Electron microscopy reveals Prochlorococcus cells of roughly 500 nanometres. Photo: Luke Thompson/Chisholm Lab & Nikki Watson/Whitehead, MIT
The smallest and most abundant photosynthesising organism in the ocean turns out to be poorly equipped to cope with heat. In tropical waters, production could halve by 2100.
De cyanobacterium Prochlorococcus proves remarkably vulnerable to further warming. An international study published in Nature Microbiology (8 September) shows that its growth rate peaks at around 28 degrees Celsius and declines sharply above that; at 31 degrees it is even comparable to the low values seen in much cooler waters of 19 degrees.
Predicting
Over ten years, researchers collected data from approximately 800 billion phytoplankton cells spread across 200,000 kilometres during ninety expeditions through the tropical and subtropical Pacific Ocean. Using continuous measurements of cell size and chlorophyll fluorescence, they mapped the growth of Prochlorococcus. Models predict that production in the tropics could fall by 17 to 51 per cent by 2100.
Benefiting
Globally, this amounts to a production reduction of 10 to 37 per cent, with the tropics suffering the heaviest losses. The finding is remarkable, because earlier laboratory and modelling studies had suggested that the bacterium would benefit from warmer, nutrient-poor water. The vulnerability is linked to its compact genome: many genes involved in the stress response have been lost over the course of evolution.
Prochlorococcus is estimated to account for around 5 per cent of global photosynthesis, making it a cornerstone of the marine ecosystem. The fact that the related cyanobacterium Synechococcus is more heat-tolerant points to shifts in the composition of picoplankton, with consequences for carbon sequestration and the marine food web.