Macroalgal microbiome biogeography is shaped by environmental drivers rather than geographic distance.

Our results suggest that although host macroalgae exhibit a relatively 'typical' biogeographic pattern of declining similarity with increasing geographic distance, the microbiome is more variable, and is primarily shaped by environmental conditions. Our findings suggest that the Baas Becking hypothesis of "everything is everywhere, the environment selects" may be a useful hypothesis to understand biogeography of macroalgal microbiomes. As environmental conditions change in response to anthropogenic influences, the processes structuring the microbiome of macroalgae may shift while those governing the host biogeography are less likely to change. As a result, increasingly decoupled host-microbe biogeography may be observed in response to such human influences.