Aquatic Fungi: Overlooked No More

 Although fungi are recognised for their ecological importance within terrestrial environments, comparatively little is understood about their functions within aquatic environments. Aquatic fungi comprise only a tiny fraction of identified marine clones, which is probably why they have been somewhat overlooked by microbiologists. However, researchers are beginning to uncover that fungi contribute more to aquatic ecology than once thought, especially with regard to the oceans. Recent studies utilising modern high-throughput sequencing techniques have demonstrated larger proportions, as well as greater taxonomic diversity, of marine fungi than previously described. Consequently, the field is of growing interest to researchers.          

 The fungal group Chytridiomycota is acknowledged as ecologically significant in freshwater systems. They are well known as facilitators of the transfer of organic carbon from phytoplankton into zooplankton, through saprophytic and parasitic activity. Yet, these important decomposers have been seldom reported within marine studies. The findings of one recent study however, found Chytridiomycota-like sequences to dominate samples from a diverse range of aquatic environments, including marine!  

 Comeau et al. performed a broad analysis of fungal diversity within marine and freshwater systems across both Arctic and temperate climates. Samples of sea-ice were also acquired from locations within the Arctic. A total of 319 samples were collected from 103 sites, between 2003 and 2011, generating an extensive database of more than 3.6 million sequences. Fungal diversity was then assessed by examining the V4 region of 18S rRNA (DNA and RNA).

 Data analysis revealed all samples to contain fungal sequences. A total of 44 fungal genera were identified, portraying a great richness in fungal diversity. Whilst there were no significant correlations between the different fungal taxa and factors such as season, depth, or salinity, fungal abundance was found to be slightly higher within warmer regions, indicating a preference for higher temperatures. Appraisal of fungal diversity was conducted by clustering fungal sequences into Operational Taxonomic Units (OTUs) at 98% shared identity. Clustering of OTUs revealed minimal overlap between environmental niches (freshwater, marine and sea-ice) demonstrating a strong degree of environmental specificity.    
   

 The most notable outcome of this study was the aforementioned dominance of Chytridiomycota-like sequences within almost all samples, including marine where they have been previously reported in much lower abundance. Within this group, the dominating sequences were closely matched to an uncultured clone, CFL161DB09, which originates from an Arctic seawater sample. Whilst these sequences are grouped together with the Chytridiomycota within this study, they actually represent novel lineages of chytrid. Moreover, Comeau et al. were able to show that there were significant positive correlations between the yield of Chytridiomycota-like sequences within samples, and both chlorophyll a concentration and yield of diatom sequences. Whilst the ecological functions performed by these novel chytrid lineages are still undetermined, these findings suggest that they probably act as algal parasites, akin to previously studied freshwater chytrids.

 Overall, this study works to highlight the requirement for more extensive investigation of aquatic fungi, especially within marine environments. Long overlooked, it is emerging that this fraction of the microbial eukarya contribute far more to the ecology of aquatic environments than traditionally believed, with possible key roles as decomposers and nutrient cyclers. In order to gain understanding of these potential roles, physiological and ecological studies of novel chytrids should be pursued.                                 

Reviewed Paper:

Comeau, A. M., Vincent, W. F., Bernier, L. & Lovejoy, C. (2016). Novel chytrid lineages dominate fungal sequences in diverse marine and freshwater habitats. Scientific Reports, 6: 30120