Abstract

The holobiont concept recognizes that macroalgae functioning, in an ecological setting, cannot be understood without considering interactions with their associated microbiome. In this study, the structure of bacterial communities on the giant kelp Macrocystis pyrifera from two different genetic populations was analyzed in localities with contrasting environmental factors. A northern population subjected to permanent and seasonal upwelling with contrasting nutrient and temperature fluctuations; and a southern population located in the interior sea of Chiloe near salmon farms, characterized by temperature variations between spring and summer and nutrients concentrations modified by salmon excretion of inorganic nitrogen to coastal waters. Bacterial diversity was estimated based on the V1-V3 regions of the 16S rRNA gene. Differences between the epiphytic bacterial communities of macroalgae and the seawater were encountered in terms of a-diversity, with the 27.5% of the genera present in both seawater and the surface of the kelp. The genus Citromicrobium dominated the epiphytic communities associated with M. pyrifera, but was almost absent in seawater. There were seasonal differences (spring-summer) in the structure of epiphytic bacterial communities associated with M. pyrifera, which was more evidenced in northern compared to southern populations, mainly by changes in the relative abundances of some bacteria taxa. The main environmental driver modulating the community structure of epiphytic bacterial communities was temperature, which was evidenced in both macroalgal populations. During the summer season the relative abundances of some genera such as Cyanobacterium and Ralstonia increased in the bacterial communities only in the northern population, and Marinomonas and Pseudomonas in the southern population. We concluded that seasonal variation in temperature explains the epiphytic bacterial community structure; and in northern populations, nutrient availability is negatively correlated with significant changes in bacterial composition.