Effects of Extreme Rainfall on a Dominant Seaweed Are Mitigated by Its Microbiota

Extreme weather events are becoming more intense and frequent, driving unprecedented ecological changes globally. The effects of such extreme events can be particularly profound if they affect the performance of habitat-forming organisms (trees, corals, kelp). Further, the emergence of the "holobiont" concept in biology suggests that these impacts can occur directly on the habitat-forming "host" and/or via disruption of their associated microbiota. Following a one-in-100 year rainfall event along the coast of Sydney, Australia, we examined the effects of rainfall (in the field) and lowered salinity (in the lab) on the performance and reproductive output of a dominant, habitat-forming intertidal seaweed, Hormosira banksii. We then examined the ability of surface-associated microbes to mitigate host responses to extreme rainfall via microbial manipulations in the field. Extreme rainfall and reduced salinity (< 25 ppt) negatively affected host reproductive output. Manipulative field experiments using a combination of antimicrobial treatments applied once (pulse) or regularly (press) showed that disruption of Hormosira's microbiota after extreme rainfall affected host photosynthesis and, more importantly, inhibited the post-event recovery of host reproductive output. Press disruption of the host-microbiota prevented recovery of normal (control) levels of reproductive output and photosynthesis for over 4 months. These experiments demonstrate that host-associated microbiota can play a significant role in mediating responses of habitat-forming seaweeds to extreme weather events, with consequences for key components of fitness. Given the increased frequency of flooding and storm events experienced by many systems, the microbiome may provide a key role in influencing habitat resilience to stress.