It’s hard to imagine an earthly environment that is more inhospitable than the dark and frigid waters beneath Antarctica’s sea ice. Yet scientists have discovered evidence that vast blooms of microscopic organisms, known as phytoplankton, are flourishing beneath several feet of compact ice around this remote continent, a finding that emphasizes the versatility of life on Earth and hints at the existence of unexplored ecosystems at the poles, reports a new study.
Phytoplankton are tiny photosynthetic organisms that come in a dazzling variety of shapes and sizes, though most are too small to be seen with the naked eye. These lifeforms are usually found close to the ocean surface, where they can soak up the sunlight they need for energy, and they can proliferate into blooms that can stretch for hundreds of square miles when conditions are right.
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Now, scientists led by Christopher Horvat, a senior lecturer in physics at the University of Auckland and a visiting assistant professor at Brown University, have discovered phytoplankton blooms lurking under the compact sea ice of Antarctica, despite assumptions that these environments “prohibit upper ocean photosynthesis” due to a scarcity of sunlight, among other hardships, according to a study published on Thursday in the journal Frontiers in Marine Science.
Horvat and his colleagues began their search for under-ice Antarctic blooms after members of the team found phytoplankton living beneath the sea ice all the way at the other pole, in the Arctic, which are flourishing in part due to the dramatic effects of human-driven climate change on the region.
“We’ve all been a part of current research in the Arctic, where there’s been a lot of focus because these blooms were a surprising observation,” Horvat said in an email to Motherboard. “In 2020 me and [study co-author] Lisa Matthes worked on a paper that showed a substantial portion of the Arctic was productive outside of the period of rapid change we’re in now.”
“Then I moved to New Zealand and met [co-authors Sarah Seabrook and Antonia Cristi] who work on culturing phytoplankton from the Southern Ocean, which are incredibly responsive to light,” he continued. “Together we started thinking that maybe the Southern Ocean has all the qualities needed to support photosynthesis under the sea ice.”
To solve this mystery, the researchers sent submersible instruments, called BGC-Argo floats, on a total of 2,197 dives under Antarctica’s ice to collect samples from 2014 to 2021. The team also ran models estimating the amount of light that might make it through the compact ice to hungry photosynthesizers in the ice-covered waters.
The results revealed that “nearly all (88%) such measurements recorded increasing phytoplankton biomass before the seasonal retreat of sea ice” and “a significant fraction (26%) met an observationally-determined threshold for an under-ice bloom,” according to the study.
The team reached these numbers by screening the samples for a certain pigment that is shared by all phytoplankton. Meanwhile, the climate models confirmed that more sunlight is able to directly permeate through Antarctic ice compared to similar environments in the Arctic, thereby allowing blooms to flourish in hidden regions of the Southern Ocean.
“Certainly we didn’t expect them to be as widespread as the climate models and float data suggest they are, but the more we look at the data the more ‘obvious’ it becomes,” Horvat said. “Sea ice in the Southern Ocean is quite different to sea ice in the Arctic, and this means there can be a lot more light reaching the upper ocean.”
Phytoplankton activity in the Arctic is definitely influenced by human-driven climate change, which is caused by the greenhouse gasses released from our fossil fuel consumption. The Arctic is warming four times faster than the rest of the globe, causing a rapid disappearance of sea ice in northern polar waters as global temperatures tick upward. The loss of sea ice is a major change that has exposed phytoplankton to more sunlight, causing a surge in their population that can be linked to human activity.
However, the effects of human-driven climate change on Antarctic environments are much murkier, though some links have been established. For this reason, Horvat said it was “hard to know” how climate change might affect these Southern Ocean blooms in the future.
“We are currently uncertain about the trajectory of Antarctic sea ice” which is “another important reason to focus on sea ice and coupled climate modeling,” he noted.
While scientists tease out the complicated relationships between climate and life here on Earth, the existence of these blooms also raise intriguing questions about the odds that organisms could emerge in similar environments on other worlds. For instance, Jupiter’s moon Europa and Saturn’s moon Enceladus both contain subsurface oceans, making them promising targets in the search for extraterrestrial life, though no sunlight would be able to penetrate through their icy shells.
“I think the lesson here, and of similar observations by New Zealand researchers of amphipods under the Antarctic ice sheet, show that life doesn’t require all that much to persist and even thrive,” Horvat said. “Whether this translates to other planetary bodies, where the solar radiation is many times lower, I can’t say.”
Horvat and his colleagues are actually looking to space for another reason: Satellite data. The team hopes to use observations from NASA’s ICESat-2, a spacecraft launched in 2018, to get a better read on these mysterious phytoplankton blooms, among other unexplored ecosystems that may be concealed under the ice.
“Our primary questions go beyond simply observing these blooms but actually watching them develop in real-time,” Horvat said. “So we’re hoping to get involved in sea-going campaigns in the Southern Hemisphere spring to see these events unfold.”
“One really exciting thing, led by [study co-author] Kelsey Bisson is the use of the NASA ICESat-2 laser altimeter to actually see through the sea ice, and observe these blooms across the Arctic and Southern Oceans,” he concluded. “We’re working hard at making that a possibility with some colleagues at NASA right now!”
