Microbial species, community, and network responses across strong environmental gradients: Does the ‘everything is everywhere’ paradigm hold up when compared to macroscopic organisms?

Clara M. Arboleda-Baena and Sergio A. Navarrete, Estación Costera de Investigaciones Marinas, Center for Applied Ecology and Sustainability (CAPES), Millenium Nucleus NUTME, Pontificia Universidad Católica de Chile, Las Cruces, Chile, Mara A. Freilich, Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, Cambridge, MA, Claudia B. Pareja and Rodrigo De la Iglesia, Laboratorio de Microbiología Marina, Departamento de Genética Molecular y Microbiología, Facultad de Ciencias Biológicas, Pontificia Universidad Católica de Chile, Santiago de Chile, Chile, Ramiro Logares, Instituto de Ciencias del Mar – CSIC, Barcelona, Spain

Background/Question/Methods and Results/Conclusions
The way strong environmental gradients shape multispecific assemblages has allowed us to examine a suite of ecological and evolutionary hypotheses about structure, regulation, and community responses to fluctuating environments. But whether the highly diverse co-occurring, free-living microorganisms are shaped in similar ways as macroscopic organisms, across the same gradients, has yet to be addressed in most ecosystems. The ‘everything is everywhere’ hypothesis suggests they are not, at least not to the same extent. Here we characterize the structure of intertidal microbial biofilm communities and compare the intensity of zonation at ‘species’ level, changes in taxonomic diversity and composition at the community level, and network attributes, with those observed in co-occurring macroalgae and invertebrates. At the level of species and OTUs, for dominant macro and microorganisms respectively, microbes showed less variability across the tidal gradient than macroorganisms. At the community-level, however, microbes and macro-organisms showed similarly strong patterns of tidal zonation, with major changes in composition and relative abundances across tides. Moreover, the proportion of ‘environmental specialists’ in different tidal zones was remarkably similar in micro and macroscopic communities, and taxonomic richness and diversity followed similar trends, with lower values in the high intertidal zone. Network analyses showed similar connectivity and transitivity, despite the large differences in absolute richness between the groups. A high proportion of positive co-occurrences within all tidal zones, and mostly negative links between the high and low tidal zones were observed among habitat specialist taxa of micro- and macro-organisms. Thus, our results provide partial support to the idea that microbes are less affected by environmental variability than macroscopic counterparts. At the species-level, the most common microbe species exhibit less variation across tides than most common macroscopic organisms, suggesting the former perceive a more homogeneous environment and/or are more resistant to the associated stress. At the community-level, most indicators of community and network structure across the gradient are similar between microbes and macro-organisms, suggesting that despite orders of magnitude differences in richness and size, these two systems respond in similar ways to stress gradients, giving rise to zonation patterns.