Abstract
Associations between marine seaweeds and bacteria are widespread, with endobiotic bacterial-algal interactions being described for over 40 years. Also within the siphonous marine green alga Bryopsis, intracellular bacteria have been visualized by electron microscopy in the early ‘70s, but were up to now never molecularly analyzed. To study this partnership, we examined the presence and phylogenetic diversity of microbial communities within the cytoplasm of two Bryopsis species by combining fluorescence in situ hybridization (FISH), denaturing gradient gel electrophoresis (DGGE) and 16S rRNA gene clone libraries. Sequencing results revealed the presence of Arcobacter, Bacteroidetes, Flavobacteriaceae, Mycoplasma, Labrenzia, Phyllobacteriaceae and Xanthomonadaceae species. Although the total diversity of the endobiotic communities was unique to each Bryopsis culture, Bacteroidetes, Mycoplasma, Phyllobacteriaceae, and in particular Flavobacteriaceae bacteria, were detected in several Bryopsis samples collected hundreds of kilometres apart. This suggests that Bryopsis closely associates with well-defined endophytic bacterial communities of which some members possibly maintain an endosymbiotic relationship with the algal host.
Highlights
Marine macroalgal-bacterial associations range from beneficial, harmful or neutral, over obligate or facultative, to ecto- or endophytic interactions [1]
Fluorescence in situ hybridization To confirm the observation of endogenous bacteria in Bryopsis made by Burr and West [5], Bryopsis sections were hybridized with the universal bacterial EUB338 probe mix labelled with Cy3
Since the Bryopsis thalli were not surface sterilized before fixation, the EUB338 probe mix hybridized with epiphytic bacterial rRNA on the cell wall (Figures 2B–C)
Summary
Marine macroalgal-bacterial associations range from beneficial, harmful or neutral, over obligate or facultative, to ecto- or endophytic interactions [1]. Besides reports of bacterial endosymbionts associated with red algal galls [2,3,4], endophytic bacteria have been microscopically observed in the vacuolar as well as cytoplasmatic regions of various bryopsidalean green algae, including Bryopsis, Penicillus, Halimeda, Udotea and Caulerpa [5,6,7,8,9,10] These seaweeds are composed of a single, giant tubular cell and form an interesting biotic environment for bacterial communities. In Bryopsis ‘bacteria-like particles’ have been visualized in the cytoplasm by means of transmission electron microscopy in vegetative thalli as well as in the gametes, the latter suggesting vertical transmission of the endophytic bacteria [5] This implies a stable and specific relationship between the algal host and its endobionts in which both partners may provide mutualistic ecological benefits. The endophytic bacteria in Bryopsis may possess ecologically significant functions and bioactive potential since Bryopsis is a substantial source of bioactive compounds such as therapeutic kahalalides which may be of bacterial origin [12,13]
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