Marine phytoplankton are much more resilient to future climate change than previously thought

A study published in Science Advances by an international team of scientists provides clear evidence that marine phytoplankton are much more resilient to future climate change than previously thought
.

Scientists combined data from the long-term Hawaiian Ocean Time Series Program with new climate model simulations conducted by one of South Korea's fastest supercomputers to reveal a mechanism known as nutrient uptake plasticity, allowing seaweed to adapt to and cope with marine conditions
that will occur in the coming decades due to global ocean warming.

Phytoplankton are tiny algae that float on the surface of the ocean (Figure 1) that form the basis of
marine food webs.
During photosynthesis, these algae absorb nutrients (e.
g.
, phosphate, nitrates), absorb dissolved carbon dioxide and release oxygen, which accounts for about
50% of the oxygen we breathe.
Therefore, understanding how seaweed will respond to global warming and the associated decline in nutrients in upper seawater is critical
to understanding the long-term habitability of our planet.

How the annual global phytoplankton production will change over the next 80 years remains highly uncertain
.
The latest report from the Intergovernmental Panel on Climate Change (IPCC) states that the uncertainty is -20% to +20%, which means uncertainty
about whether phytoplankton will increase or decrease in the future.

The impact of global warming on the upper layers of the oceans is greater than on the deep layers of the
oceans.
Warmer waters are lighter, so the upper ocean will become more layered in the future, which reduces the mixing
of nutrients from the ground into the sun-exposed layers where phytoplankton inhabit.
Early research suggests that future depletion of nutrients near the sea surface is expected to lead to a significant reduction in marine phytoplankton production, with wide-ranging and potentially catastrophic impacts
on marine ecosystems and climate.

But according to a new study, advances in science may not happen
.
New analysis of phytoplankton data from the Hawaii Ocean Time Series Project shows that productivity can be sustained
even under conditions of very nutrient scarcity.
"Under these conditions, individual phytoplankton cells can replace phosphorus
with sulfur.
At the community level, one may see further shifts to taxa that require less phosphorus," said David Carr, a co-author of the study and professor of oceanography at the University of Hawaii and co-founder of the Hawaii Marine Temporal Research Project, to illustrate the concept
of phytoplankton plasticity.
Further evidence to support plasticity comes from the fact that in the subtropics, surface water has lower nutrient concentrations, and algae absorb less
phosphorus per amount of carbon stored in cells than the global average.

To investigate how this unique metabolic "hack" will affect global ocean productivity in the coming decades, the team ran a series of climate model simulations
using community Earth system models (version 2, CESM2) on their supercomputer Aleph.
By turning off phytoplankton plasticity in their model, the authors were able to qualitatively reproduce the results of previous models that global productivity had fallen by about 8%.

However, when the plasticity parameter was turned on in their model, in a way that captured observations from the past 30 years near Hawaii, computer simulations showed that global productivity would increase by up to 5 percent
by the end of the century.
Dr Eun Young Kwon, lead author of the study and a researcher at the IBS Centre for Climate Physics at Busan National University in South Korea, said: "However, regionally, these future productivity differences are likely to be much larger, up to 200%
in the subtropics.
" With this additional productivity gain, the ocean can also absorb more carbon dioxide from the atmosphere and eventually sequester it below
the surface of the ocean.

Inspired by the results of sensitivity computer model simulations, the authors then looked at 10 other climate models whose data were used in the last 6 climate change reports using ^{the IPCC} assessment report
.
The results confirmed the author's initial conclusions
.
"Models that lack plasticity tend to predict overall declines in primary yields in the 21st century^.
Dr Sreeush said: "Century, and those explaining the ability of phytoplankton to adapt to low nutrient conditions show an average growing global productivity
.
"

"Although our study demonstrates the importance of biological buffers against global-scale ecological change, this does not mean that phytoplankton are immune to
human-induced climate change.
" For example, worsening ocean acidification will reduce the calcification rate of certain types of phytoplankton, which could lead to large-scale changes
in ecosystems.
Dr Kwon Eun Young warned
.
These factors are neither well understood nor well reflected
in climate models.

"Future Earth system models will require improved observation-based phytoplankton to respond to multiple stressors, including warming and ocean acidification
.
This is necessary to predict the future of marine life on our planet," said Professor Axel Timmerman, co-author of the study and director of the IBS Centre for Climate Physics
.