Exchange Dynamics Reveal Significant Accumulation of Dimethylated Sulfur by Mediterranean Benthic Communities

Heidi L Burdett

doi:10.3389/fmars.2017.00431

Abstract

One fifth of Mediterranean waters can be classified as shelf – much higher than the global average. Consequently, the shelf / coastal zone plays a proportionally greater biogeochemical role than in the major oceans, including the support of a wide range of range of endemic or culturally important species and ecosystems. However, despite their known importance in regulating ecosystem function and the marine sulphur cycle, our understanding of the dynamics of dimethylated sulphur compounds such as dimethylsulphide (DMS) and dimethylsulphoniopropionate (DMSP) in Mediterranean benthic habitats is limited. Here, a community-level approach was adopted to quantify DMS and DMSP dynamics in Mediterranean ecosystems including seagrass (Posidonia oceanica) meadows, coralligene (an algal carbonate reef found along the Mediterranean shelf) and macroalgal stands. It was found that P. oceanica and coralligene are likely to act as significant benthic stocks of DMSP in the coastal / shelf environment. ‘Hotspots’ of water column DMS and DMSP processing were observed where net benthic production was high (e.g. P. oceanica meadows), demonstrating that benthic communities are able to modify DMS biogeochemistry in the overlying water column. High variability between, and within, habitat types illustrates the importance of ecosystem structure and light availability in determining benthic DMS and DMSP accumulation, and highlights a previously under-appreciated complexity in benthic dimethylated sulphur dynamics.

Highlights

The Mediterranean Sea is the largest and deepest semi-enclosed sea in the world, providing coastline to 21 states and serving as a tourist destination for 200 million people every year (Aquarone et al, 2010)
The highest observed intracellular concentrations were for the dead P. oceanica mat during the day, followed by the sand at 15 m and M. alternans that make up the coralligène framework (Table 1)
The intracellular DMSP concentrations reflect the seagrass “holobiont”. Whilst this likely contributed to the observed high variability between samples due to heterogeneity in epiphyte composition, it means the concentrations presented here are more representative of the natural community found in situ, and are more comparable to other higher plants (e.g., Spartina alterniflora), where extracellular accumulation of DMSP is known to occur (Dean and Kiene, 1992)

Summary

Introduction

The Mediterranean Sea is the largest and deepest semi-enclosed sea in the world, providing coastline to 21 states and serving as a tourist destination for 200 million people every year (Aquarone et al, 2010). The Mediterranean’s varied geological history, including full isolation from primary oceans and subsequent abrupt flooding (Garcia-Castellanos et al, 2009), has shaped its current characteristics: one fifth of Mediterranean waters can be classified as shelf—much higher than the 7% global average (Pinardi et al, 2006). This has resulted in high cultural and biological diversity (Coll et al, 2010). The subsequent impact on benthic-pelagic coupling will almost certainly affect ecosystem structure and function, but these processes are not yet adequately understood for the present day (Griffiths et al, 2017), limiting the accuracy of future projections

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