Coccolithophorids are minute planktonic microbes that drift with the ocean current. While they remain unseen without the aid of a microscope, their presence back through geological time is evident the world over in the form of vast limestone deposits.
It’s hard to imagine that the white chalk cliffs of Dover, up to 500 metres thick in some places, were formed from the remains of these tiny microbes accumulating at the rate of about half a millimetre per year. Across the globe and through geological time enormous deposits have been, and continue to be, laid down removing CO2 from the atmosphere. The CO2 is locked away in the plate-like structures that the plankton produce, and eventually form the chalk rock. Sequestering CO2 on such a vast scale has had a long-term impact on Earth’s climate, changing conditions for life and avoiding the runaway rise in temperatures, as experienced on Venus, that our planet might have otherwise experienced.
Scientists have long-recognised the process behind the formation of the huge limestone deposits, but why the plankton forms the chalky plates that form their ‘shells’ in the first place has remained a mystery. Various theories have suggested they are for defence against grazers, active as sun-screens or to enhance photosynthesis. Now a group of scientists from Swansea University, Plymouth Marine Laboratory and the Marine Biological Association have concluded that these tiny organisms are bioengineering their immediate environment in order to enhance their growth. The scientists considered Emiliania huxleyii, globally the most abundant bloom-forming species - a single large bloom may contain up to one thousand billion billion individuals visible from space, and so it is a key species in global carbon cycles.
All organisms modify their environment to some extent as a result of removal of resources such as nutrients, and releasing waste. Marine phytoplankton like Emiliania also remove CO2 during photosynthesis to support their growth. In doing so the chemistry of the seawater is modified, resulting in an increase in pH, the water becoming basified (more alkali). Like all organisms living intimately with their surroundings survival is enhanced in a stable environment, and indeed the scientists found that growth at stable pH was of benefit to the organism. So, by increasing pH Emiliania appears to place itself at risk. However, the process of calcification leads to acidification of the water. Adequately controlled, acidification during calcification has scope to balance basification during photosynthesis, stabilizing the plankton’s environment to enhance growth.
The modelling studies carried out by the scientists show that the rates of calcification relative to photosynthesis seen in cocclithophorids, like Emiliania, are consistent with calcification providing a mechanism to bioengineer their environment to stabilise pH. Such activity would be most advantageous in periods of high cell density during blooms. However, the scientists also conclude that if ocean acidification continues, natural selection is likely to favour strains of coccolithophores that calcify less and will thus not ‘over acidify’ their environment. In consequence, claims Professor Kevin Flynn who led the research, we cannot assume that anthropogenically released CO2 will be sequestered into chalk at the same rate per cell of Emiliania in the future as has occurred in the past.