According to a University of Sydney and UNSW study published in New Phytologist, heat stress from marine heatwaves transforms beneficial co-existences between seagrasses and hidden soil bacteria into toxic relationships, stunting marine plant growth and impairing climate resilience. Researchers found that rising water temperatures alter below-ground microbial communities, increasing the abundance of bacteria that produce hydrogen sulphide—a compound toxic to seagrasses.
How Marine Heatwaves Trigger Toxic Microbes
Seagrasses act as vital fish nurseries, purify coastal waters, and store immense amounts of blue carbon. However, their underground ecosystems remain largely overlooked compared to terrestrial plant soil microbiomes. In an underwater gardening experiment, biologists discovered a delicate balance of bacteria controlling soil chemistry and plant health around seagrass roots.
Under increased water temperatures, this balance breaks down. Higher heat favors bacterial species that generate hydrogen sulphide. Plants previously exposed to warmer conditions suffer the most from these microbial shifts. According to the study, seagrass growing in sediments from warm areas produces 34 percent less biomass when natural sediment microbes remain undisturbed.
Did you know? Just as microalgal symbionts drive the health of coral reefs, bacterial symbionts nestled at the roots and sediment of seagrasses dictate whether the marine plants survive or decline under environmental stress, according to Dr. Renske Jongen.
Decades of Industrial Warming Mimic Future Climate Scenarios
To test how rising temperatures impact seagrasses and microbes, researchers utilized a real-world climate experiment in Myuna Bay, Lake Macquarie. Since 1984, the Eraring Power Station has continuously discharged warm estuarine water into the lake, keeping certain areas up to three degrees warmer than ambient temperatures for nearly four decades.
“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. Renske Jongen from the University of Sydney’s School of Life and Environmental Sciences.
The research team transplanted native coastal seagrass (Zostera muelleri) into the lakebed. By extracting and analyzing DNA from the sediment and root zones, the team mapped how bacterial compositions shifted across different temperatures, uncovering the surge in growth-suppressing bacterial communities.
Implications for Coastal Restoration and Conservation
Marine heatwaves have already thinned once-lush seagrass meadows spanning from tropical Queensland down to the temperate waters of Tasmania. Because seagrass declines often go unnoticed until it is too late, understanding subsurface dynamics is critical for future conservation.
“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. Experts suggest that coastal restoration initiatives must look beyond selecting heat-tolerant plant species and actively address below-ground microbial communities before transplanting meadows.
Frequently Asked Questions
What causes seagrass to decline during marine heatwaves?
While direct heat stress impacts the plants, rising temperatures also alter soil microbes around the roots. This shift increases hydrogen sulphide-producing bacteria, which poison the plant and reduce its biomass production by up to 34 percent.
Why is Myuna Bay important for climate research?
Decades of warm water discharge from the Eraring Power Station have created conditions that mimic projected marine heatwaves and future ocean temperatures expected by 2090, allowing scientists to study long-term impacts on marine life.
How can this research improve seagrass restoration?
Scientists recommend that restoration projects not only select heat-tolerant seagrass varieties but also assess and manage sediment bacterial communities before transplanting meadows into degraded areas.