Turf algae destroys marine ecosystems – Cover story of the journal “Science”

The warming of the oceans in temperate climate zones is leading to the death of kelp forests. At the same time, red algae are increasingly spreading, for example in northern Spain. This transformation from dense, high-growth kelp forests to extensive mats of red algae, as can also be observed in the Gulf of Main on the east coast of the USA, has serious consequences: There is a loss of biodiversity, a change in the flow of energy and nutrients in the reef systems and fundamental changes in the chemical ecology of coastal ecosystems.

A new study, published in the renowned journal Science, shows for the first time how red algae release chemicals that can damage young seaweed plants. This was discovered by a team lead by Shane Farrell and Doug Rasher from the Bigelow Marine Laboratory in Maine and the group of Daniel Petras at the University of Tübingen and the University of California, Riverside. Petras was head of a junior research group in the Cluster of Excellence CMFI at the University of Tübingen in 2021-2024, where parts of the work were carried out. According to the researchers, a worrying feedback loop is emerging: more turf means increased production of harmful chemicals that impede the natural recovery of kelp forests and further accelerate their collapse.

This chemically mediated interaction is also known as allelopathy. The researchers refer to it as chemical warfare. The results of the study impressively demonstrate how climate change is altering ocean ecosystems and making the regeneration of kelp forests on the rapidly warming coast of Maine increasingly difficult.

Researchers from the University of Maine, the University of Tübingen, the Perry Institute for Marine Science and Harvard University are involved in the study. Working in interdisciplinary teams, they combine extensive field studies, chemical analyses and laboratory experiments to better understand the dynamic changes in these valuable ecosystems.

‘It's great to see how our non-targeted metabolomic tools can shed new light on the fascinating chemical complexity caused by changing environments such as invasive algae,’ says Petras. ‘This becomes particularly meaningful when we combine our chemical data with functional information, such as kelp survival.’

In a series of laboratory experiments, the researchers then tested the effects of all the chemicals present in the water around the turf algae-dominated reefs and the specific chemicals released by the five most common species of red algae on the gametophytes, an early life stage of the kelp. The experiments showed that the organisms' chances of survival dropped dramatically - in some cases by up to 500% - when exposed to the chemicals released by the algae. This confirms that the new chemical environment is directly responsible for kelp mortality.

‘Our study is the first to show that chemical warfare can support the recovery potential of cold-water kelp forests. And surprisingly, some of the same types of molecules we identified in reefs in turf algae areas are also involved in the recovery dynamics of tropical coral reefs,’ Rasher said. ‘This shows that we still have a lot to learn about chemical warfare in temperate reefs, the organisms and molecules involved, and how this process varies around the world.’

Previous work by Rasher's research group confirmed that ocean warming is the primary cause of kelp forest collapse in the Gulf of Maine. The new findings show that invasive algae species can keep an ecosystem in a degraded state and prevent kelp forest recovery.

‘Once the algae become established, curbing global carbon emissions and reversing ocean warming is not enough to restore kelp forests in Maine,’ Farrell said. ‘Because of these feedback mechanisms, we need to intervene locally to remove the red algae before the kelp can actually recover.’

(Source: Press release Bigelow Lab/ Leah Campbell)