Marine diatoms contribute significantly to photosynthesis in our oceans and act as the base of the food chain in many of the most productive fisheries. The process of photosynthesis in phytoplankton is affected by cell size, as the rate at availability of carbon dioxide (CO2) is much lower around large diffusion-limited cells.

To investigate how large marine phytoplankton, such as diatoms, overcome the problems of diffusion limitation, we have developed techniques to measure the concentrations of inorganic carbon at the surface of individual diatom cells. By using tiny ion-selective microelectrodes, we were able to measure pH, oxygen and carbonate ion concentrations around the large diatom Odontella sinensis during photosynthesis. The results indicate that photosynthesis leads to dramatic changes in pH at the cell surface of individual cells. By combining these findings with cellular modelling approaches, we were able to demonstrate how large cells can overcome the diffusion limitation of CO2 using the enzyme external carbonic anhydrase.

These findings will help researchers understand how phytoplankton of different size may respond to the predicted changes in the availability of CO2 in future oceans.

© Marine Biological Association

An individual cell of the large diatom Odontella sinsensis showing the position of microelectrodes to measure pH and carbonate ion concentration at the cell surface

Chrachri A, Hopkinson BM, Flynn K, Brownlee C, Wheeler GL (2018)

Dynamic changes in carbonate chemistry in the microenvironment around single marine phytoplankton cells. Nature Communications. 9:74