Post provided by Dr Rosie Sheward

The 2021 UN Climate Change Conference (COP26) is being held Glasgow this week, and now more than ever before, the pressure is on for world leaders to agree on climate action to keep global warming below 1.5°c. In the lead up to the conference, we’re asking our editors and authors to discuss their research at the interface of climate and ecology. In this post, Rosie Sheward of the Goethe-University Frankfurt shares how combining data from living and fossil phytoplankton communities can help us to bridge the range of timescales and ecosystem complexity involved in understanding how marine organisms are impacted by climate change.

Powering ocean life

When you think of marine ecosystems, the organisms that you can’t even see with the naked eye are probably the last to come to mind.

However, primary production by the oceans’ microscopic phytoplankton fuels all marine animal life and shapes the marine ecosystem services that sustain life as we know it on Planet Earth. Phytoplankton are single photosynthetic cells, rarely bigger than the width of a human hair, that live for just a few days. But this brief existence played out by trillions of individuals each day powers the functioning of our marine ecosystems.

When climate change alters the balance of temperature, nutrient, chemical and light conditions that phytoplankton need to thrive, phytoplankton abundance and distribution also shift in response. This has ramifications across food webs and for the biological carbon pump and carbonate counter pump that are major driving forces of the global carbon cycle.

With ever-increasing climatic and anthropogenic pressures on marine ecosystems, “What are the consequences of climate change for marine phytoplankton?” has never been a more urgent, and challenging, question to answer.

Laboratory experiments, where a species is exposed to controlled changes in an ocean condition like temperature, are a popular way to investigate how sensitive major phytoplankton species are to environmental change. They are particularly good at identifying the cellular processes involved in any physiological response. But lab experiments can never realistically mimic the timeframes and complexity of ocean climate change.

Long-term plankton time-series and ship-board sampling programs contribute data for how productivity and species distributions have responded to climate change over the last decades. However, these records still fall short of capturing evolutionary timescales and only represent a comparatively short snapshot of climate conditions. To capture the response of phytoplankton to a wide range of rates and extremes of climatic change, we must look further back in time.

Small and mighty: marine phytoplankton may be microscopic, but their phytoplankton blooms can be seen from space. This coccolithophore bloom spread across the Barents Sea in 2021. Credit: NASA Earth Observatory.

Learning from the past

Ocean sediments are an exceptional archive of the phytoplankton communities that existed during different past climate states. When an ocean sediment core is drilled, each layer of sediment reveals microfossils that accumulated from slowly sinking plankton remains over thousands to millions of years. These fossil records play a key role in understanding the links between climate, phytoplankton, and marine ecosystem functions more broadly on a range of timescales that complement ocean sampling and lab experiments.

I focus on a calcifying phytoplankton group called coccolithophores. Their fossil record comprises the tiny calcium carbonate plates that form their exoskeleton and spans from around 220 million years ago to the present day. This timeframe captures coccolithophore evolution under both warmer- and cooler-than-Today climates and how climate change on timescales from thousands to millions of years has shaped the ecology and evolution of this important group.

Using size to look at function

Analysing these fossil records reveals the community composition, biodiversity, and evolutionary success or demise of different lineages through geological time. But interestingly, when you compare the datasets collected from fossils with those from modern coccolithophores they are usually very different. This is simply because we can’t always measure the same parameters on both living, growing cells and fossil remains!

Morphological traits, like cell size, are an exception to this because identical measurements can be made on both living and fossil cells. Size is also commonly used as a functional trait because it has such strong links to essential physiological and ecological processes, like nutrient uptake and grazing. The size structure of phytoplankton communities and the relative amount of smaller to larger cells ultimately drives abundance and primary production, distribution and diversity.

My research aims to unify the types of data collected from both fossil and living coccolithophore communities by measuring cell size and other morphometric traits from fossils, sea surface samples, and laboratory cultures. This involves developing new methods for reconstructing size from fossils as intact cells are relatively uncommon in deep-sea sediments.

Currently, I am investigating the size structure of subtropical, temperate and sub-polar coccolithophore communities in the Pacific Ocean during the Oligocene, 33 and 23 million years ago. At this time, atmospheric CO2 levels were similar to those projected for our near future if carbon emissions are not rapidly and dramatically reduced. I work alongside scientists from around the world who bring their expertise in paleoceanographic proxies, marine biogeochemistry and biological oceanography to the project.

Ultimately, our work aims to highlight how the response of marine primary producers to changing ocean conditions drives shifts in key ecosystem functions that relate to the carbon cycle and trophic webs.

Bridging scales of climate change and ecosystem complexity. Left image shows coccolithophore cells from a laboratory experiment, right image shows a fossilised coccolithophore cell from ~26 million years ago observed in deep-sea sediments from the South Pacific Ocean. The scale bar represents 5/1000ths of a mm. Credit: Rosie Sheward

At first glance, variability in phytoplankton community structure through time may not have an obvious link to policy. However, primary production and the ecological processes that shape it are essential parts of understanding how climate change has the potential to alter the ecosystem services that phytoplankton and ultimately our oceans provide. Phytoplankton must therefore be considered as part of the evidence base used to inform appropriate marine management and protection strategies.  

For me, COP26 is an opportunity to lift my head from the microscope and reflect again on why I initially became a marine scientist: connections. The infinite relationships that link our natural world with the organisms that inhabit it holds endless fascination to me. But it’s also a reminder not to forget the human connection to our work, which inspires our understanding of the marine ecosystems that support people’s wellbeing, now and hopefully for generations to come. During COP26 and beyond, we should all remember the stories told each day by our oceans: in large numbers, even the smallest cells have a global impact.

Rosie is a Research Associate in Phytoplankton Ecology working in the Micropaleontology and Paleoceanography Group at the Goethe-University Frankfurt, Germany. Her research is currently funded by the Deutsche Forschungsgemeinschaft (DFG) and uses sediments drilled through the International Ocean Discovery Program.

Rosie is also a Project co-Leader in the VeWA consortium (Past Warm Periods as Natural Analogues of our High CO2 Climate Future) funded by the LOEWE programme of the Hessen Ministry of Higher Education, Research and the Arts, Germany.

Read more about her work here and find her on Twitter @RosieSheward.