Differential response of planktonic primary, bacterial, and dimethylsulfide production rates to static vs. dynamic light exposure in upper mixed-layer summer sea waters

Abstract

Abstract. Microbial plankton experience short-term fluctuations in total solar irradiance and in its spectral composition as they are vertically moved by turbulence in the oceanic upper mixed layer (UML). The fact that the light exposure is not static but dynamic may have important consequences for biogeochemical processes and ocean-atmosphere fluxes. However, most biogeochemical processes other than primary production, like bacterial production or dimethylsulfide (DMS) production, are seldom measured in sunlight and even less often in dynamic light fields. We conducted four experiments in oligotrophic summer stratified Mediterranean waters, where a sample from the UML was incubated in ultraviolet (UV)-transparent bottles at three fixed depths within the UML and on a vertically moving basket across the same depth range. We assessed the response of the phyto-and bacterioplankton community with physiological indicators based on flow cytometry singe-cell measurements, fast repetition rate fluorometry (FRRf), phytoplankton pigment concentrations and particulate light absorption. Dynamic light exposure caused a subtle disruption of the photoinhibition and photoacclimation processes associated with ultraviolet radiation (UVR), which slightly alleviated bacterial photoinhibition but did not favor primary production. Gross DMS production (GP DMS ) decreased sharply with depth in parallel to shortwave UVR, and displayed a dose-dependent response that mixing did not significantly disrupt. To our knowledge, we provide the first measurements of GP DMS under in situ UV-inclusive optical conditions.

Assessing the potential for dimethylsulfide enrichment at the sea surface and its influence on air–sea flux

“…These production rates are still 1-2 orders of magnitude lower than many of the calculated PR SSM for B1 (Table 1), but nevertheless confirm the potential for rapid DMS accumulation in response to increased light stress. Deck board incubations of SSM and SSSW from B2 and B3 stations showed that DMS production in the light was approximately double that in the dark (Cliff Law, personal communication, 2016), consistent with other reports (Galí et al, 2013). The highest net production rate of 3.7 nmol L −1 h −1 in the light (Cliff Law, personal communication, 2016) was again substantially lower than the calculated PR SSM in Table 1.…”

“…The vertical DMS profile in nearsurface waters in B2 and B3 was uniform (see Fig. 4), indicating that DMS production and loss terms, such as ventilation, bacterial oxidation, and photolysis, were in balance (Galí et al, 2013). Furthermore, the profiles do not show significant near-surface depletion in [DMS], which has been previously reported and attributed to ventilation and photolysis (Kieber et al, 1996).…”

“…This is supported by the "DMS summer paradox" where higher DMSP and DMS levels have been observed in shallow mixed layers that are exposed to high light levels (Simó and Pedrós-Alió, 1999). Laboratory and field experiments have also demonstrated that DMS has a positive, dose-dependent response to solar radiation (Galí et al, 2013;Sunda et al, 2002;Vallina et al, 2007). In particular, gross DMS production is stimulated by ultra-violet radiation (UVR), which causes a reduction in algal cell integrity and enhanced release of DMSP, DMS, and cleavage enzymes, and also upregulation of intracellular DMSP cleavage (Galí et al, 2013).…”

“…Laboratory and field experiments have also demonstrated that DMS has a positive, dose-dependent response to solar radiation (Galí et al, 2013;Sunda et al, 2002;Vallina et al, 2007). In particular, gross DMS production is stimulated by ultra-violet radiation (UVR), which causes a reduction in algal cell integrity and enhanced release of DMSP, DMS, and cleavage enzymes, and also upregulation of intracellular DMSP cleavage (Galí et al, 2013). No relationship was observed between either [DMS SSM ] or [DMS 6.0 m ] with incident solar radiation in the current study, although this was confounded by differences in other factors such as phytoplankton biomass and community composition.…”

Limitation of dimethylsulfoniopropionate synthesis at high irradiance in natural phytoplankton communities of the Tropical Atlantic

“…If DMSP is released from cells exposed to UVR in common with other components of the DOC, the resulting increased dissolved DMSP availability potentially boosts DMS production. Several studies conducted in high‐light oceanic waters have shown enhanced DMS production in incubations exposed to near surface levels of PAR and UVR (Toole et al , Galí et al ). Although, these studies also showed that UVR enhancement of DMS production is to some extent balanced by increased rates of DMS photolysis.…”