Wait—There are Fungi Living in the Ocean?

What kinds of organisms do you think of when you imagine a beach? You probably think of dolphins, starfish, sea anemones, or maybe even sharks... but did fungi make your list? Marine (ocean) fungi are different from the mushrooms you normally see on land. One of the reasons scientists did not know about marine fungi for so long is that no one has ever seen a marine mushroom! Instead, the fungi in the ocean are microscopic and live “secret” lives.

So far, <1% of marine fungi have been identified, although scientists estimate that there are over 10,000 types of marine fungi living in Earth’s oceans [1]. Some of these fungi are called obligate marine fungi, because they have only been found living in the ocean. Many obligate marine fungi have adapted specifically to life in the ocean and some even form important relationships with other organisms, like seaweeds, sponges, corals, and seagrasses. Marine fungi can be found everywhere, from the waves breaking on the beach, down to the hydrothermal vents of the deep seafloor. However, when examining the ocean, scientists have also found fungi that commonly live on land. Scientists are unsure whether these land-based fungi are playing active roles in the ocean, or whether they are just using the ocean to travel to distant lands. Fungi that can survive both on land and in the ocean are called facultative marine fungi.

While most marine fungi are microscopic and no marine mushrooms have been observed to date, there is one place where scientists have found macroscopic marine fungi, which means organisms that can be seen with the naked eye [2]. These fungi exist in the form of lichens, which are a symbiosis between bacteria and fungi. In their symbiosis, these microscopic organisms work together to form the macroscopic lichen. You can see lichens living on rocks along the coastlines. Next time you go to the beach at low tide, look at the rocks and try to find some marine lichens (Figure 1)!

What Do Fungi Need to Survive in the Ocean?

Living in the ocean is not easy. There are several adaptations that marine fungi need to be able to survive in the ocean environment. Adaptations are traits that organisms gain to help them survive better in a certain environment. Adaptations needed by marine fungi include high salt tolerance (the ocean is salty!), protection from the sun’s dangerous ultraviolet light, a way to stay alive when food is limited, ways to withstand the high pressure of living in the deep sea, and the ability to spread and reproduce in the ocean ecosystem. The need for so many adaptations makes facultative marine fungi particularly interesting. To transition from life on land to life in the ocean, these fungi need to be good at a lot of things.

Are Marine Fungi Friends or Foes?

One way marine fungi survive in the ocean is by associating with other organisms, like the symbiosis that forms marine lichens. However, these relationships are complicated and are not always positive. Some marine fungi are parasites, meaning they live inside or on other organisms and, like tiny vampires, take nutrients away from that host organism. Parasitic marine fungi play an important role in the ocean by killing other marine microbes, releasing their nutrients into the seawater as food for other organisms [3].

In addition to parasitic fungi, other marine fungi can also cause diseases. For example, a marine fungal pathogen called Fusarium is killing sea turtles and infecting their eggs. These pathogenic fungi grow in sea turtle nests, covering the surface of the eggs and making chemicals that break down the eggshell. The infected eggs become yellowish-blue and die. The eggs generally become infected through contact with contaminated beach sand. However, scientists have recently seen that pathogenic fungi can be transported from land into the ocean by floating particles of plastic and by contaminated plant parts, such as wood and leaves, that are carried to sea turtle nests by the water and wind [4]. These fungal foes are reducing the numbers of marine turtles that hatch. By working to detect Fusarium in the environment, scientists are trying to understand how to prevent turtles from getting fungal infections.

However, lichens teach us that not all marine fungi are foes—some are friends! Many fungi that live on land have important relationships with plants, but for a long time scientists did not know if marine plants had any fungal friends. Scientists have recently found that some seagrasses form relationships with marine fungi [5, 6]. While the specifics of these friendships are still being researched, the symbiosis looks similar to that of land plants and their fungi, so scientists believe the relationships are likely to help seagrasses thrive.

Why Have I Never Heard of Marine Fungi?

If marine fungi are new to you, do not worry—many scientists have not heard of marine fungi either! Studying these fungi is difficult. First, many marine fungi are microscopic, making them hard to find and study. While scientists have had some success taking ocean samples to the laboratory to try to grow and study marine fungi (Figure 2), these fungi often need very specific food, which can be tricky to make in the lab. How can scientists study something they cannot see or grow?

To get around this challenge, scientists are now using DNA sequencing to look for marine fungi. This technique can be performed on ocean samples without the need to see or grow the fungus. DNA sequencing tells scientists which specific fungi are found in a certain place. Scientists can then compare this information to databases that were made from studying the fungi that live on land. However, this technique is still challenging because many marine fungi are newly discovered and do not match these databases. By collaborating and sharing methods and data, scientists can work together to describe marine fungi and bridge gaps in their knowledge of which marine fungi are present and what they might be doing in the ocean.

Another challenge is answering the question of whether facultative marine fungi should be considered true marine fungi. Scientists often find facultative marine fungi in samples collected far away from land [7]. While DNA sequencing has allowed scientists to survey marine fungal diversity in different locations, it cannot tell them what the fungi are doing in these locations, making it hard to know whether facultative marine fungi are truly living in the ocean. Many obligate marine fungi have recently gained the adaptations that allow them to survive in the ocean and they have close relatives that live on land, making it difficult to identify fungi that are actively growing in the ocean from those that are temporary tourists using the ocean to travel to distant lands.

Are Marine Fungi Important For Our Planet?

Even though scientists are still learning about marine fungi, they already know these organisms have crucial roles in the ocean. Some marine fungi can break down dead plants and animals by feeding on them. This process recycles nutrients, like carbon, back into the ocean, making the nutrients available for other marine life. Some fungi can even break down harmful substances like plastic or oil that find their way into the ocean. For example, ocean sites contaminated by oil spills can be cleared up by marine fungi that are effective at breaking down these complex compounds [3, 7]. Although a marine fungi-based solution to clean up plastic pollution has yet to be tried, plastic-eating fungi already exist in the oceans [3]. These fungi will be a great help in solving environmental pollution. Marine fungi also produce fascinating natural products called secondary metabolites, which might help fight human diseases. Scientists are researching whether secondary metabolites might be useful as medicines.

In summary, although scientists are still figuring out all the roles marine fungi play, we know that these ocean organisms are just as important as the fungi on land, and that studying them may lead to life- and planet-saving solutions. So, the next time you think about fungi, remember they are not just on land—they live in the ocean, too (Figure 3)!

Glossary

Obligate Marine Fungi: ↑ Fungi that can only live in a marine environment.

Facultative Marine Fungi: ↑ Fungi that live in freshwater or on land, but that can also grow and survive in marine environments.

Symbiosis: ↑ A close, long-term relationship between two or more organisms.

Adaptation: ↑ A natural process in which an animal or plant becomes better suited to its environment by gaining new traits that make it more successful.

Parasite: ↑ Any organism that lives inside of or attached to another organism which it uses for food, like a vampire, harming the organism it infects.

Pathogen: ↑ Any organism that can cause a disease or harm the organism it infects.

DNA Sequencing: ↑ The laboratory process of reading the genetic code in an organism’s DNA.

Secondary Metabolites: ↑ Special chemicals made by fungi and bacteria to protect themselves, better survive in a new environment, or compete with others.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

JA was supported by the NSF REU the National Summer Undergraduate Research Project, Award No. 2149582. JS is a CIFAR Fellow in the program Fungal Kingdom: Threats and Opportunities and was partially supported by NSF awards EF-2125066 and IOS-2134912. CE was supported by the National Science Foundation (NSF) under a NSF Ocean Sciences Postdoctoral Fellowship (Award No. 2205744).