The mysterious life of Antarctic phytoplankton

A curious thing is happening underneath the Antarctic oceans: the food chain is changing.

Last year, in McMurdo Sound, it changed in some unexpected ways.

“In 2022 there was quite an interesting season where the sea ice in McMurdo Sound formed, some of it at its normal time, and then some of it about six months later,” says Jacqui Stuart, a PhD student and researcher at New Zealand’s Cawthron Institute.

She took environmental DNA (eDNA) samples from both the normally-timed sea ice and the latecomer to see what the phytoplankton – the workhorse of the ocean – were up to.

There were big shifts in what was thriving under the ice.

The precise details are under wraps until Stuart’s PhD thesis is published, possibly next year, but the broad picture is the number of phytoplankton under the late sea ice was normal.

The diversity, however, was not.

Phytoplankton is an umbrella term given to a huge range of tiny plant species and Stuart found there just weren’t as many species under the ice, which could become a problem for the larger creatures that eat them.

The sea ice in McMurdo Sound grows a particular kind of fragile matrix called platelet ice, created by supercooled water melted underwater from the bottom of ice shelves.

This ice, which would normally be a couple of metres deep under the sea ice, creates an “upside down reef” in which phytoplankton flourish. But last year the late ice measured anything from 10 centimetres to a metre and a half, says Stuart. 

“The algae that live in the platelet ice don’t come from nowhere. They have to exist in the water column when the sea ice forms,” she says.

“Potentially those species [that I found] prefer that environment.”

Indeed, ice dives between 2014 and 2021 identified unexpected populations of phytoplankton living under sea ice around Antarctica, just waiting for their chance to bloom. 

McMurdo Sound is just a tiny portion of the Antarctic coastline, but Stuart’s eDNA samples aren’t the only evidence suggesting something funny is happening under the water.

Phytoplankton blooms are starting earlier and lasting longer in areas where sea ice is taking longer to grow, as they can take advantage of more access to light, found a team led by Dr Sandy Thomalla at South Africa’s Council of Scientific & Industrial Research.

Conversely, blooms elsewhere in the Southern Ocean are starting on average 50 days later and finishing earlier – factors the researchers fear will have consequences for ocean food chains.

Follow the gourd!

If there is a god, it might be phytoplankton-shaped.

Phytoplankton are microscopic organisms that sit at the very bottom of the food chain and on which all other ocean, and some land-based creatures, depend for their lives.

They turn inorganic matter that is brought up on currents from the deep ocean into food – themselves – which then is eaten by the next size up, zooplankton, which are eaten by fish, which are eaten by bigger fish and so on.

But the microorganisms are more than just the grass of the sea.

About 800-600 million years ago, cyanobacteria colonised the ocean and through photosynthesis began making oxygen. In doing so they created, and continue to maintain, the atmosphere life on Earth enjoys.

Today, they fix about half of the carbon dioxide in the atmosphere and are responsible for half of oxygen production, but make up only 1% of global biomass.

Furthermore, endosymbiotic theory suggests the more than 20,000 species of phytoplankton that we know of evolved from cyanobacteria. 

Phytoplankton also shapes the chemical makeup of the ocean and the atmosphere. They consume carbon from the atmosphere via photosynthesis, then store the carbon in the form of dead fish and fish faeces.

Maybe they thrive, maybe they don’t

Among the many mysteries surrounding phytoplankton, no one is quite sure what effect the extreme natural events caused by more carbon dioxide in the atmosphere will have on phytoplankton, and hence the food chain.

Phytoplankton biomass is expected to increase with more carbon dioxide in the air (provided they have enough nutrients).

The area around Antarctica is the most uncertain in the world in terms of biochemistry.

And several studies, like this one in Frontiers in Marine Science journal in 2014, theorise that warmer oceans will reduce the amount of phytoplankton, because warmer layers of water suppress upwellings that bring up nutrients from deeper waters for the microorganisms to eat.

Then “The Blob” happened, a hot mass of water in the Pacific in 2014-2016 where larger phytoplankton, such as diatoms eaten by krill, disappeared and were replaced in some areas by toxic cyanobacteria and smaller species. In other places, upwellings continued and diatoms flourished.

A new Australian climate model published in Nature Climate Change in March suggested those smaller phytoplanktons are still accessible as food, but to gelatinous groups of zooplankton, such as salps and larvaceans, at the expense of small crustacean omnivores such as krill and copepods which need bigger, more substantial phytoplanktons.

“It comes at a cost: these groups are gelatinous, having about five per cent of the carbon contained in omnivorous zooplankton such as krill and copepods,” lead author Dr Ryan Heneghan said at the time.

“In terms of nutrition, this would be like replacing a rib-eye steak with a bowl of jelly.”

But he told Cosmos that Antarctica is a more complex place.

“The area around Antarctica is the most uncertain in the world in terms of biochemistry,” he says.

“What has been observed in large marine heatwaves is a change in the composition of phytoplankton to smaller phytoplankton and an explosion of gelatinous phytoplankton.

“On the other hand with melting sea ice, you might see more upwelling in Antarctic regions which could mean higher productivity as well.”

Which brings us back to…

Stuart’s surprising finding of a lack of diversity under the late McMurdo sea ice might have consequences for the health of creatures that rely on them for food.

“I like to think of the microalgae community as the produce section in the supermarket,” she says.

“We’ve got our broccoli and our bananas and our pears and lots of different things with lots of different nutritional values. It’s all produce at the end of the day, but you’re not going to get the same nutrients from a banana that you’re going to get from a broccoli.

“Just think, we wouldn’t be super stoked if one year there was just broccoli and yams all winter.”

The problem, as she sees it, is we don’t yet know which changes will be important because so little is known about phytoplankton.

The microalgae community as the produce section in the supermarket.

And because they are the foundation of the global food chain – humans source about 17 per cent of edible meat from the sea – it’s unclear what the consequences will be from any change in the dominant species of phytoplankton in places like Antarctica.

“How much protein does humanity try to get from the oceans every year? And where is the energy for them to grow coming from? It’s this big chain, this big connection. If we’re seeing that at that foundational level, and we don’t understand it, I don’t know if it’s going to be good or bad things that happen.”

While the plight of Emperor penguins tugs at heartstrings, and the rate of growth of sea ice is confusing everyone, the life and times of phytoplankton in the Southern Ocean is still very much a mystery.