TAKE a walk along a seashore almost anywhere in the world and you will probably see colourful patches of life growing on rocks, sea walls and driftwood. These are lichens, mutualistic partnerships between fungi and algae. In Britain, that might include the brightly coloured orange sea lichen and the yellowish maritime sunburst lichen, plus many other drabber species.
Lichens are usually seen as land-living organisms and, indeed, the vast majority are, growing ponderously on inland rocks, tree trunks, leaves and soil. But coastal ones aren’t accidental vagrants from the land: they are marine-adapted species found only on or very near the shore. Until recently, they were thought to be outliers in the almost completely landlubbing kingdom of the fungi. Not any more. “In every marine ecosystem we look at, we find fungi,” says Michael Cunliffe at the University of Plymouth, UK.
The study of marine fungi is now mushrooming, though their exact role and importance in marine ecosystems remain something of a mystery. Nevertheless, there are high hopes that they could help to rescue us from two of the great scourges of the 21st century: antibiotic resistance and plastic pollution.
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What are marine fungi?
The existence of marine fungi has been known for many years. The earliest descriptions were published in the mid-19th and early 20th centuries, but these were largely ignored by mainstream mycologists. The species discovered were mostly near-shore inhabitants of seaweed roots and driftwood, a niche that seemed so, er, nichey as to not warrant much attention. They were also dismissed as refugees from the land that had somehow eked out a life in seawater.
In 1944, however, renowned palaeontologist proved otherwise: yes, some land and freshwater fungi can also grow and breed in seawater, but some are obligate marine species, meaning they can only live and reproduce in this environment. Still, such discoveries barely moved the mycological dial.
There was another minor surge in interest in 1979, when Jan and Erika Kohlmeyer at the University of North Carolina at Chapel Hill published a book called . They, along with Brigitte Volkmann-Kohlmeyer, had spent years scouring the world’s beaches for bits of driftwood and gently teasing fungi out of them. In total, they collected 25,000 specimens, including 149 new species of obligate marine fungi, which are now deposited in the . “It’s beautiful, exquisite work,” says Cunliffe. Nonetheless, the study of marine fungi remained a backwater.
Fungi in the sea
Then along came molecular analysis, and a new porthole on discovery was opened. The Kohlmeyers had relied on the old-school method of collecting fungi in the field and culturing them in Petri dishes. This works up to a point, but many groups of fungi can’t be cultured, especially those that form mutualistic or parasitic relationships with other organisms. Molecular techniques are far more sweeping. For example, one key approach, known as environmental DNA (eDNA) analysis, allows you to identify all the species in a particular place from their distinct genetic sequences. It often relies on DNA that is secreted into the surroundings, such as water, so you don’t actually need to collect the organisms themselves. In the past 20 years, almost 2000 more species of marine fungi have been discovered using various DNA methods. “The application of molecular tools has been a revolution,” says Cunliffe.
Such DNA sampling has revealed that all of the , everywhere and in large quantities. “They exist in every marine habitat where researchers have bothered to look: from hydrothermal vents, subsurface deep-sea sediments and Arctic ice, to surface waters, salt marshes and sandy beaches at low tide,” wrote Amy Gladfelter at the University of North Carolina at Chapel Hill in 2019. Subsequent research has also shown that they are , from the top to the bottom of the water column. Almost all marine fungi we know of are microscopic. The most concluded that of the 6 billion tonnes of carbon in marine biomass, about 5 per cent is fungal. Most of the rest is in multicellular animals, which means that fungi are a major component of oceanic plankton. So what is this biological dark matter up to?
One possibility is that most of it is mere detritus. After all, terrestrial fungi produce vast numbers of spores, which disperse into the air and fresh water, and some will inevitably end up in the ocean. The species discovered by the Kohlmeyers and their predecessors were obligate marine fungi. But that isn’t necessarily the case for those discovered by molecular techniques. “Most of the marine fungi – certainly out in open water – are closely related to species that we find in non-marine ecosystems,” says Cunliffe. And it wasn’t clear whether they are alive because eDNA analysis doesn’t distinguish between dead or dormant organisms and living ones.
So, to resolve the issue, Cunliffe and his team used a more refined technique called mRNA analysis. RNA is the intermediate between DNA and proteins, so if fungal RNA is found, it must come from organisms that are metabolically active, and hence alive. The research provides . That would make these incomers at least “facultative” marine fungi, meaning they can survive – and possibly reproduce – in seawater, but only as a plan B.
