The Time Traveller’s Guide to the Salt Marsh: Marine to terrestrial shifts in fungal communities

Temperate salt marshes are characterised by halophilic angiosperms diurnally subjected to tidal inundation. As habitats, they represent areas of high productivity and carbon sequestration. Previous studies have shown fungi to play an integral role in terrestrial biogeochemical cycling by positively (or negatively) interacting with carbon cycling in associated plants. It is relatively unknown whether fungi play an analogous role in salt marsh ecosystems and how such fungal communities are established. Therefore, Dini-Andreote and colleagues (2016) investigated large-scale fungal community succession in an ambitious salt marsh survey.

Sediment deposition has gradually extended the Dutch island of Schiermonnikoog eastwards, creating a temporally younger salt marsh habitat than that in the West. Such environments are ecologically designated as ‘chronosequences’ -  habitats where the only assumed variable across the study area is age. The ~8km cline at Schiermonnikoog represents a temporal gradient of over 105 year and was thus selected for investigation in this study. Sediment samples were collected along the temporal gradient and fungal ITS regions were sequenced using Illumina Mi-Seq to characterise the fungal community. Bioinformatic analysis identified 917 total OTU’s across 16 classes, mostly belonging to the Dikarya. Contrary to the authors’ initial hypotheses, the β-diversity of early-successional communities was comparable to older samples (although fungal abundance was up to 100x lower). The earlier communities were, however, considerably more dynamic and fluctuated at small spatiotemporal scales relative to older samples, suggesting stable assemblages had not yet been established.

Most fascinatingly, OTU’s from earlier, more tidally-inundated sites were mostly composed of unidentified aquatic taxa (20.7% of which belonged to the parasitic Rozellomycota) with few Dikarya while mature communities showed filamentous and edaphic (soil-associated) characteristics. Distance-based linear modelling suggested that the predicting factors of fungal community structure were organic matter (OM) concentration and soil structure. This lead the authors to conclude that salt marsh fungal communities are established from dynamic marine consortia, which evolve into stable terrestrial assemblages as soil structure and angiosperm enrichment becomes established.

As with any large metabarcoding study, artefacts were present. The authors humbly acknowledge that the ITS primers employed were biased against chytrid-like signatures, which may explain their under-representation in this study. As well as this, abundance was enumerated using ITS signature quantification, which may have been obscured by fungal taxa with multiple gene copies - perhaps a problem that could be overcome in further studies using FISH coupled to flow cytometry.

In regards to further research, the authors’ conclusion that OM and soil structure were predictors of community identity warrants further investigation into the physiochemical modelling of angiosperm colonisation and their fungal symbionts. Multivariate changes in habitat over time may very well be attributed to chemical and structural niche creation by pioneer angiosperms, which may themselves be the drivers of fungal change. I would also be keen to see this phenomenon investigated in reverse, should an appropriate chronosequence be identified. Anthropogenic sea level rise is predicted to encroach into tidal wetlands (Galbraith et al, 2002) and studying the response of salt marsh fungal communities to salinization may show this succession in reverse.

In conclusion, this study is the first large-scale and high-resolution survey of fungal community succession in salt marsh soils and provides an impressive and comprehensive basis for further study.

Reviewed Paper: Dini-Andreote, F., Pylro, V. S., Baldrian, P., van Elsas, J. D., & Salles, J. F. (2016). Ecological succession reveals potential signatures of marine–terrestrial transition in salt marsh fungal communities. The ISME journal. http://www.nature.com/ismej/journal/v10/n8/full/ismej2015254a.html

Wetland Inundation: Galbraith, H., Jones, R., Park, R., Clough, J., Herrod-Julius, S., Harrington, B., & Page, G. (2002). Global climate change and sea level rise: potential losses of intertidal habitat for shorebirds. Waterbirds, 25(2), 173-183.  http://www.bioone.org/doi/abs/10.1675/1524-4695(2002)025%5B0173:GCCASL%5D2.0.CO%3B2