Effects of epibiotic bacteria on leaf growth and epiphytes of the seagrass Posidonia oceanica

Abstract

Beneficial effects of microorganisms on plant productivity are well known from terrestrial ecosystems, but no corresponding effects on marine plants have been described. We tested the hypothesis that the bacteria Marinomonas posidonica and M. mediterranea, which have been described as a forming component of the microbiota on Posidonia oceanica, influence leaf growth and epiphytic communities of this seagrass. We carried out 4 treatments: (1) seedlings inoculated with M. mediterranea or (2) M. posidonica and seedlings to which (3) sterilized seawater (sterile control) or (4) fresh seawater (field control) was added. Two experiments were performed: a growth assay in the laboratory under sterile conditions measuring leaf area after 3 mo, and an epiphyte assay in which the seedlings were transferred to the field. Leaves were sampled after 3 mo to analyse the community structure of epiphytes. The growth experiment showed that inoculation with M. posidonica enhanced leaf growth of P. oceanica compared with the controls to which no bacterial strain was added. In contrast, inoculation with M. mediterranea did not stimulate leaf growth. Likewise, inoculation with M. posidonica induced changes in epiphyte structure and can be expected to have a regulatory effect on macroepiphyte structure.