|Abstract: ||Abstract: The previous special issue of Botanica Marina on Marine Mycology was published in 2010 (Jones and Pang 2010), when the number of known marine fungi was 530 species (Jones et al. 2009). Jones et al. (2015) updated this figure to 1112 species (in 472 genera), which included newly described taxa, with the inclusion of the Chytridiomycota, marine yeasts and a broader definition of what constitutes a marine fungus (Pang et al. 2016). The documentation of new marine fungi and new records brings the current total to 1206 (www.marinefungi.org, 2017). Jones (2011) regarded these figures as an underestimate and suggested there may be as many as 10,000 marine fungi, indicating where these missing fungi might be found. However, controversy surrounds the diversity of marine fungi with Richards et al. (2012) concluding that fungi are low in both diversity and abundance in marine environments. They questioned “Are we overlooking a large diversity of fungi?” Furthermore, Tisthammer et al. (2016) stated that very little is known about the global distribution of marine fungi and that marine fungi are understudied when compared to other microorganisms.
Marine fungi are heterotrophic organisms, relying on degrading organic substrates for growth and reproduction, including plant-based substrata, macroalgae, animal remains (Vrijmoed 2000). These substrates are abundant in coastal environments and, therefore, research on marine fungi has been mainly focussed on fungi fruiting on substrata in these environments, where they play a significant role in nutrient cycling (Pang et al. 2016). Recent culture-independent analyses of fungal sequencing obtained from seawater and sediment samples have revealed a high abundance/richness of sequences related to the Ascomycota and the Basidiomycota and possible new lineages of fungi early in the evolution of Fungi using different regions of the ribosomal RNA genes (Richards et al. 2015, Picard 2016, Hasset et al. 2017). Interpretation of the results from molecular studies has to be cautious. Seawater is a dispersal medium, not a growth substrate, while sediment represents a niche for accumulation of fungal propagules. Fungi in these substrates may not represent the active marine populations as many fungal propagules may be of freshwater/terrestrial origin. Using RNA-tag sequencing and fluorescent staining of samples, Richards et al. (2015) found low fungal abundance in upper seawater column samples. This is not surprising as low organic matter in seawater does not support a stable population of fungi; the exceptions are those that occur on phytoplankton. Also, recent molecular studies have repeatedly discovered the dominance of the Ascomycota and the Basidiomycota in seawater/sediments, without acknowledging the individual species, thus making comparisons between different studies difficult (Jones et al. 2015, Richards et al. 2015, Picard 2016, Hasset et al. 2017).
There is therefore a gulf in appreciation about what is known about marine fungi using three approaches in their documentation: (a) substrate based studies, (b) isolation from seawater and (c) DNA sampling of water, sediments, often at great depths in the ocean. Although marine fungi are worldwide in their distribution, there are great differences in their diversity in tropical, temperate and cold water habitats. Culture-independent studies have been used only recently to study the diversity of marine fungi and much greater sampling is required to obtain a full picture of their occurrence and distribution. Another area that requires greater study is the much-neglected documentation of parasitic fungi and fungal-like organisms parasitizing plankton and marine invertebrates. This may be attributed to the lack of fresh material of epizootics and the techniques available for their study. This special issue has gathered up active marine mycologists to give updates on marine mycology in the areas related to ecology, phylogeny, pathology and applied aspects. They have also highlighted the gaps of knowledge in marine mycology, suggesting further work in ecologically and industrially important but understudied groups of fungi.
The recent research effort in Taiwan, the Middle East, Thailand and Arctic areas has increased our understanding of the diversity of marine fungi in these regions. In this special issue, Suetrong et al. report the diversity of marine fungi in eastern and southern Thai mangroves, while Räma et al. have prepared a list of marine fungi in the Arctic region. Supaphon et al. have investigated the phylogenetic diversity of fungi associated with a few seagrass species collected in Thailand. Exploration of wider geographical locations always results in new marine fungi. Abdel-Wahab et al. report five new marine fungi collected from various countries including Japan, Malaysia, Saudi Arabia, Thailand and Turkey. Lulworthia grandispora is transferred to a new genus Sammeyersia based on morphology and sequence analysis in the same study. Jones et al. also make two new combinations: Aniptodera lignatilis to Aniptosporopsis gen. nov. and A. longispora to Paraaniptodera gen. nov.; a taxonomic review of the Halosphaeriaceae is provided in the same paper.
Marine fungi are known to produce bioactive secondary metabolites and fatty acids. Zhong et al. optimize fermentation conditions including nitrogen and carbon sources and fermentation temperature to investigate microbial oil production by a Fusarium sp. isolated from seawater. Unagul et al. evaluate the polyunsaturated fatty acid profile of thraustochytrids isolated from fallen mangrove leaves in Thailand. They have also tested several methods for cryopreservation of the thraustochytrids. From an environmental point of view, marine fungi produce degradative enzymes to remove chemical pollutants, and Velmurugan et al. have evaluated the ability of a Paecilomyces sp. to degrade several polycyclic aromatic hydrocarbons and their biochemical response using a proteomic approach.
Recent studies have suggested the prevalence of the Chytridiomycota in seawater; this group alone accounted for more than 60% of the rDNA sequences sampled in six near-shore sites around Europe (Massana et al. 2015, Richards et al. 2015) and, in Arctic and sub-Arctic coastal habitats, they have been described as the most abundant fungal group (Hassett and Gradinger 2016, Hassett et al. 2017). Many of these species cause diseases of marine organisms in natural environment and in mariculture. Scholz et al. report on the effects of environmental parameters on infection prevalence in diatoms while Gleason et al. and Collier et al. have provided reviews of the diseases of molluscs caused by Perkinsozoa and Labyrinthulomycota.