Elective affinities or random choice within the seagrass holobiont? The case of the native Posidonia oceanica (L.) Delile and the exotic Halophila stipulacea (Forssk.) Asch. from the same site (Limassol, Cyprus)

Seagrasses provide many benefits to our ecosystems, they effectively capture and store carbon, act as wave breakers, and produce huge amounts of oxygen to name a few. They are the only flowering plants which grow in marine environments, supporting a highly diverse community of marine vertebrates and invertebrates. Seagrasses have associated microbial communities which they create a functional unit with, known as a halobiont. These diverse microbial communities on their surface may establish mutualistic relationships which both the microbes and the seagrass benefit from. The halobiont adapts and responds as a whole to any environmental changes, with the plants known to respond to changes and stressors over days or weeks whereas the associated microbes have the potential to respond faster.

The seagrass Posidonia oceanica is native to Cyprus and is an endemic seagrass species in the Mediterranean Sea. However, in the last century since the opening of the Suez Canal, a non-native tropical seagrass Halophila stipulacea has also established itself here.

In a study published in the Journal of Aquatic Botany, we collaborated with colleagues from Tor Vergata Rome University, ISPRA, Dead Sea and Arava Science Centre, Ben-Gurion University of the Negev, Cyprus University of Technology, University of Milano-Biocca and e-Campus University, to compare the epiphytic microbial communities associated with Posidonia oceanica and Halophila stipulacea, Mediterranean native and exotic seagrass species, respectively, growing side by side in monospecific patches within the port of Limassol (Cyprus, Eastern Mediterranean Sea).

To evaluate whether the environment rather than the host species and/or its physiological condition play a role in shaping the seagrass epiphytic microbial community, the environmental microbial communities (seawater and sediment) and seagrass associated ones were determined by using 16S rRNA gene amplicon sequencing. Plant ecological status was evaluated by morphological (biometry), structural (density) and biochemical (pigment/phenol content) descriptors. In both species, leaf associated microbial communities are clearly similar to seawater microbes; conversely, microbes associated with H. stipulacea roots/rhizomes differ from the microbial communities in surrounding sediment. In both seagrasses, Pseudomonadaceae was the most abundant family on leaves, but each species harboured unique microbial families. To our best knowledge, this is the first study on these two neighbouring seagrass species, coupling plant ecological status with associated microbial communities. Results demonstrated that each seagrass responded differently to the same environmental conditions and selected different epiphytic microbial communities, supporting their putative use as ecological indicators.

Co-author Demetris Kletou, from the Marine and Environmental Research Lab MER, said:

“Epiphytic microbial communities influence the functioning and ecological resilience of seagrass species. By studying their associations with the protected Posidonia oceanica and the non-indigenous species Halophila stipulacea we described differences and provided insights about unique microbial families that might benefit each species. This is a novel study and could be used as baseline as well as promote the putative application of microbes as ecological indicators in future seagrass monitoring efforts“.

The full study can be read in the following link:

https://doi.org/10.1016/j.aquabot.2021.103420