Chasing Bacteria in the Green Alga Bryopsis Plumosa

Abstract

Bryopsis is a siphonous green marine macroalga characterized by a complex life cycle including both sexual and asexual reproductive stages. While the diverse bacterial communities associated with Bryopsis have been extensively studied, little is known about the stability of these bacterial communities. We investigated the bacteria associated with Bryopsis spp. from different sites along the European coast. Our aim was to understand the stability of the bacterial communities over time and host reproduction. We therefore focused on tracking bacteria across selected life stages in Bryopsis spp. We characterized the Bryopsis-associated bacteria using high throughput 16S rRNA amplicon sequencing. Symbiont stability was tested by characterizing cultures which had been in the lab for an extended amount of time. Bacterial transmission during sexual reproduction was monitored by screening Bryopsis gametes using transmission electron microscopy (TEM). The potential for the acquisition of novel bacteria during protoplast formation, an asexual reproduction strategy, was tested using fluorescent bacteria. Proteobacteria (mostly Alphaproteobacteria and Gammaproteobacteria), Bacteroidetes, and to a lesser extent Cyanobacteria and Firmicutes dominated the communities associated with Bryopsis. In situ Bryopsis samples hosted the highest bacterial species richness while communities present in the Bryopsis cultures which had been maintained in the lab for at least six months were the most diverse. Significant differences in the alpha and beta diversity indices of the bacterial communities hosted by the different Bryopsis sample types and reproductive stages in our study illustrated limited bacterial symbiont stability over time and life cycle transitions. Spatial variations were also observed among communities associated with in situ Bryopsis strains. Bacterial communities associated with hosts from a single location, Marseille (France) were more similar than those found on hosts from other sites. Vertical bacterial transmission was not observed using TEM on Bryopsis gametes. Fluorescent bacteria were not stably acquired (horizontal transmission) during Bryopsis protoplast formation, suggesting that Bryopsis employs a selectivity mechanism against foreign bacteria during protoplast formation which possibly influences horizontal bacterial transmission. The phylogenies of Bryopsis did not reflect the similarity between bacterial communities associated with those hosts, implying a lack of a host-symbiont evolutionary signal which is characteristic of transient symbioses. Our study sheds light on the stability of the bacterial communities associated with Bryopsis sp. and how this is impacted by cultivation and the potential for (directed) horizontal and vertical transmission. Functional characterization of these bacterial communities is recommended to improve the understanding of factors determining bacterial stability in Bryopsis.