Greater host influence and promiscuity: how an invasive seaweed host has advantages over co-occurring natives.

The surface microbiome of seaweed hosts is a multi-domain biofilm regulated by host-microbe and microbe-microbe interactions. The extent to which hosts influence these interactions, and potentially affect their resilience and invasion success, remains unclear. We experimentally tested whether hosts with invasion history exert greater influence over their biofilms than native hosts. Biofilm formation on proxy surfaces adjacent to one invasive (Gracilaria vermiculophylla) and two native (Fucus serratus, Fucus vesiculosus) co-occurring hosts was monitored and compared to mature epiphytic biofilms of the same hosts. Only Gracilaria's Proxy Biofilms (PBs) were significantly different in community composition compared to control surfaces. Gracilaria's PBs also showed the highest similarity to their adjacent algae sharing certain bacterial taxa that were absent in control treatments, indicating that colonization of the proxy surface was influenced by the host. Gracilaria and its proxy biofilms showed highest similarity in microbial network variables, suggesting a higher ability of the invader to influence connectivity and microbial associations within its biofilm. Meanwhile Gracilaria's mature biofilm also showed higher variability in its prokaryotic composition over experiments, which was also reflected in a less robust microbial network in both Gracilaria and its proxy biofilms. This suggests that in addition to stronger influence in the invasive host, it was also more promiscuous towards potential symbionts from the environment. Ultimately, through examining microbial interactions, in line with previous research we found that host influence and promiscuity may play an important role in seaweed hosts to acclimate to different environmental condition and successfully thrive in new ecosystems.