In addition to their important effects on nitrogen (N) cycling via excretion and assimilation (by macrofauna and macroflora, respectively), many macrobiota also host or facilitate microbial taxa responsible for N transformations. Interest in this topic is expanding, especially as it applies to coastal marine systems where N is a limiting nutrient. Our understanding of the diversity of microbes associated with coastal marine macrofauna (invertebrate and vertebrate animals) and macrophytes (seaweeds and marine plants) is improving, and recent studies indicate that the collection of microbes living in direct association with macrobiota (the microbiome) may directly contribute to N cycling. Here, we review the roles that macrobiota play in coastal N cycling, review current knowledge of macrobial-microbial associations in terms of N processing, and suggest implications for coastal ecosystem function as animals are harvested and as foundational habitat is lost or degraded. Given the biodiversity of microbial associates of macrobiota, we advocate for more research into the functional consequences of these associations for the coastal N cycle.

Foundation species are important components of ecosystems because they provide habitat and ameliorate stressful conditions for residents. Comparisons of congeneric foundation species have mostly been limited to comparisons of native and invasive species, with less attention paid to multiple native species. Surfgrasses (Phyllospadix spp.) are ubiquitous foundation species on the coast of Oregon, USA, protecting resident invertebrates from waves and providing them with access to sandy substrate in an otherwise rocky habitat. Two native surfgrass species, P. scouleri and P. serru latus, have superficially similar morphological characteristics and co-occur within the same rocky intertidal zones. We investigated whether these native congeneric species function similarly as foundation species by comparing the 2 species' morphology, sediment accretion and associated resident macroinvertebrates at 3 capes that vary in oceanographic conditions. The results show that although the macroinvertebrate abundance was the same between surfgrass species, macroinvertebrate species richness, composition and functional groups varied considerably, with more infauna and deposit feeders found within P. serru la tus. P. serrulatus also had fewer tillers and rhizomes, and lower biomass per given area, but greater sediment accretion than its congener P. scouleri. One notably strong result was the difference in macroinvertebrate abundance among capes, with Cape Perpetua having 2.5-3 times more animals per given area than Capes Foulweather or Blanco. Overall, we conclude that although the 2 co-occurring surfgrass congeners provided functionally different habitat for resident macroinvertebrates, regional oceanographic processes (i.e. upwelling and productivity) may be more influential in determining the overall abundance and productivity of these diverse animal communities.

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