Rising ocean temperatures due to global warming have already been linked to coral reef deaths, destructive storms, shifting species distributions and harmful algal blooms. Now, a team of Australian researchers is adding a new and similarly daunting concern to that list: the spread of disease in "habitat-forming" seaweeds that are critical to marine health.
Scientists fear that the widespread loss of these seaweeds could have disastrous effects on creatures that rely on them for food and protection, such as sea hares, sea urchins and dozens of fish and invertebrate species.
"Seaweeds are the 'trees' of coastal temperate systems," said Peter Steinberg, a marine biologist at the University of New South Wales and director of the Sydney Institute of Marine Science, who helped lead the research that was published in the journal Global Change Biology last month.
"They provide the food and habitat for many of the other organisms that live there. Without them, these systems are radically different," he said.
Earlier studies documented rapid decline and disease in seaweeds during the past two decades, but this analysis was the first to examine whether climate change is driving illness in habitat-forming stands that provide life to vast numbers of marine organisms.
In a 2008 study, for instance, biologists failed to locate the seaweed Phyllospora comosa along a 45-mile stretch of New South Wales, Australia — despite evidence to suggest that the species covered the coastline 50 years ago and would still be there.
A previous paper published in 1995 in the journal Science found that off Australia's coast the amount of coralline algal pathogen, a bacteria that infects coral and other habitat-forming plants, jumped from zero to 100 percent in just one year.
The new study by Steinberg and colleagues from the University of New South Wales in Sydney focused on Delisea pulchra, a type of red algae, or seaweed, found in an area around Australia, New Zealand and Antarctica considered to be a global warming hot spot. Ocean temperatures in that region have already increased at rates well above the global average — roughly 3.6 degrees Fahrenheit in the last century, due to the strengthening of the East Australian Current system that flows south toward the South Pole.
In normal conditions, D. pulchra produces molecules known as halogenated furanones that bind to bacterial receptor sites, acting as a kind of chemical defense against infection.
Through field and lab observations, however, researchers discovered that in warmer waters — in this case, in temperatures ranging from 57 to 79 degrees Fahrenheit — the seaweeds showed higher levels of disease, or "bleaching."
They also found that seaweeds injected with antibiotics in the hot waters experienced less disease than those in similar temperatures that were left untreated, indicating that increased bacterial activity was driving disease.
The researchers believe that higher temperatures stress the seaweeds so much that they can no longer allocate resources to support their chemical defense systems, leaving their bacterial receptor sites open to attack.
Rising temperatures also stimulate microbial populations to flourish and become more virulent, and to aggressively seek out surfaces on which to attach.
With a weaker defense mechanism and stronger bacteria, seaweed can become diseased, or bleached, with relative ease, Steinberg said.
In this way, the process is not unlike coral bleaching, he said, which occurs when algae that live in the tissues of coral polyps are killed off from abnormally warm water.
"In terms of climate change and climatic variation it has been known for some time that similar mechanisms operate on corals and seagrasses," Thomas Wernberg, a marine biologist at the University of Western Australia's Oceans Institute, who was not involved in the research, told SolveClimate News.
"These experiments extend this knowledge to a group of organisms that are more widespread and probably much more ecologically and socioeconomically important for the marine environment on a global scale," Wernberg said. "[These findings are] an exciting discovery with great implications."
Implications for Managing Seaweeds
Without these seaweeds, grazing species of fish and invertebrates would lose a vital food source. These same creatures — along with encrusting animals that must anchor themselves onto a surface for survival, such as barnacles, sea sponges and corals — would also lose their habitat.
According to Steinberg, the findings, the first of their kind, may help researchers and conservationists more effectively manage the seaweeds in a warming climate.
"Historically, disease has been challenging to study relative to other ecological factors like competition, predation or nutrients because of many of the difficulties associated with studying bacteria or other microbes in the field," said Steinberg.
"But once you understand the mechanisms by which climate change impacts organisms, it generally helps you manage these systems. If you know the factors that cause or facilitate disease, you can put effort into protecting those populations that are less affected, such as those in colder water or outside the range of particular pathogens."
Steinberg also noted that some pathogens in coral systems seem to be associated with bacteria-rich run-off from land-based energy pollution, meaning that the presence of some bacteria can potentially be managed through marine protected areas.
Sandra Brooke, a marine biologist and director of coral conservation for the Marine Conservation Biology Institute who was not involved in the research, agrees.
"We are now starting to see the effects of historic fossil-fuel combustion and cannot prevent the outcome," she said.
"The only way we can mitigate the effects of global climate change is by making vulnerable ecosystems as healthy as they can be, [which will make them] more resilience to disease, predation and other stressors that may occur as a result of temperature increase and ocean acidification. This should be the objective of future management actions, through implementation of protected areas and improvement of water quality in coastal zones."
More Research to Come
Steinberg and his colleagues are currently investigating how climate change influences the chemical defense systems of several other dominant seaweeds along the Australian coast.
Wernberg, the marine biologist at the University of Western Australia's Oceans Institute, expects Steinberg and his colleagues to find similar disease-temperature connections as they broaden their research to include other seaweeds.
For example, he said, this year Western Australia experienced a "marine heatwave" with seawater temperatures 3.6 to 7.2 degrees Fahrenheit above normal for a prolonged period. Anecdotal evidence suggests that this heat wave was associated with significant amounts of bleaching of kelps.