Marine fungi

Marine fungi are species of fungi that live in marine or estuarine environments. They are not a taxonomic group but are grouped together because of their habitat. Obligate marine fungi grow exclusively in the marine habitat and are capable of being 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. According to the Gulf Coast Research Laboratory, University of Southern Mississippi, there are 444 species of marine fungi, including 7 genera and ten species of basidiomycetes, and 177 genera and 360 species of ascomycetes. The remainder of the marine fungi are mitosporic or asexual fungi.^[1] Many species of marine fungi are known only from spores and there are likely to be a large number of species 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. They can be found in habitats ranging from ocean depths and coastal waters to mangrove swamps and estuaries with low salinity levels.^[3] The fungi can be saprobic or parasitic on animals, saprobic or parasitic on algae, saprobic on plants or saprobic on dead wood.^[1]

Young salmon with fungal disease

Types of marine fungi

Factors which 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]

Primary marine fungi, species which have originated in the sea, are all obligate parasites. These include fungi that cause diseases in marine organisms.^[4]

Some secondary marine fungi, species which have ventured into the sea from terrestrial habitats, are free living and 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][5]

In 2011, Richards and others studied the phylogeny of marine fungi by the analysis of small subunit ribosomal DNA. Thirty six new marine lineages were found, the majority of which were chytrids but also some filamentous, hyphus-forming species and some higher, multicellular fungi. The majority of the species found that were already known to science were ascomycetous and basidiomycetous yeasts. ^[6]

The secondary metabolites produced by marine fungi have high potential for use in biotechnological, medical and industrial applications.^[7]

Mangroves

Mangroves in Puerto Rico

The greatest number of known species of marine fungi are from mangrove swamps ^[1] which support a range of different species of fungi. In a 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 to enable spores of intertidal ascomycotous fungi to colonise the plant. It was found that the snail ate 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 in the invertebrate kingdom outside the class Insecta.^[8]

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.^[9][10]

Timber

Driftwood on Jekyll Island, Georgia

Many marine fungi are very specific as to which floating or submerged timbers they colonise. A range of species of fungi attack beech while oak has a quite different range. When a fungal propagule lands on a potentially acceptable piece of timber, it will grow if no other fungi are present. If the log is already colonised by a different fungal species, growth will depend on whether that fungus produces anti-fungal chemicals or if the new arrival has resistance to such substances. Careful field observations may be able to provide pointers as to which fungi may have biotechnological potential and are worthy of further investigation regarding the secondary metabolites they produce and the antifungal properties of these. In a similar way, it was shown in a study that 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. Further investigation would be required to establish whether this fungus had biotechnological potential.^[4]

Detection of fungi in timber is not straightforward and may involve incubation at a suitable temperature in a suitable water medium for a period of six months to upward of eighteen months.^[4]

Algae

Rhyzophydium littoreum is a marine chytrid, a primitive fungus that infects green algae in estuaries. Its thallus obtains nutrients from the alga but the zoospores need to survive in the open water, a low nutrient environment, until a new host is encountered. A study was done to establish the range of nutrients that was required in the water for the survival and growth of the sporangia.^[4]

A species of Fusarium that has been isolated from a seaweed has been found 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 cyanobacterium. There are several lichens found in marine environments including Arthopyrenia halodytes, Pharcidia laminariicola, Pharcidia rhachiana and Turgidosculum ulvae.^[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 but it is thought that these were partly due to the captive conditions rather than the virulence of the disease. Transmission between animals in the open sea is limited making it unlikely that fungal disease epizootics will occur. Infectious agents in 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 being kept in cages in marine environments may be affected by a number of different fungal infections. Exophiala salmonis causes an infection with growth of hyphae in the kidneys causing a swelling of the abdomen. There is a cellular response by the fish which tries to isolate the fungus by walling it off. Another uncommon fungal infection is Icthyophonus which may be symptomless or cause spiralling behaviour and curvature of the spine. Spread is encouraged by overcrowding. Dermocystidium infections cause lesions on the gills and may cause high mortality in fry and young fish. Branchiomyces also affects the gills and other opportunistic fungi such as Saprolegnia may subsequently attack the damaged tissue.^[16]

Fungal infections of fish often occur in ornamental aquaria where spores are usually present in the tank. They usually affect the epidermis, fins or mouth of the fish and give a fluffy, white appearance. An infection may follow an injury which allows the spores to develop. Systemic infections can occur caused by fungi in the genus Icthyophonus. Incidence of infections is encouraged by poor water quality, overcrowding, poor hygiene and the accumulation of organic waste in the tank. Treatment can involve segregation in a hospital tank, whole tank treatment or the use of gentian violet. Prevention of further problems involves improving the tank environment.^[17]

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 prevents them from infection by the pathogenic fungus 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 Lagenidium callinectes.^[18]

See also

Marine life portal Fungi portal

References