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. About 444 species of marine fungi have been described, including 7 genera and ten species of basidiomycetes, and 177 genera and 360 species of ascomycetes. The remainder of the marine fungi are chytrids and mitosporic or asexual fungi.^[1] Many species of marine fungi are known only from spores and it is likely a large number of species have yet to be discovered.^[2] It is impracticable to culture many of these fungi but their nature can be investigated by examining sea water samples and undertaking rDNA analysis of the fungal material found.^[2]

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.^[3] Marine fungi can be saprobic or parasitic on animals, saprobic or parasitic on algae, saprobic on plants or saprobic on dead wood.^[1]

Mature salmon with fungal disease

Types of marine fungi

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.^[3]

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.^[2][4]

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.^[5]

The secondary metabolites produced by marine fungi have high potential for use in biotechnological, medical and industrial applications.^[6]

Mangroves

Mangroves in Puerto Rico

The greatest number of known species of marine fungi are from mangrove swamps.^[1] In one study, blocks of mangrove timber and pieces of driftwood of Avicennia alba, Bruguiera cylindrica and Rhizophora apiculata were examined to identify the lignicolous (wood-decaying) fungi they hosted. Also tested were Nypa fruticans, a mangrove palm and Acanthus ilicifolius, a plant often associated with mangroves. Each material was found to have its own characteristic fungi, the greatest diversity being among those growing on the mangrove palm. It was surmised that this was because the salinity was lower in the estuaries and creeks where Nypa grew, and so it required a lesser degree of adaptation for the fungi to flourish there. Some of these species were closely related to fungi on terrestrial palms. Other studies have shown that driftwood hosts more species of fungus than do exposed test blocks of timber of a similar kind. The mangrove leaf litter also supported a large fungal community which was different from that on the timber and living material. However, few of these were multicellular, higher marine fungi.^[3]

Other plants

The sea snail Littoraria irrorata damages plants of Spartina in the sea marshes where it lives, which enables spores of intertidal ascomycetous fungi to colonise the plant. The snail eats the fungal growth in preference to the grass itself. This mutualism between the snail and the fungus is considered to be the first example of husbandry among invertebrate animals outside the class Insecta.^[7]

Eelgrass, Zostera marina, is sometimes affected by seagrass wasting disease. The primary cause of this seems to be pathogenic strains of the protist, Labyrinthula zosterae, but it is thought that fungal pathogens also contribute and may predispose the eelgrass to disease.^[8][9]

Timber

Driftwood on Jekyll Island, Georgia

Many marine fungi are very specific as to which species of floating and submerged timbers they colonise. A range of species of fungi colonise beech while oak supports a different community. When a fungal propagule lands on a suitable piece of timber, it will grow if no other fungi are present. If the log is already colonised by another fungal species, growth will depend on whether that fungus produces antifungal chemicals and whether the new arrival can resist them. The chemical properties of colonizing fungi also affect the animal communities that graze on them: in one study, when hyphae from five different species of marine fungi were fed to nematodes, one species supported less than half the number of nematodes per mg of hyphae than did the others.^[10]

Detection of fungi in timber may involve incubation at a suitable temperature in a suitable water medium for a period of six months to upward of eighteen months.^[10]

Algae

Rhyzophydium littoreum is a marine chytrid, a primitive fungus that infects green algae in estuaries. It obtains nutrients from the host alga and produces swimming zoospores that must survive in open water, a low nutrient environment, until a new host is encountered.^[10]

A species of Fusarium isolated from a seaweed has been shown to contain a chemical that seems to have cytogenic, anti-cancer effects.^[11] Another fungus, Ascochyta salicorniae, found growing on seaweed is being investigated for its action against malaria,^[12] a mosquito-borne infectious disease of humans and other animals.

Lichens

Lichen covered rocks on the Isles of Scilly

Lichens are mutualistic associations between a fungus, usually an ascomycete, and an alga or a cyanobacterium. Several lichens, including Arthopyrenia halodytes, Pharcidia laminariicola, Pharcidia rhachiana and Turgidosculum ulvae, are found in marine environments.^[1] Many more occur in the splash zone, where they occupy different vertical zones depending on how tolerant they are to submersion.^[13] Fossil marine lichens 600 million years old have been discovered in the late Neoproterozoic marine phosphate rocks in the sedimentary, fossil-rich Doushantuo Formation in China.^[14]

Vertebrates

Whales, porpoises and dolphins are susceptible to fungal diseases but these have been little researched in the field. Mortalities from fungal disease have been reported in captive killer whales; it is thought that stress due to captive conditions may have been predisposing. Transmission among animals in the open sea may naturally limit the spread of fungal diseases. Infectious fungi known from killer whales include Aspergillus fumigatus, Candida albicans and Saksenaea vasiformis. Fungal infections in other cetaceans include Coccidioides immitis, Cryptococcus neoformans, Loboa loboi, Rhizopus sp., Aspergillus flavus, Blastomyces dermatitidus, Cladophialophora bantiana, Histoplasma capsulatum, Mucor sp., Sporothrix schenckii and Trichophyton sp.^[15]

Salmonids farmed in cages in marine environments may be affected by a number of different fungal infections. Exophiala salmonis causes an infection in which growth of hyphae in the kidneys causes swelling of the abdomen. A cellular response by the fish aims to isolate the fungus by walling it off. Fish are also susceptible to fungus-like oomycetes including Branchiomyces which affects the gills of various fishes, andSaprolegnia which attacks damaged tissue.^[16]

Invertebrates

The American lobster (Homarus americanus), like many other marine crustaceans, incubates its eggs beneath its tail segments. Here they are exposed to water-borne micro-organisms including fungi during their long period of development. The lobster has a symbiotic relationship with a gram-negative bacterium that has anti-fungal properties. This bacterium grows over the eggs and protects them from infection by the pathogenic fungus-like oomycete Lagenidium callinectes. The metabolite produced by the bacterium is tyrosol, a 4-hydroxyphenethyl alcohol, an antibiotic substance also produced by some terrestrial fungi. Similarly, a shrimp found in estuaries, Palaemon macrodactylis, has a symbiotic bacterium that produces 2,3-indolenedione, a substance that is also toxic to the oomycete Lagenidium callinectes.^[17]

See also

References