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Dispersal limitation and colonization of new land by symbiotic microorganisms

Periodic Reporting for period 1 - DISPMIC (Dispersal limitation and colonization of new land by symbiotic microorganisms)

Reporting period: 2016-09-01 to 2018-08-31

The study of how biological communities assemble and maintain themselves is one of the key questions in ecology. In view of the global biodiversity crisis and the species movements that are being recorded as a response to climate change, the study of how dispersal of species shape communities is more relevant than ever to evaluate potential impacts of global change in biodiversity. Particularly, due to the fast change in climate conditions, we must know if the dispersal of species is fast enough to allow them to reach new location to avoid extinction. In other words, we must disentangle whether dispersal of key biological groups is limited or not.
In the specific case of the plant-symbiont arbuscular mycorrhizal (AM) fungi, the lack of knowledge on the processes that determine their communities and the impact of potential dispersal limitation is joined to the importance that this fungal group has for plant establishment and growth. The aim of this project has been the disentangling of the nature of ecological processes that shape species composition in AM fungal communities. The classical deterministic view, in which species composition is driven by environmental factors, has been faced in the last decades by theories pointing out towards community assemblies shaped by random factors. Dispersal, if limited or not, plays an important role in configurating communities either in a random or a deterministic way, respectively.
The research tasks performed during the project period included the study of three primary successional sites in Denmark: the artificial island Peberholm, built in 1998; a system of islands of recent formation (1945-2006); and a dune system generated since 1995. These environments are pieces of land recently formed and hence absent of any terrestrial life, allowing the study of the colonization by organisms and the effects of their dispersal, in the process known as primary succession. By selecting sites with different times since formation, it was possible to trace temporal succession of communities. In these temporal gradients, the plant communities and their associated arbuscular mycorrhizal (AM) symbiotic fungi were recorded.
The first objective of the project covered the study of the AM fungal traits that drive their dispersal. The focus was done into the AM fungal spore traits, since these are the key structures for dispersal. It was expected an imprint in the spore shapes/sizes due to the recent dispersal events: i.e. at the beginning of succession only species with smaller spores (i.e. higher dispersal potential) could colonize the sites. A database of spore traits was developed by digging in published species descriptions and these values were matched with the data from the Peberholm island by comparing the DNA sequences of described species with those found in the island. Species inhabiting in the young island of Peberholm produced single spores (not in groups), in agreement with the hypotheses. In the system of islands, the results revealed a pattern towards having smaller and less diverse spore sizes in the younger sites, which became more variable when advancing in succession.
The second objective focused in disentangling the relative contribution of dispersal and deterministic processes (environmental filtering) along primary succession. This was achieved by quantifying the variation in species compositions due to spatial factors, which is assumed to be caused by dispersal limitation, and soil environment as deterministic factors. This implied a spatially explicit sampling design carried out in Peberholm and the neighbor island Saltholm (ca. 4,000 years-old). The results revealed a huge percentage of the variation in community composition in Peberholm driven by spatial factors that was independent on any analyzed soil variable. This result, is also in agreement with the existence of a certain dispersal limitation, i.e. not every species can reach every place.
The third objective covered the study of how the change on the symbiosis (AM fungi-plants) features varied along the succession, in search for extrapolating the turnover of plants and AM fungi to the change of ecosystem functioning. For this, the variation of AM colonization features was subjected to a trait-based analysis of the plant community means and compared with variation of leaf traits in the islands and the dune system. The trends revealed a transition in the plant life history strategies, from communities dominated by slow, conservative, growth modes, to faster and more ruderal plant communities. The use of mycorrhizal features at plant community level added further understanding to the functioning of plant communities: an increase in the functional diversity of mycorrhizal features (in comparison to leaf traits) pointed out that biotic interactions between plant species could be driven their community assembly, and this results was not evident when looking at leaf traits alone.
The results of this project have been presented in two international congresses and are being processed to be published in peer-review scientific journals (a first manuscript has been accepted for publication). A PhD course was organized to teach students on molecular and statistical approaches to the study of the assembly of microbial communities, as well as other four students carried out research projects on this topic. Finally, international collaborations were established with research groups in Europe and overseas (Brazil and Chile) with the aim of continue the open research line.
The results of this project points out towards a clear effect of dispersal limitation on the community assembly of arbuscular mycorrhizal (AM) fungi. Although patterns of AM fungal community composition congruent with the existence of dispersal limitation had been reported previously, no studies had been addressed the causes and the mechanic behind the process. Moreover, no clues existed about the potential consequences of dispersal limitation during primary succession of this fungal group. This project found imprints of dispersal limitation in the assembly of AM fungal communities, pointing out to spore traits as responsible of differences in dispersal rate across AM fungal species. Despite of this novel result, this project contributed to the advancement on the application of ecological approaches: trait-based approaches and neutral model generation. Trait-based approaches are challenging when studying microbial communities due to the difficulties to isolate and measure individual microbial species. The use of neutral models has been presented here as a tool to face observed communities with random expectations, in an attempt to generalize the use of these techniques as a backbone in the ecological studies of microbial communities.
The main impact of this project focus on generation of basic knowledge, needed for the advancement of our understanding about the assembly of biological communities, and particularly on microbes. Now that we know about the dispersal limitation in this key group of symbiotic fungi, further studies will be necessary to evaluate the potential consequences when facing a scenario of climate change where biological communities will be forced to move their geographical ranges. Although this is a first step in this line, increasing the understanding in dispersal processes of organisms will be key for planning mitigation measure to climate/global change.