Marine fungi

Morphological diversity of fungi collected from a marine sponge, Ircinia variabilis[6]

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.[5]

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.[3][7]

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.[8]

The secondary metabolites produced by marine fungi have high potential for use in biotechnological, medical and industrial applications.[9]

Mangroves

The greatest number of known species of marine fungi are from mangrove swamps.[2] In one study, blocks of mangrove wood 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 wood of a similar kind. The mangrove leaf litter also supported a large fungal community which was different from that on the wood and living material. However, few of these were multicellular, higher marine fungi.[5]

The sea snail Littoraria irrorata. The shell on this one is covered with the lichen Pyrenocollema halodytes.

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.[10]

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.[11][12]

Driftwood

Many marine fungi are very specific as to which species of floating and submerged wood 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 wood, it will grow if no other fungi are present. If the wood 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.[13]

Detection of fungi in wood may involve incubation at a suitable temperature in a suitable water medium for a period of six months to upward of eighteen months.[13]

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.[13]

Another fungus, Ascochyta salicorniae, found growing on seaweed is being investigated for its action against malaria,[14] a mosquito-borne infectious disease of humans and other animals.

Lichen covered rocks

Lichens are mutualistic associations between fungi, usually an ascomycete with a basidiomycete,[15] and an alga or a cyanobacterium. Several lichens, including Arthopyrenia halodytes, Pharcidia laminariicola, Pharcidia rhachiana and Turgidosculum ulvae, are found in marine environments.[2] Many more occur in the splash zone, where they occupy different vertical zones depending on how tolerant they are to submersion.[16] 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.[17]

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.[18]

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, and Saprolegnia which attacks damaged tissue.[19]

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.[20]

Ascomycota, Basidiomycota, and Chytridiomycota have been observed in sediments ranging in depth from 0 to 1740 meters beneath the ocean floor. One study analyzed subsurface samples of marine sediment between these depths and isolated all observable fungi. Isolates showed that most subsurface fungal diversity was found between depths of 0 to 25 meters below the sea floor with Fusarium oxysporum and Rhodotorula mucilaginosa being the most prominent. Overall, the ascomycota are the dominant subsurface phylum.[21] Almost all fungal species recovered have also been observed in terrestrial sediments with spore-sourcing indicating terrestrial origin.[21][22]

Contrary to previous beliefs, deep subsurface marine fungi actively grow and germinate, with some studies showing increased growth rates under high hydrostatic pressures. Though the methods by which marine fungi are able to survive the extreme conditions of the seafloor and below is largely unknown, Saccharomyces cerevisiae shines some light onto adaptations that make it possible. This fungus strengthens its outer membrane in order to endure higher hydrostatic pressures.[21]

Several sediment-dwelling marine fungi are involved in biogeochemical processes. Fusarium oxysporum and Fusarium solani are denitrifiers both in marine and terrestrial environments.[21][23] Some are co-denitrifying, fixing nitrogen into nitrous oxide and dinitrogen.[22]  Still others process organic matter including carbohydrate, proteins, and lipids. Ocean crust fungi, like those found around hydrothermal vents, decompose organic matter, and play various roles in manganese and arsenic cycling.[24]

Sediment-bound marine fungi played a major role in breaking down oil spilled from the Deepwater Horizons disaster in 2010. Aspergillus, Penicillium, and Fusarium species, among others, can degrade high-molecular-weight hydrocarbons as well as assist hydrocarbon-degrading bacteria.[24]

Marine fungi have been observed as far north as the Arctic Ocean. Chytridiomycota, the dominant parasitic fungal organism in Arctic waters, take advantage of phytoplankton blooms in brine channels caused by warming temperatures and increased light penetration through the ice. These fungi parasitize diatoms, thereby controlling algal blooms and recycling carbon back into the microbial food web. Arctic blooms also provide conducive environments for other parasitic fungi. Light levels and seasonal factors, such as temperature and salinity, also control chytrid activity independently of phytoplankton populations. During periods of low temperatures and phytoplankton levels, Aureobasidium and Cladosporium populations overtake those of chytrids within the brine channels.[25]

Marine fungi produce antiviral and antibacterial compounds as metabolites with upwards of 1000 having realized and potential uses as anticancer, anti-diabetic, and anti-inflammatory drugs.[26][27]

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  Chytrid parasites of marine diatoms[30]
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  Salmon with fungal disease