Icy Cold Marine Fungi – Domination of Cryptomycota and Chytridiomycota

With the help of high throughput sequencing, DNA analysis have opened doors to a vast diversity of fungi, but a number of aquatic fungi are still missing from the fungal lineage, namely Cryptomycota and Chytridiomycota (the early diverging lineages) (Rojas-Jimenez et al., 2017). Lepelletier et al (2014) mentions how only a few studies have reported Chydrids in the marine environment and a limited number of marine Chytrids have been properly identified. This has led to the understanding that the diversity of fungi is relatively higher in terrestrial systems compared to marine and freshwater systems.

Members of the basal phyla, Chytridiomycota and Cryptomycota, have mechanisms such as their mobile, parasitize and saprophytic capabilities that allow them to survive in aquatic environments. These fungi, along with Dikarya, contribute to the overall functioning of the ecosystem as they take part in the carbon and nutrient cycle (Rojas-Jimenez et al., 2017).

The sampling sites were five ice-covered lake basins in the McMurdo Dry Valleys, Antarctica - Lake Miers, Lake Fryxell, Lake Hoare, and the West and East lobes of Lake Bonney. Several environmental parameters were measured - temperature and conductivity, many ions, dissolved oxygen, DOC, bacterial production, PPP and chlorophyll concentrations. DNA and RNA extraction, and cDNA synthesis were undertaken following protocols/kits and V7 and V8 regions of the 18S rRNA gene were amplified using specific primers. The samples were sequenced on an Illumina MiSeq sequencer and were deposited into the NCBI Sequence Read Archive. The 18S rRNA gene sequences were quantified and were assigned to OTUs and each taxa were also assigned to OTUs. Finally, the BLAST tool was used to obtain an accurate taxonomic assignation of the eukaryotes. Statistical analysis was performed in R.

From 4.99 m sequences that were analysed, 787,937 were classified as fungi and the DNA and RNA derived suggests that the fungal communities were active. Cryptomycota and Chytridiomycota were the most abundant fungal taxa in the sites. With reference to fungal reads and OTUs; Cryptomycota represented 72% and 44%; and Chytridiomycota represented 26% and 40% respectively. There were significant differences in fungal richness and community composition among the five lakes, with Lake Miers exhibiting the highest in all the depth layers, due to the freshwater habitat, warmer temperatures, and location – being situated in a valley with a higher altitude. The interaction between salinity and depth is what shapes the communities and diversity in the area. Rojas-Jimenez et al (2017) observed that with an increase in depth there was a significant difference between the richness of both habitats. Deep waters that form the monimolimnia (non-mixing layer of the lake) had a higher proportion of fungi than the mixolimnia (mixing layer of the lake).

Freshwater contained a significantly higher proportion of fungi and higher species richness than brackish waters (but an unequal abundance distribution) and there was a clear separation between the populations, eg: Blastocladiomycota was only found in freshwaters. As there were differences in community composition between lakes, depths and habitats, it is safe to say that some fungal taxa have a preference for certain niches.

Using the network analysis technique strong positive relationships between Chytrids and Cryptophyta (the most abundant primary producer) were visualized. Chytrids also had associations with Prasinophytae, Basidiomycota, Rhizaria, Zygomycota, Chloropastida, Chlorophyta and Stramenopile.

Recent findings of Chytridiomycota dominating freshwater and marine communities led Rojas-Jimenez et al (2017) to believe that basal fungal communities dominate undisturbed aquatic systems, whereas higher fungal communities (Dikarya) dominate terrestrial systems as well as aquatic systems affected by terrestrial and anthropogenic input.

This study has many potential areas for further research such as understanding how parasitic fungi may play a crucial role in maintaining phytoplankton biomass (considering the lack of grazers in this habitat) and the role fungi plays in energy, nutrient and carbon transferring by exhibiting these relationships. Also, the sample sites used have microbes living in extreme conditions that have not had anthropogenic influence, which makes it a good model to understand how fungi are able to cope with lower temperatures, osmoregulation and high ion levels.

It is interesting to note that Rojas-Jimenez et al (2017) did not observe any associations between Chytrids and Antarctic diatoms or dinoflagellates, since Chytrids have known to infect freshwater diatoms (Bruning., 1991), marine diatoms (Scholz et al., 2014) and dinoflagellates (Lepelletier et al., 2014), and also Arctic diatoms (Hassett and Gradinger., 2016). As we are entering a world where more and more molecular tools are available, it would be beneficial to implement different tools to further understand these fungal groups, with respect to their biology and ecology. As this is the first study emphasizing the dominance of Cryptomycota, along with Chytridiomycota in an aquatic ecosystem, it does a good job highlighting the dynamics of the early diverging fungi.

Reference:

Rojas-Jimenez, K., Wurzbacher, C., Bourne, E., Chiuchiolo, A., Priscu, J. and Grossart, H. (2017). Early diverging lineages within Cryptomycota and Chytridiomycota dominate the fungal communities in ice-covered lakes of the McMurdo Dry Valleys, Antarctica. Scientific Reports, [online] 7(1). Available at: https://www.nature.com/articles/s41598-017-15598-w#article-comments [Accessed 6 Jan. 2018].

Additional references:

Lepelletier, F., Karpov, S., Alacid, E., Le Panse, S., Bigeard, E., Garcés, E., Jeanthon, C. and Guillou, L. (2014). Dinomyces arenysensis gen. et sp. nov. (Rhizophydiales, Dinomycetaceae fam. nov.), a Chytrid Infecting Marine Dinoflagellates. Protist, [online] 165(2), pp.230-244. Available at: http://www.sciencedirect.com/science/article/pii/S1434461014000170.

Bruning, K. (1991). Infection of the diatom Asterionella by a chytrid. II. Effects of light on survival and epidemic development of the parasite. Journal of Plankton Research, 13(1), pp.119-129.

Scholz, B., Küpper, F., Vyverman, W. and Karsten, U. (2014). Eukaryotic pathogens (Chytridiomycota and Oomycota) infecting marine microphytobenthic diatoms - a methodological comparison. Journal of Phycology, [online] 50(6), pp.1009-1019. Available at: http://onlinelibrary.wiley.com/doi/10.1111/jpy.12230/full.

Hassett, B. and Gradinger, R. (2016). Chytrids dominate arctic marine fungal communities. Environmental Microbiology, [online] 18(6), pp.2001-2009. Available at: http://onlinelibrary.wiley.com/store/10.1111/1462-2920.13216/asset/emi13216.pdf?v=1&t=jc4qdznz&s=e300e3f5d2e6413a73bf78d3832aba5ff1ae33ff.