Smaller than a grain of sand, phytoplankton are key to aquatic health

Cup sea water in your hands and you will be holding a bustling world of single-cell organisms – thousands of them.

Much like creatures of an undersea metropolis, microscopic photosynthetic microbes – phytoplankton – quietly float through the ocean, enhancing water quality. As the foundation for the ocean ecosystem, phytoplankton work tirelessly to fuel marine food webs and consume large amounts of carbon dioxide on scales equivalent to forests. But this is not all they can do! These tiny plants may turn organic contaminants into less toxic chemicals.

Sounds simple, but it’s not. The processes involved remain elusive.

Synthetic chemicals in the environment

Water pollution, once an invisible, silent threat, is now a top environmental concern worldwide.

‘Millions of tonnes of synthetic organic chemicals are used for industrial, agricultural and consumers’ purposes annually. These compounds partially find their way to the aquatic environment, impairing water quality and undermining aquatic life,’ said Giulia Cheloni, an environmental scientist studying phytoplankton’s responses to carbon-based contaminants.

These pollutants are described as contaminants of emerging concern because of their potential risk to human health and ecological impacts. They can be found in personal care products like fragrances, disinfectants and sunscreen agents, as well as household items such as solvents, fabric protectors and flame retardants.

Can powerful phytoplankton clean up contaminants?

Scientists are studying how organic contaminants affect phytoplankton.

‘Phytoplankton are not defenceless against chemical pollution: when exposed to contaminants, they may activate cellular responses to reduce their toxicity,’ said Cheloni, who is conducting research under PHYCOCYP, a project undertaken with the support of the Marie Skłodowska-Curie Actions programme.

This process is called biotransformation. Xenobiotic molecules, namely molecules that are not naturally produced within the organisms, such as pesticides, could be metabolised by phytoplanktonic cells. The latter activate specific enzymes that cause contaminants to become less toxic and more easily eliminated from the organism.

Until now, exactly how this happens has remained elusive. ‘The aim of PHYCOCYP is to further mechanistic understanding of the biotransformation processes in phytoplankton and investigate how they affect their tolerance to organic contaminants,’ noted Cheloni.

Dissecting the biotransformation pathways will boost scientists’ understanding about which enzymes are activated in the process and what kind of chemicals they may transform.

This information is vital to understanding exactly how phytoplankton transforms different classes of organic contaminants in natural environments.

‘The ability to transform organic contaminants into safer products renders phytoplankton a promising candidate in water treatment plants,’ added Cheloni.

Preparing the ground for CYP

To better understand this process, PHYCOCYP will probe how a family of enzymes called Cytochrome P450 (CYP) affects the phytoplankton’s ability to transform organic contaminants. Found in all kingdoms of life including animals, plants, bacteria and even in a few viruses, CYPs play a prominent role in stress responses and xenobiotic degradation.

However, our knowledge of how CYPs act in phytoplankton lags far behind that of other organisms. This is why researchers will build on earlier genetic advances into phytoplankton and apply a genome editing approach to generate mutant species without active CYPs.