, with no oxygen – what could be viewed as the greatest environmental pollution event to affect Earth. The removal of carbon dioxide also decreased the levels of this greenhouse gas, which lowered the planet’s temperature.

Emiliania huxleyi: a single-celled phytoplankton that produces coccoliths. Alison R Taylor/Wikimedia, CC BY

Where did all that carbon go? One sink was into organic carbon that we now extract as gas and oil. Much of this material originated from the burial of microscopic photosynthetic organisms, the phytoplankton.

Phytoplankton form the base of the marine foodweb: they produce fatty acids, some of which, once they get transferred into fish, form an important part of the human diet. If they become buried for millions of years, the fatty acids are converted into oil. Millions of years of phytoplankton growth have produced the oil that humanity is so rapidly extracting and converting back to atmospheric carbon dioxide.

Buried phytoplankton

Most of this CO₂ is not in this form, however – most is phytoplankton buried as chalk, a form of limestone; think of the white cliffs of Dover. Perhaps fortunately for Earth, humans have no great need for chalk, so this reservoir of carbon dioxide remains safely locked up. But why did biology make limestone in the first place?

Many organisms, including ourselves, use calcium carbonate for their skeletons. Even microscopic organisms make hard parts made of this material, and some of those structures could certainly be viewed as having a skeletal function, to protect the organism, or to hang components from. However, much of the limestone represents deposits made by phytoplankton that have no obvious need for a skeleton. So why did they go through the effort of making calcium carbonate; indeed why are they still doing it, and will climate change affect the process?

The striking white cliffs of Dover. Wikimedia/Michael Rowe, CC BY

Large growths of the phytoplankton that deposit calcium carbonate, called “coccolithophorids”, can be seen from space; their blooms are so large that the white chalky plates, the “coccoliths”, reflect light that is sensed by satellites. Why they make this material has baffled scientists for decades. Some have suggested that the plates help protect the organisms, which are only some five-millionth of a metre in diameter, from viruses or grazers. Others have suggested that making calcium carbonate may enhance the supply of carbon dioxide for photosynthesis by modifying seawater chemistry. Evidence for such explanations has been refuted or at is least not substantiated, however. But a new idea developed by our team lead by Swansea University provides an alternative explanation, which – even if it is not the basis for the evolution of coccolith production – must affect the ecology of these Earth-shaping organisms.

A balancing act

During photosynthesis, phytoplankton remove carbon dioxide from the water and the acidity of the water then decreases. This decrease (an increase in pH) is basification, the opposite of acidification. Humans are causing ocean acidification through excess carbon dioxide from burning coal, oil and gas dissolving in the sea and volcanoes also contribute. Neither basification nor acidification is good for phytoplankton growth, but while ocean acidification is largely human-made, basification during phytoplankton growth is perfectly natural and is something that all phytoplankton must endure. Actually, this is not true: coccolithophorids do not have to endure it, and this is why.

The white clouds in the water are actually light reflected from billions of coccoliths. Wikimedia

Calcification, such as the production of coccoliths, also removes carbon dioxide, as does photosynthesis. However, the chemistry behind calcification leads in total to a decrease in pH: it causes an acidification event. During our research, we used detailed mathematical models of the growth of coccolithophorids to test the theory that basification caused by photosynthesis would be matched by acidification caused by calcification. The result of such an event would be a stable pH that promoted coccolithophorid growth. The models demonstrated that the ratio of photosynthesis to calcification seen in the real organisms, and in the way that they respond to changing environmental conditions, matched predictions from the model.

Coccolithophorids do not do what most other organisms do, take what they need and throw waste out that damages their environment for the next generation. These organisms can bioengineer their environment by mopping up the waste, here, by countering basification. They make the seawater in which they live a more stable environment.

Our research also considered the implications of ocean acidification for coccolithophorids, and the result is not such good news. To balance pH in the future ocean, less calcification is needed; this means they cannot help sink so much of the carbon dioxide humans pump to the atmosphere. So, we may have found the reason why coccolithophorids help make cliffs of chalk, but also why they won’t be doing it so much in the future.