Environment vs geographic location in influencing the biogeography of fungi on a global scale

Marine fungi are both a diverse and important part of ocean ecosystems, however compared to many other microbes, fungi are unstudied, due to this understudy much less is known about fungi biogeographic patterns and the main factors which influence them both geographical distance and habitat type have been shown to be important in influencing distribution, a recent paper by Tisthammer et al (2016), was the first to look at these factors on a global scale to assess their relative importance in determining community composition the paper also looked at if there was a different in fungi assemblages between sediment and water column samples.

Methods

·         The authors used data from the International Census of Marine Microbes Dataset, including information about environmental parameters (e.g. temperature, salinity, and concentrations of phosphate, nitrate, dissolved oxygen and silicate) for the sample sites. For some sites there was incomplete data for environmental conditions, this was overcome by using information from the world ocean atlas to estimate these environmental parameters. To ensure accurate analysis, when reliable data was not available for sample these samples were excluded.

·         Samples were separated into pelagic or benthic in order to compare the community composition between the habitats.

·         The International Census of Marine Microbes Dataset used primers to amplify the regions of the ribosomal small subunit V9 which the authors used in the study.

·         Operational taxonomic units were determined at 97% similarity.

·         In total 42 pelagic and 14 benthic samples which contained 10,793 fungal sequences were used.

Findings 

The authors found a clear difference in the marine fungal community composition between the pelagic and benthic samples. They found that very few OTU were shared between these two realms of the 739 OUT present in the dataset only 114 were found to occur in both the water column and sediment site.

As hypothesised by the authors the environmental variables between sites were a significant predictor in determining the marine fungal community structure and played a much stronger role then geographic location (73% compared to 18%). Furthermore by looking at the effect of individual environmental variables it was found that depth was the best explanatory variable, explaining 24.3% of the total explainable variance dissolved oxygen was ranked second (23.5%) and nitrate third (22.9%).

The study used one of the most extensive datasets to date to examine the global distribution of marine fungal communities, although previous studies showed that both geographic distance and habitat are important predictors of fungal community composition this study was the first to address which of the two contributed more to community differences, previous studies also had limited geographic scales (<1000 km). However even studies that did examine larger geographic ranges tended to focus on specific habitats such as mangroves, or hydrothermal vents in contrast with the present paper which investigated a range of habitats to allow differences between environmental parameters to be more widely accessed. There were still limits to the study for example the microbial techniques used were insufficient for capturing a number of rare OUT, it is noteworthy to mention that even with this limit the conclusions and community similarity and divergence are unlikely to be significantly impacted, however I think that future studies could focus on addressing this bias in sampling by using techniques that are able to detect these rare OUT using primers with a higher specificity for fungal taxa to provide a more complete picture. The authors used secondary data and therefore had no control of where the data was collected which lead to sampling locations being unevenly distributed making it harder to draw conclusions. The authors also run into trouble with the sequence region they used due to the conserved nature of the SUU v9 region there were limits to the resolution to lower taxonomic ranks. The study also only included free living fungi I think it would be interesting if fungi associated with macroscopic host were considered in future studies as this would allow us to see the implications to the ecological dynamic this association facilitates. Overall even with the limitations of the study this study has helped in our understanding of fungal distribution and the factors that influence it on a global scale and contributed a new understanding of factors affecting fungal distribution.

Tisthammer, K., Cobian, G. and Amend, A. (2016). Global biogeography of marine fungi is shaped by the environment. Fungal Ecology, 19, pp.39-46.