Environmental stressors alter the composition of seagrass phyllosphere microbial communities

Seagrass meadows are among the most valuable habitats in the world as they are used by a wide range of marine organisms and provide important ecosystem services. With increasing human populations and coastal development, seagrasses are under increased stress and coverage is declining worldwide. This is the first experiment to test the effects of two known seagrass stressors, increased temperature and reduced light availability, on the composition of seagrass blade surface microbial communities, which is a relatively understudied community. Analysis of 16S rRNA amplicon (iTag) sequence data revealed that both of these stressors significantly altered microbial community structure, including both taxonomy and abundance, on the blade surfaces of the tropical seagrass Thalassia testudinum. The highest temperature and lowest light treatments showed higher abundances of phyla not commonly reported as indigenous members of seagrass phyllosphere communities, including members of the bacterial phyla Ca. PAUC34f, Ca. Modulibacteria, and Chlamyidae. Despite these compositional difference among treatments, no significant differences in overall microbial diversity or richness were found. These results suggested seagrass phyllosphere microbial communities have the capacity to change significantly and relatively quickly in response to changing environmental conditions due to anthropogenic activity. Further studies are needed to determine if these direct environmental effects on the microbial community or indirect effects that feedback through the seagrass host.