Plankton – organisms carried on the tides – are the foundation of the marine food chain. They are also part of an intricately balanced system that helps keep us all alive. As well as helping produce much of the oxygen we breathe, they are a crucial part of the global carbon cycle.

“The big thing that we’re seeing is warming,” say Ostle, coordinator of the Pacific CPR Survey, as she demonstrates the plankton recorder off the coast of Plymouth in Britain.

The CPR Survey has documented a decisive shift of plankton towards both the poles in recent decades, as ocean currents change and many marine animals head for cooler areas.

Smaller warm water plankton are also replacing more nutritious cold water ones, often also with differing seasonal cycles, meaning the species that feed on them need to adapt or move too.

“The big worry is when change happens so quickly that the ecosystem can’t recover,” says Ostle, adding that dramatic temperature spikes can lead “whole fisheries to collapse”.

With nearly half of humanity reliant on fish for some 20 per cent of their animal protein, this could be devastating.

Plankton is a catch-all term from the Greek for “drifting” and encompasses everything from photosynthesising bacteria many times smaller than the width of a human hair, to jellyfish with long trailing tendrils.

There are two main types: phytoplankton, diverse plantlike cells commonly called algae; and zooplankton, animals including krill and the larvae of fish, crabs and other marine creatures.

Phytoplankton photosynthesise using the sun’s rays to turn C02 and water into sugars, starches and oxygen. Scientists estimate the seas produce around half the oxygen on Earth, and that is mostly thanks to phytoplankton.

They are also crucial to the ocean’s “biological carbon pump”, which helps the sea lock away at least a quarter of C02 emitted by burning fossil fuels.

While trees store carbon in wood and leaves, phytoplankton store it in their bodies. It passes through the food web, with phytoplankton consumed by zooplankton which, in turn, are eaten by creatures from birds to whales.

“Pretty much everything you can think of in the sea at some stage of its life cycle will eat plankton,” says CPR Survey head David Johns. When organic matter from dead plankton or their predators sinks to the ocean floor it takes carbon with it.

Climate change has “exposed ocean and coastal ecosystems to conditions that are unprecedented over centuries to millennia with consequences for ocean-dwelling plants and animals around the world”, says the UN’s Intergovernmental Panel on Climate Change (IPCC) in a leaked draft report on climate impacts, due to be published next year, which predicts “escalating impacts on marine life”.

While phytoplankton are relatively resilient and are likely to continue to shift territory as the seas warm, the IPCC expects that deteriorating conditions in the oceans will ultimately lead to an overall decline this century.

Meanwhile, research published in Nature last month found that iron carried in smoke from huge 2019 and 2020 wildfires in Australia sparked a giant swell of phytoplankton thousands of miles away, which could have sucked up substantial amounts of C02.

Blooms can be seeded by nutrients from sandstorms or volcanic eruptions and these “natural processes” have inspired David King, founder of the Centre for Climate Repair at the University of Cambridge in the UK. King supports a hotly debated idea to “fertilise” plankton blooms by sprinkling iron on the surface.

The theory is that this would not only help suck up more C02, but lead to a surge of ocean life, which would eventually help increase whale populations that have been devastated by hunting.

More whales equal more whale poo, which is full of the nutrients plankton need to bloom and which King hopes could restore a “wonderful circular economy” in the seas.

A pilot project will try the technique in an area of the Arabian Sea carefully sealed off in a “vast plastic bag”, but King acknowledges that the idea raises fears of unintended consequences: “We certainly don’t want to deoxygenate the oceans and I’m pretty confident we won’t.”

Ocean organisms have been photosynthesising for billions of years – long before land plants. But we still have much to learn about them. It was only in the 1980s that scientists named the planktonic bacteria prochlorococcus, now thought to be the most abundant photosynthesiser on the planet.

Some “drifters” it turns out can swim, while others are masters of communal living.

Take the partnership between corals and plankton – it is so important that when the partnership breaks down because of warming, the corals bleach and die. Or Acantharea, a single cell shaped like a snowflake that can gather photosynthesising algae and manipulate them into an energy-generating “battery pack”, says Johan Decelle, of the French research institute CNRS and the University of Grenoble Alpes.

They have been “overlooked” because they dissolve in the chemicals used by scientists to preserve samples.

To study plankton under a high-resolution electron microscope, Decelle used to collect samples at the French coast and drive for hours back to Grenoble with them in a special cool box. But this year he worked with the European Molecular Biology Laboratory on a pioneering project bringing hi-tech freezing virtually onto the beach.

This enables the study of these delicate organisms as close as possible to their natural environment. By contrast, Continuous Plankton Recorders end up mashing their samples into “roadkill”, says Ostle.

The value of the survey, which began in 1931 to understand how plankton affected herring stocks, comes from decades of data. Scientists have used it to look back to track climate changes and it played an important role in the recognition of microplastics.

From the boat in Plymouth, the water appears calm as sunlight slides across its surface. But every drop is teeming with life.

“There’s just a whole galaxy of things going on under there,” Ostle says.