Abstract
Posidonia oceanica is the only reported seagrass to produce significant amount of dimethylsulfoniopropionate (DMSP). It is also the largest known producer of DMSP among coastal and inter-tidal higher plants. Here we studied i) the weekly to seasonal variability and the depth variability of DMSP and its related compound dimethylsulfoxide (DMSO) in P. oceanica leaves of a non-disturbed meadow in Corsica, France, ii) the weekly to seasonal variability and the depth variability of DMSP to DMSO concentration to assess the potential of the DMSP:DMSO ratio as indicator of stress, and iii) the relationships between DMSP, DMSO and the DMSP:DMSO ratio with potential explanatory variables such as light, temperature, photosynthetic activity (effective quantum yield of photosystem II) and leaf size. The overall average concentrations of organosulfured compounds in P. oceanica leaves were 130 ± 39 μmol.gfw-1 for DMSP and 4.9 ± 2.1 μmol.gfw-1 for DMSO. Concentrations of DMSP and DMSO in P. oceanica leaves were overall distinctly higher and exhibited a wider range of variations than other marine primary producers. Concentrations decreased from a maximum in autumn to a minimum in summer; they changed little with depth. Potential explanatory variables except the leaf size, i.e., the leaf age were little or not related to measured concentrations. To explain the seasonal pattern of decreasing concentrations with leaf aging, we hypothesized two putative protection functions of DMSP in young leaves: antioxidant against reactive oxygen species and predator-deterrent. The similar variation of the two molecule concentrations over time and with depth suggested that DMSO content in P. oceanica leaves results from oxidation of DMSP. The DMSP:DMSO ratio remained constant around a mean value of 29.2 ± 9.0 µmol:µmol for the non-disturbed harvested meadow regardless of the time of the year, the depth or the leaf size. As suggested for the salt march plant S. alterniflora, we hypothesized the DMSP:DMSO ratio could be considered as indicator of stress in seagrasses exposed to environmental or anthropogenic stressors. More research would now be needed to confirm the functions of DMSP and DMSO in seagrasses and how the DMSP:DMSO ratio will vary under various disturbances.
Highlights
The synthesis and metabolism of dimethylsulfoniopropionate (DMSP) has been studied for 70 years (Challenger and Simpson, 1948)
The maximum mean DMSP concentration measured in this work was 205 ± 58 μmol.g−fw1, for an overall average in P. oceanica leaves of 130 ± 39 μmol.g−fw1
These values are much higher than the maximum mean DMSP concentration of 33.9 μmol per g of oven-dried leaf, i.e., 6.8 μmol.g−fw1 (P. oceanica leaves contain about 80% water) reported by Borges and Champenois (2015), and of the same order of magnitude as the DMSP concentrations of fresh and frozen leaf samples reported by Borges and Champenois (2017)
Summary
The synthesis and metabolism of dimethylsulfoniopropionate (DMSP) has been studied for 70 years (Challenger and Simpson, 1948). DMS, via transfer from the ocean to the atmosphere, could have a cooling effect on climate and could help to compensate for warming from “greenhouse effect” (Lovelock and Maggs, 1972; Charlson et al, 1987). This climatic role has been found to be much more complex than originally thought (Quinn and Bates, 2011), it explains the interest of research on the production and fate of these organosulfured compounds in the marine environment. P. oceanica beds are major coastal ecosystems (Gobert et al, 2006; Boudouresque et al, 2012), highly productive (Champenois and Borges, 2019b), provide many goods and services (Campagne et al, 2015; Mtwana Nordlund et al, 2016) and have considerable environmental, financial, and heritage value (Vassallo et al, 2013; Campagne et al, 2015)
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