Even so, this shows incredible resilience and adaptability because living in seawater presents two severe challenges: salinity and pressure. Fungi appear to be up to the task in part because they have extremely thick and durable cell walls, which protect them from harsh external environments. They are also able to adjust these cell walls in response to almost any challenge. Cunliffe and his team have experimented on marine fungi and found that they can deal with radical alterations in pressure and salinity, responding within minutes by changing the shape and composition of their cell walls. Fungi are also metabolically flexible and able to extract energy from almost anything. Again, they quickly change, shifting metabolic gear to deal with unexpectedly finding themselves in seawater. “It appears that fungi that live in surface environments on land can easily adapt to marine environments,” says Frank Kempken at the University of Kiel, Germany.
The transition from terrestrial to marine living, called “crossing the salt barrier”, is very challenging, biologically speaking. Of all complex organisms, fungi are the most adept at making this leap. A recent analysis found that living things have over the course of evolution. But fungi are the champions, making at least 39 and as many as 47 crossings in their evolutionary history. Indeed, they originally evolved and diversified in the ocean, starting around a billion years ago, , and colonised the land and fresh water around 600 million years ago. There they diversified again, giving rise to around 3 million species. But the switch back to ocean dwelling is only ever a gust of wind away.
How do aquatic fungi survive?
Exactly what today’s marine fungi are living on is a bit of a mystery. Fungi secrete enzymes to digest macromolecules outside their cells and then transport the smaller products inside. That makes sense if you live in soil or leaf litter, but it is more difficult to pull off in water. This was one reason why their presence in the open ocean had long seemed implausible, according to Kempken. But recent research suggests that they have overcome this problem , small particles of organic matter sinking slowly from the surface to the depths. Another discovery, yet to be published, is that many species of seaweed rely on symbiotic fungi, forming a kind of “reverse lichen” where the alga is dominant but the fungus supplies vital services. It would appear that many, or most, marine fungi are symbionts or parasites.
Whether they are all reproducing, however, isn’t clear. But even if they aren’t, itinerant fungi seem to play an important role in marine ecosystems. On land, their feeding habits make them the main recyclers of organic matter, extracting compounds from dead plants and animals and injecting them back into the system. Ditto in the sea… maybe. “We’ve provided evidence that they’re potentially involved in globally important processes, like the biological carbon pump, where organic carbon from surface waters moves to the deep ocean,” says Cunliffe. In other words, they help to reduce the amount of carbon in the atmosphere and so limit global warming.
There is probably more. According to Eva Breyer at the University of Vienna, Austria, marine fungi seem to have a , including those involving carbohydrates, amino acids and lipids. The significance of this in terms of global nutrient cycling is still an open question. It is one of the many mysteries that still surround marine fungi.
Marine fungi as medicine
There is another big question occupying mycologists: can they be useful to us? Terrestrial fungi have historically been a prolific source of antimicrobial compounds – think Alexander Fleming and penicillin – and marine fungi . “They secrete all these different types of metabolites to try and knock down competitors,” says Cunliffe. “We’ve done some preliminary work screening them against bacteria that might cause infection to people, and some of them seem to be pretty good.”
Marine fungi could also be an answer to another scourge of our modern world: pollution. Fungal blooms occur where there has been an oil spill and, by breaking down the hydrocarbons, they accelerate the clean-up. Fungi have also been found living in the vast ocean collection of plastic refuse known as the Great Pacific Garbage Patch and, given their metabolic flexibility, it is thought that they can digest this material. If so, marine fungi could prove an invaluable ally when it comes to getting rid of floating waste.
The more we learn about marine fungi, the more fantastical they seem. But there is one more discovery that would really be magic: marine mushrooms. Sadly, none has been found to date, and their absence makes sense if you think about what a mushroom is, says Cunliffe. “It’s the spore dispersal mechanism. Spores disperse in air, so there’s no reason why there would be marine mushrooms.”
But never say never. In 2010, mycologists unexpectedly , in a stream in Oregon. The claim remains controversial. But it isn’t impossible that something similar exists in the oceans too. “The marine environment is so poorly sampled,” says Cunliffe. “It’s fractions of a percentage that we’ve covered.” The idea of ocean mushrooms may seem fanciful, but perhaps we just haven’t looked deep enough.
Graham Lawton is a features writer at 91av
