Temporal variations in microbial metacommunity structure and assembly processes in response to shifts in environmental conditions are poorly understood. Hence, we conducted a temporal field study by sampling rock pools in four-day intervals during a 5-week period that included strong changes in environmental conditions due to intensive rain. We characterized bacterial and microeukaryote communities by 16S and 18S rRNA gene sequencing, respectively. Using a suite of null model approaches (elements of metacommunity structure, Raup-Crick beta-diversity and quantitative process estimates) to assess dynamics in community assembly, we found that strong changes in environmental conditions induced small but significant temporal changes in assembly processes and triggered different responses in bacterial and microeukaryotic metacommunities, promoting distinct selection processes. Incidence-based approaches showed that the assemblies of both communities were mainly governed by stochastic processes. In contrast, abundance-based methods indicated the dominance of historical contingency and unmeasured factors in the case of bacteria and microeukaryotes, respectively. We distinguished these processes from dispersal-related processes using additional tests. Regardless of the applied null model, our study highlights that community assembly processes are not static, and the relative importance of different assembly processes can vary under different conditions and between different microbial groups.

Distinguishing the importance of different community assembly mechanisms is an emerging topic in microbial ecology and much focus has been placed in recent years on investigating how contemporary environmental conditions, dispersal and stochastic processes influence the spatial turnover of communities. However, historical events, such as past environmental conditions or dispersal events, can be important as well. We provide a short summary of the processes that can lead to so-called legacy effects, where past biotic or abiotic factors influence the composition of present-day communities. Priority effects, which arise if early colonizers gain advantage over later-arriving species, can lead to persistent legacy effects. In contrast, time-lags in environmental selection can lead to transient legacy effects. Dispersal rates as well as factors that influence the adaptability of species to changing environmental conditions should be important factors that determine the relative importance of contemporary selection versus historical processes and whether legacy effects are likely to be permanent or temporary. Working with microbial communities offers the advantage of feasible time series studies and multi-generation experiments, and can therefore make important contributions to a novel systematic framework on how historical processes shape complex metacommunities in nature.

No abstract

10Temporal variations in microbial metacommunity structure and assembly processes in 11 response to shifts in environmental conditions are poorly understood. Hence, we conducted a 12 temporal field study by sampling rock pools in four-day intervals during a 5-week period that 13 included strong changes in environmental conditions due to intensive rain. We characterized 14 bacterial and microeukaryote communities by 16S and 18S rRNA gene sequencing, 15 respectively. Using a suite of null-model approaches to assess dynamics in community 16 assembly, we found that strong changes in environmental conditions induced small but 17 significant temporal changes in assembly processes and triggered different responses in 18 bacterial and microeukaryotic metacommunities, promoting distinct selection processes. 19Incidence-based approaches showed that the assemblies of both communities were mainly 20 governed by stochastic processes. In contrast, abundance-based methods indicated the 21 dominance of historical contingency and unmeasured factors in case of bacteria and 22 microeukaryotes, respectively, which we distinguished from dispersal-related processes using 23 additional tests. Taken together, our study highlights that community assembly processes are 24 not static, and the relative importance of different assembly processes can vary under different 25 conditions and between different microbial groups. 26

The immigration history of communities can profoundly affect community composition. For instance, early‐arriving species can have a lasting effect on community structure by reducing the invasion success of late‐arriving ones through priority effects. This can be particularly important when early‐arriving communities coalesce with another community during dispersal (mixing) events. However, the outcome of such community coalescence is unknown as we lack knowledge on how different factors influence the persistence of early‐arriving communities and the invasion success of late‐arriving taxa. Therefore, we implemented a full‐factorial experiment with aquatic bacteria where temperature and dispersal rate of a better adapted community were manipulated to test their joint effects on the resistance of early‐arriving communities to invasion, both at community and population level. Our 16S rRNA gene sequencing‐based results showed that invasion success of better adapted late‐arriving bacteria equaled or even exceeded what we expected based on the dispersal ratios of the recipient and invading communities suggesting limited priority effects on the community level. Patterns detected at the population level, however, showed that resistance of aquatic bacteria to invasion might be strengthened by warming as higher temperatures (a) increased the sum of relative abundances of persistent bacteria in the recipient communities, and (b) restricted the total relative abundance of successfully established late‐arriving bacteria. Warming‐enhanced resistance, however, was not always found and its strengths differed between recipient communities and dispersal rates. Nevertheless, our findings highlight the potential role of warming in mitigating the effects of invasion at the population level.