Their estimate is a conservative one, he added, and it’s possible even fewer photons got through. “The ice cover is quite heterogeneous,” he explained. Because some parts of the sheet might allow more light through than others, the research team selected the upper thresholds of their light measurements. “In the end there’s some variety, and we really want to be on the safe side — to not stake on the lower limit where we’re not 100% certain that this is really the amount of light.”
Pairing Fuchs’ light data with Hoppe’s microalgae observations clinched it: At the end of March, right when the barest amount of sunlight returned, the microalgae not only had their photosynthetic machinery up and running but were also growing and building biomass. Her team concluded that they’d made the first-ever field observation of photosynthesis at just around the theoretical minimum — where the amount of light was an order of magnitude lower than what had been observed in nature before.
Sleep No More
Hoppe was excited to observe photosynthesis at or near the minimum amount of light that could power life. But the finding raised a question: How could dormant cells be ready to turn their machinery on at the very moment that spring’s first light trickled through the ice?
Her team found that during the darkest periods of polar night, the microalgae didn’t show a measurable uptick in carbon uptake — they were neither growing nor photosynthesizing. Yet they weren’t totally dormant either. The cells kept running on low power. Then, as soon as the light levels rose enough to support active carbon fixation in late March, the algae were ready to explode into action.
“It’s sort of like a seedbed or an inoculation issue,” Campbell said. “That ability to productively exploit really low light improves your ability to survive and then be ready to go fast when the light goes back.”
The researchers aren’t entirely sure how the microalgae managed to stay alive and out of dormancy through the darkest times. Some, such as diatoms, can consume dissolved organic nutrients directly from the water. Perhaps they could eke out a living from stray photons that passed through cracks in the ice or were emitted by some bioluminescent creature. Or perhaps polar algae have evolved unique mechanisms that can keep their metabolism running on low at frigid temperatures so that they’re ready to activate at first light.
Such adaptations might be important to the ecology of the region, said Kevin Flynn, a plankton specialist at Plymouth Marine Laboratory who was not involved in the study. “The organisms may be getting ready earlier than we think,” he said. The finding is “important work that’s a reality check about what nature really does.”
However, he isn’t entirely convinced that the cells’ late-March growth occurred through photosynthesis. “The appearance of chlorophyll does not mean that they are photosynthesizing to obtain that growth,” he said. “They may simply be making more chlorophyll from organics and in preparation for photosynthesizing. Because as the season goes, there will be light. And the organism which is ready for it quicker than the others is going to go the quickest.”
On the other hand, Campbell thinks it’s possible that the algae might be photosynthesizing even earlier than Hoppe’s team suggested. Their estimates of light levels were conservative, he said, and photosynthesis may have been occurring well in advance of the kind of biomass accumulation that’s easy to measure. It is feasible to him, then, that “these things are right at or touching below that biochemical thermodynamic limit,” he said.
The findings paint a new picture of life in the Arctic’s polar night and possibly beyond. Life may not be packed entirely into a few short months of summer; rather, the waters may be productive — or, at the very least, still living — throughout the year. This, Hoppe said, could rewrite our understanding of Arctic organisms’ life cycles, interactions and energy reserves.
She wonders, too, whether Arctic phytoplankton’s ability to ride out near-absolute darkness might be shared by some algae in the colder, darker waters of the deep sea. If she’s right, the zone of productive ocean may be deeper than anyone thought. “If polar phytoplankton were able to evolve these mechanisms,” Hoppe suggested, “I’m sure phytoplankton in other areas of the ocean can do the same.”
