Marine fungi

Marine fungi are species of fungi that live in marine or estuarine environments. They are not a taxonomic group, but share a common habitat. Obligate marine fungi grow exclusively in the marine habitat while wholly or sporadically submerged in sea water. Facultative marine fungi normally occupy terrestrial or freshwater habitats, but are capable of living or even sporulating in a marine habitat. Marine fungi account for about 5% of the total ocean biomass. About 2,149 species of marine fungi have been described, within eleven phyla and 856 genera, although only about 64 species have been fully genetically sequenced. Many species of marine fungi are known only from spores and it is likely a large number of species have yet to be discovered. It has been estimated that less than 1% of all marine fungal species have been described, due to difficulty in targeting marine fungal DNA and difficulties that arise in attempting to grow cultures of marine fungi. It is impracticable to culture many of these fungi, but their nature can be investigated by examining seawater samples and undertaking rDNA analysis of the fungal material found. Different marine habitats support very different fungal communities. Fungi can be found in niches ranging from ocean depths and coastal waters to mangrove swamps and estuaries with low salinity levels. Marine fungi can be saprobic or parasitic on animals, saprobic or parasitic on algae, saprobic on plants, or saprobic on dead wood. There has been some debate as to what exactly a marine fungus should be defined as. A definition used previously was "individuals with a long-term presence and metabolic activities in a marine habitat." A more commonly used definition now is from Ka-Lai et al. 2016: "any fungus that is recovered repeatedly from marine habitats because: 1) it is able to grow and/or sporulate (on substrata) in marine environments; 2) it forms symbiotic relationships with other marine organisms; or 3) it is shown to adapt and evolve at the genetic level or be metabolically active in marine environments."

Overview Terrestrial fungi play critical roles in nutrient cycling and food webs and can shape macroorganism communities as parasites and mutualists. Although estimates for the number of fungal species on the planet range from 2.2 to 3.8 million species, likely fewer than 10% of fungi have been identified so far, being around 150,000. To date, a relatively small percentage of described species are associated with marine environments, with ~2,200 species retrieved exclusively from the marine environment. Nevertheless, fungi have been found in nearly every marine habitat explored, from the surface of the ocean to kilometers deep in ocean sediments. Fungi are hypothesized to contribute to phytoplankton population cycles and the biological carbon pump and are active in the chemistry of marine sediments. Many fungi have been identified as commensals or pathogens of marine animals, like corals, sponges, and other marine organisms like plants, and algae. Despite their varied roles, remarkably little is known about the diversity of this major branch of eukaryotic life in marine ecosystems or their ecological functions. Fungi represent a large and diverse group of microorganisms in microbiological communities in the marine environment and have an important role in nutrient cycling. They are divided into two major groups; obligate marine fungi and facultative marine fungi. Obligate marine fungi are adapted to reproduce in the aquatic environment, while facultative marine fungi can grow in aquatic as well as terrestrial environments. Marine fungi are called marine-derived fungi when their facultative or obligate state is not certain. Marine fungal species occur as saprobes, parasites, or symbionts and colonize a wide range of substrates, such as sponges, corals, mangroves, seagrasses and algae. Factors that influence whether or not marine fungi are present in any particular location include the water temperature, its salinity, the water movement, the presence of suitable substrates for colonization, the presence of propagules in the water, interspecific competition, pollution and the oxygen content of the water. Some marine fungi which have ventured into the sea from terrestrial habitats include species that burrow into sand grains, living in the pores. Others live inside stony corals, and may become pathogenic if the coral is stressed by rising sea temperatures. In 2011 the phylogeny of marine fungi was elucidated by analysis of their small subunit ribosomal DNA sequences. Thirty six new marine lineages were found, the majority of which were chytrids but also some filamentous and multicellular fungi. The majority of the species found were ascomycetous and basidiomycetous yeasts. The secondary metabolites produced by marine fungi have high potential for use in biotechnological, medical and industrial applications. The ocean constitutes about 99% of the volume in the biosphere. Oceanic microbes comprise 70%–90% of the biomass of marine biota and are the main drivers of marine biogeochemical cycles. They are responsible for determining the amount of carbon fixed during photosynthesis, the subsequent release of carbon back to CO2 through respiration, and ultimately the sequestration of carbon in the deep ocean over millennial scales. Present estimates of marine microbial biomass primarily rely on bacteria, archaea, and protists. Pelagic (planktonic) fungal biomass has been less studied and only superficially characterized, despite their recently documented ubiquitous presence and active participation in the marine carbon and nitrogen cycles. As of 2025, only a few studies are available on pelagic fungal biomass, restricted to distinct coastal areas, and an accurate representation of the vast open-ocean environment is still lacking. These studies are based on different methods, each with their own limitations hampering robust intercomparison of results. Among the biomass of organisms/groups estimated in the ocean, fungi accounted for the highest uncertainty, ranging more than two orders of magnitude, indicating the need for a direct estimate of fungal biomass in the ocean. There is need for large-scale sampling across biogeographical provinces in the open ocean to ensure precise quantification of pelagic fungi in the microbial biomass.

… excerpt ends here. Continue reading the full article.