Macroalgae Alter Water Column Communities and the Surface Microbiota of their Neighbors

Macroalgae variably promote and deter microbial growth through release of organic carbon and anti-microbial compounds into the water column. Consequently, macroalgae influence the microbial composition of the surrounding water column and biofilms on nearby surfaces. Here, we test the hypothesis that the surface microbiota of actively growing Nereocystis leutkeana meristem fragments (NMFs) is altered when co-incubated with mature blades of Nereocystis leutkeana, Mastocarpus sp., or both. We assess both the water column and NMF surface microbiota to determine whether shifts on NMF surfaces are driven by shifts in the surrounding water column community. Laboratory incubated macroalgae retain species- specific communities that resemble those from wild seaweeds. We find that water column community composition and richness changes significantly with co-incubated macroalgal type in a species specific manner. In contrast, NMF surface communities exhibit small compositional shifts and no significant changes in richness across treatments. Co-incubation with macroalgae results in enrichment or reduction of fewer taxa on NMF surfaces compared to the water column, but the direction of shifts is generally consistent. Thus, macroalgae sur- face microbiota are influenced by changes in the water column community, but appear to be more resistant to change. Overall, these results support the idea that macroalgae surfaces are highly selective, and demonstrate that modulations of macroalga microbiota operate within an overarching paradigm of host species specificity.