The Role of Macrobiota in Structuring Microbial Communities along Rocky Shores the Role of Macrobiota in Structuring Microbial Communities along Rocky Shores 3 4 Removal Rates with a Tracer Experiment with Enriched

The role of macrobiota in structuring microbial communities along rocky shores. PeerJ 2:e631 Rocky shore microbial diversity presents an excellent system to test for microbial habitat specificity or generality, enabling us to decipher how common macrobiota shape microbial community structure. At two coastal locations in the northeast Pacific Ocean, we show that microbial composition was significantly different between inert surfaces, the biogenic surfaces that included rocky shore animals and an alga, and the water column plankton. While all sampled entities had a core of common OTUs, rare OTUs drove differences among biotic and abiotic substrates. For the mussel Mytilus californianus, the shell surface harbored greater alpha diversity compared to internal tissues of the gill and siphon. Strikingly, a 7-year experimental removal of this mussel from tidepools did not significantly alter the microbial community structure of microbes associated with inert surfaces when compared with unmanipulated tidepools. However, bacterial taxa associated with nitrate reduction had greater relative abundance with mussels present, suggesting an impact of increased animal-derived nitrogen on a subset of microbial metabolism. Because the presence of mussels did not affect the structure and diversity of the microbial community on adjacent inert substrates, microbes in this rocky shore environment may be predominantly affected through direct physical association with macrobiota. INTRODUCTION 25 The dynamics and interactions of the macroscopic species on rocky shores of the northeast 26 Pacific Ocean have been well-characterized and thus have contributed significantly to our 27 understanding of coastal ecological processes (eg. little about multi-taxa microbial associations. There is increasing evidence that many marine 31 macrobiota have surface biofilms (Grossart et al. Marine Ecosystem (CCLME) maintain diverse and unique microbial communities across 37 upwelling areas (Bertagnolli et al., 2011). The relatively high productivity of this system has 38 been attributed to the seasonal upwelling of nitrate (Barber and Smith 1981), which can lead to 39 significant levels of carbon fixation. The diversity of potential plant and animal 'host' species in 40 the CCLME, their relatively large geographic range, their longevity and the provision of key 41 resources suggest they provide unique microniches capable of increasing the diversity and 42 function of microbial communities. 43 If macrobiota directly provide habitat for intertidal microbes (e.g. mussels, Pfister et al., 44 2010; algae, Miranda et al., 2013) or indirectly provide resources such as nitrogen, in the form of 45 animal-regenerated ammonium, then our understanding of coastal biogeochemistry is incomplete 46 without …