The discovery is part of a new international study published in Nature Communications that shows how the photosynthetic potential of phytoplankton in the global ocean may have previously been underestimated.
The study was carried out as part of the MOSAiC expedition at 88° northern latitude and revealed that even in the Arctic far north, phytoplankton can build up biomass through photosynthesis as early as the end of March. At this time, the sun is barely above the horizon, so it is still almost completely dark under the snow and ice cover of the Arctic Ocean.
Photosynthesis converts sunlight into biologically usable energy and thus forms the basis of all life on our planet. However, previous measurements of the amount of light required for this have always been well above the theoretically possible minimum.
Fieldwork during the MOSAiC expedition deployed three OptiCAL sensors built at SAMS that can measure light almost down to photon level and showed how photosynthesis can occur with a quantity of light that is close to this minimum.
SAMS scientist Dr. Phil Anderson, who designed and built the OptiCAL sensors, said: “The sensors picked up photosynthetic activity at conditions 10 times dimmer than previously modeled, so this was an exciting discovery. Photosynthesis in the ocean plays a huge part in producing oxygen and capturing carbon, so it is important to know if we have underestimated this process.
“Through this study, we’ve discovered that tiny ocean organisms respond to light in underwater conditions that humans would consider utter darkness. They are remarkably efficient in harnessing all of the light physically possible to make energy.
“We were delighted that the sensors worked so well, continuing to send data after the fieldwork team had left. We’re currently working on a new OptiCAL that is 100 times more sensitive to light in order to explore even darker layers of the ocean.”
As part of the MOSAiC expedition, the German research icebreaker Polarstern was frozen into the icepack of the central Arctic for a year in 2019, in order to investigate the annual cycle of the Arctic climate and ecosystem.
The team led by Dr. Clara Hoppe from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) focused on studying phytoplankton and ice algae. These are responsible for the majority of photosynthesis in the central Arctic. Unexpectedly, the measurements showed that just a few days after the end of the month-long polar night, plant biomass was built up again, for which photosynthesis is absolutely essential.
The results were particularly surprising because photosynthesis in the Arctic Ocean takes place under snow-covered sea ice, which only allows a few photons of sunlight to pass through. As a result, microalgae only had about one hundred thousandth of the amount of light of a sunny day on the Earth’s surface available for their growth.
“It is very impressive to see how efficiently the algae can utilize such low amounts of light. This shows once again how well organisms are adapted to their environment,” said Dr. Hoppe. “Even though our results are specific to the Arctic Ocean, they show what photosynthesis is capable of. If it is so efficient under the challenging conditions of the Arctic, we can assume that organisms in other regions of the oceans have also adapted so well.”
