'Not just hot water': Marine heat waves can create toxic relationship between seagrasses and microbes

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Heat stress from marine heat waves can create a toxic relationship between seagrasses and a hidden ecosystem of bacteria, transforming a previously beneficial co-existence between marine plants and microbes into a harmful one, a University of Sydney and UNSW study has found. The research is published in New Phytologist.

Seagrasses are marine flowering plants that act as fish nurseries, purify water and are crucial in coastal carbon storage. Their decline is often missed until it's too late.

The role soil microbes play in land plant health and climate resilience is well known. But for marine plants like seagrass, this science has largely been overlooked.

"It's worth paying attention to what happens in seagrass habitats as marine heat waves become more common. That information could be invaluable for conservation efforts," said lead researcher Dr. Renske Jongen, from the School of Life and Environmental Sciences.

In an underwater gardening experiment, biologists found a diverse bacterial ecosystem in the soil and around seagrass roots. The bacterial ecosystem was in a delicate balance, controlling the chemistry of the soil and seagrass health.

Under increased water temperature, tiny bacteria living in the sediment among seagrass roots can reduce seagrass tolerance to climate change, stunting its growth and its ability to cope with heat stress.

Higher temperatures favor bacterial species known to produce hydrogen sulfide, a compound toxic to seagrass, which may stunt seagrass growth. Plants previously exposed to warmer conditions suffer more from those changes in microbes.

The researchers found seagrass growing in sediments from warm areas produces 34% less biomass when the natural sediment microbes weren't disturbed.

The findings show how bacterial communities are a hidden factor in recovering and restoring seagrass.

"Just as microalgal symbionts (tiny organisms that rely on sunlight as energy) are key to the health of coral reefs, bacterial symbionts nestled at the roots and sediment of seagrasses can influence whether seagrass survives or declines," said Dr. Jongen.

"Even though seagrasses may look okay at first glance, what we've found below ground under increased temperature tells a different story."

Just as heat waves have hit terrestrial plants, marine heat waves have thinned out once lush and widespread seagrass meadows along the Australian coast. They are mainly found in shallow coastal waters and estuaries from tropical Queensland all the way down to the cool, temperate waters of Tasmania.

Microbial communities also shape marine plants' responses to environmental stress.

Heat stress isn't only about hot water. "Increased water temperatures dramatically change the ecosystem of microbes living among the seagrass roots and how microbes co-exist," said senior author Associate Professor Ziggy Marzinelli from the University of Sydney.

"Under heat stress, the microbial communities around seagrass roots shift in ways that can harm rather than help the plant."

How decades of industrial history created a real-world climate experiment

In Myuna Bay in Lake Macquarie, history has created the perfect conditions for the research team to answer the question—"what would happen to seagrasses and microbes if water temperatures increased as projected by climate change models?"

Since 1984, Eraring Power Station has continually fed a plume of warm estuarine water into the lake.

This has made some of the lake waters up to three degrees warmer than ambient temperature for nearly four decades, mimicking both marine heat waves and what future oceans could be like along the Eastern Australia coast by 2090.

"This has inadvertently created realistic conditions for the ultimate 'gardening experiment'—for us to test how seagrass and below ground microbe health is shaped by exposure to higher-than-normal ocean temperatures," said Dr. Jongen.

"Locals are aware of the temperature increase in the area. It also has a reputation as a popular fishing spot because the hot water attracts a lot of fish species and everything from sharks to turtles have been seen here."

The research team transplanted Zostera muelleri, a species of sea grass native to coastal areas of Australia, into the lakebed.

They also extracted and analyzed DNA to find the type of bacterial communities from the sediment and sediment from the seagrass roots to find how their composition changed at different temperatures.

That was when they uncovered the change in bacterial communities and especially the relative increase of bacterial species that suppressed seagrass growth.

"Our study highlights the overlooked role of microbes in tipping the balance in marine environments," said Professor Paul Gribben from the University of New South Wales.

"Seagrass restoration should not just focus on selecting species that are more heat tolerant, but also look deeper, below the ground surface—and, if needed, address microbial communities before transplanting or restoring seagrass meadows."

Publication details

Renske Jongen et al, Ocean warming indirectly affects seagrass performance through effects on sediment microbial communities, New Phytologist (2026). DOI: 10.1111/nph.71195

Journal information: New Phytologist

Key concepts

effects of climate changeestuarine ecosystemsMicrobial ConsortiaClimate Change

Provided by University of Sydney