Seasonal and spatial variability of dimethylsulfoniopropionate (DMSP) in the Mediterranean seagrass Posidonia oceanica

• First investigation of the occurrence of dimethylsulfoniopropionate (DMSP) in the leaves of Posidonia oceanica.
• Description of seasonal, spatial and within plant variability of leaf DMSP in P. oceanica.
• Discussion of the possible physiological and ecological roles of DMSP in P. oceanica.
• Evaluation of the content of DMSP in the meadow at community level (compared to phytoplankton).

We investigated for the first time the occurrence of dimethylsulfoniopropionate (DMSP) in the leaves of Posidonia oceanica and we report its variability during 4 sampling periods covering the seasonal cycle (February, June, August and November) and along a gradient from 10 m to 30 m depth. The P. oceanica leaf DMSP content expressed per mass of dry weight (dw) ranged from 0.1 to 33.9 μmol gdw−1 and averaged 5.0 μmol gdw−1. It was higher than the DMSP content of roots and rhizomes that averaged ∼0.5 μmol gdw−1. The leaf DMSP content showed seasonal variations, being highest in summer when primary production and biomass of P. oceanica were also highest. In August, the leaf DMSP content showed variations with depth, increasing from 30 m to 10 m depth. In summer, the leaf DMSP content was highest in the youngest sections of leaves (closest to base) than in the older ones (closest to apex). The seasonal and depth distribution suggest that the DMSP leaf content is positively related to irradiance, hence, we hypothesize that DMSP in P. oceanica plays a role as an antioxidant against reactive oxygen species, although we cannot unambiguously exclude other potential roles such as grazer deterrent. The average leaf DMSP content of P. oceanica is modest compared to high DMSP producing macroalgae and phytoplankton. Yet, the integrated DMSP stock associated to the meadows of P. oceanica is very large due to its enormous biomass, and at the community level it is 2 orders of magnitude higher than the potential integrated DMSP stock related to phytoplankton in the same area.