The tiny green creatures can even use the extreme twilight under the ice: photosynthesis is possible in much weaker light conditions than previously thought, according to a discovery in Arctic microalgae. This in turn suggests that biological energy production could also take place in greater depths of the oceans than previously thought. The results are therefore of considerable importance for understanding the marine carbon cycle, say the researchers.
They use light energy to produce energy-rich substances and oxygen from carbon dioxide. The ability of plants and other “green” organisms to photosynthesize forms the basis of the energy metabolism of almost all life on our planet. Due to its fundamental importance, this natural patent has been extensively researched. But surprisingly, one aspect has remained unclear so far: what is the minimum light requirement for photosynthetic organisms in order to gain enough energy to build up biomass? There have been theoretical calculations based on physical and biochemical parameters. Until now, however, it was assumed that nature was not exploiting this potential. Photosynthetic biomass production has so far only been demonstrated with light quantities that were significantly higher than the theoretically possible minimum.
On the trail of the light minimum
But now researchers led by Clara Hoppe from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) in Bremerhaven have once again explored the lower limits of photosynthesis capabilities. Their focus was on microalgae – representatives of phytoplankton and ice algae that live in and under the Arctic pack ice. The team used data obtained as part of the international MOSAiC research project. The German research icebreaker Polarstern was frozen in the Arctic Ocean at 88 degrees north latitude in order to then study the developments in the Arctic climate and ecosystem over the course of the year for almost a year.
For the study, data from measuring devices in and under the ice were evaluated, which provided information on biomass production and light conditions. “To measure such low levels of light under the harsh conditions of the Arctic winter, we had to deploy special, newly developed measuring devices in the middle of the polar night,” reports co-author Niels Fuchs from the University of Hamburg.
As the researchers report, values of the plant pigment chlorophyll, which is important for photosynthesis, as well as measurements of biomass changes initially showed that even the pale twilight after the end of the persistent polar night in March can cause the microalgae to produce substances based on photosynthesis – even though the radiation still has to penetrate the snow-covered ice. The measurements of the light intensity made it clear that only a few photons reach the microalgae. “In the end, we could be sure that there was no more light,” says co-author Dirk Notz from the University of Hamburg.
Close to the theoretical minimum value
Specifically, it was shown that a value that was at least one order of magnitude lower than previous estimates was sufficient for photosynthetic biomass production. The researchers say that the evidence is now approaching the theoretical minimum for the system’s performance. “It is very impressive to see how efficiently the algae can use such low amounts of light. This shows once again how well organisms are adapted to their environment,” says Hoppe.
As the team emphasizes, the discovery may have far-reaching significance. “Even though our results come specifically from the Arctic Ocean, they show what photosynthesis is capable of overall. If it is so efficient under the challenging conditions of the Arctic, one can assume that organisms in other regions of the oceans have adapted just as well,” says Hoppe.
This would mean that there would be enough light in deeper areas of the oceans to produce usable energy and oxygen through photosynthesis, which would then be available for higher life forms such as fish. “The low light requirements suggest that the zones in which photosynthesis can take place in the world’s oceans may extend further into the depths and over a longer period of time than previously thought, which has significant implications for estimates of global productivity,” the authors write.
Source: Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, scientific article: Nature Communications, doi: 10.1038/s41467-024-51636-8