The world’s oceans support vast populations of single-celled organisms (phytoplankton) that are responsible, through the process of photosynthesis, for removing about half of all of the carbon dioxide that is produced by burning fossil fuels – as much as the rain forests and all other terrestrial systems together. One group of phytoplankton, known as the coccolithophores, are known for their remarkable ability to build chalk (calcium carbonate, or calcite) scales inside their cells which are secreted to form a protective armour on the cell surface. On a global scale this calcification process accounts for a very significant flux of carbon from the surface ocean, and hence an important component of the global carbon cycle, as cells die and the calcium carbonate sinks to form ocean sediments. New findings have pinpointed a molecular process that may be directly affected by changes in ocean chemistry that are happening as increasing atmospheric carbon dioxide levels lead to increased levels of dissolved carbon dioxide in the oceans.

A team of scientists from the Marine Biological Association and Plymouth Marine Laboratory in the UK and the University of North Carolina Wilmington, USA report the findings in an article to be published in PLoS Biology on 21st June. The armour scales of coccolithophores are formed by transporting calcium and bicarbonate into the cell where they combine to form calcium carbonate. Calcification is a strongly pH-dependent process and it will be critical to understand how it may be affected together with the long-term effects on cycling of carbon in the oceans as carbon dioxide makes the ocean surface waters increasingly more acidic. The researchers used a combination of single cell physiology and molecular biology to probe the molecular machinery that may underlie calcification. A by-product of the calcification reaction is the formation of protons (H+) inside the cell.

“These H+ ions can potentially accumulate in the cell causing it to become acidic – a process known as metabolic acidosis.” says Alison Taylor, article author. Such metabolic acidosis occurs in a range of specialised animal cells which use a vareity of pH regulatory processes to alleviate this burden. The team showed that coccolithophores dispose of unwanted H+ by allowing them to leave the cells through specialised protein pores or ion channels that are selectively permeable to H+. This allows cells to keep pH inside the cell at acceptable levels which is in turn needed for the cells’ ability to produce calcium carbonate scales.

They also showed that coccolithophores possess a gene that encodes for a H+ channel protein. Ion channels are found in all cellular membranes. Channels that allow the passage of sodium or potassium, for example, underlie processes such as nervous activity and muscle contraction in animals. “These H+ channels belong to a unique group of transport proteins that were discovered quite recently in certain types of animal cells that experience metabolic acidosis.” explains Glen Wheeler, co-author of the study. “It turns out that H+ channel genes are also present in other groups of phytoplankton, including diatoms which produce silica rather than calcite scales. The majority of the photosynthetic phytoplankton belong to groups that are not closely related to either plants or animals. Our discovery shows that H+ channels are more widespread than previously thought and that they serve a critical function in regulating cellular pH during a range of cellular processes in evolutionarily distant organisms.” says Wheeler.

“A key finding of the work is that that H+ channel activity in coccolithophores is dependent on external pH.”, explains Colin Brownlee of the Marine Biological Association “These data showing that coccolithophores possess a pH regulatory mechanism analogous to one used by specialised animal cells are really exciting. The findings will ultimately allow a better understanding of how they will respond to the effects of increased dissolved carbon dioxide in the oceans’ surface waters.”