Characterization of Microbial Communities on Natural and Transplanted Congener Macroalgae Across the Intertidal Stress Gradient

Abiotic and biotic stresses cause distinct zonation of intertidal macroorganisms, but whether bacteria follow these same patterns is unknown. We characterized the microbiota of Fucus congeners (high zone F. spiralis, mid zone F. vesiculosus, and low zone F. distichus) to determine whether their bacterial associations are different, possibly due to the stress gradient. Additionally, F. vesiculosus individuals were transplanted to the high zone and either watered with seawater to partially mimic mid-zone conditions, or not watered, to expose transplants to high intertidal stress (e.g., desiccation). Additional individuals were back-transplanted to their native mid-zone (procedural controls). Samples of transplants and the unmanipulated, natural controls were collected over two weeks in summer. After sequencing the V4 region of the 16S rRNA gene, we normalized the total number of reads returned from minimum entropy decomposition in each sample to 17,000 by rarefaction. An adonis test of natural congener microbiota found a significant interaction between species and tissue type (Morisita-Horn index). Additional tests found no significant differences in microbiota between natural and back-transplanted F. vesiculosus or between back-transplanted and watered F. vesiculosus at the end of the experiment. The holdfasts of our unwatered transplants were significantly different from procedural controls (Jaccard Index), supporting our hypothesis that desiccation affects the relative composition of microbiota, but, surprisingly, our unwatered transplants were not significantly different from the watered ones. This suggests desiccation was not the only influential stress. There was a small difference in mean temperature between zones, but a large difference in exposure time between zones (3 - 6 h). We hypothesize that light stress may account for some of the microbial patterns.