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 (GPDMS) 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 GPDMS under in situ UV-inclusive optical conditions.
Highlights
The characteristic response times of microbial plankton match the natural variability of light exposure, which changes at different temporal scales with solar elevation, the passage of clouds, vertical mixing and even wave focusing (Gallegos and Platt, 1985)
In the coastal experiments (C1 and Coastal 2 (C2)) the samples were taken from a boat at the Blanes Bay Microbial Observatory coastal site (BBMO; 0.5 miles offshore over a water column depth of 20 m), brought to the lab, maintained within ±1 ◦C of the sea surface temperature, and incubated at the pier of the Barcelona Olympic Harbor during 4 h centered on the solar noon
The picoeukaryote fraction was likely dominated by haptophytes and pelagophytes in Oceanic 1 (O1) and Oceanic 2 (O2) according to HPLC pigment data (Pérez et al, unpublished)
Summary
The characteristic response times of microbial plankton match the natural variability of light exposure, which changes at different temporal scales with solar elevation, the passage of clouds, vertical mixing and even wave focusing (Gallegos and Platt, 1985). Exposure to high irradiance (photosynthetically available radiation, PAR) is accompanied by exposure to detrimental ultraviolet radiation (UVR) in the upper portion of the water column (Vincent and Neale, 2000). Short-term irradiance fluctuations elicit fast and reversible responses (Roy, 2000), whereas continued exposure to high PAR and UVR may elicit photoacclimation (MacIntyre et al, 2002) or permanent physiological changes, i.e., irreversible damage (Buma et al, 2001). M. Galí et al.: DMS and C cycling in dynamic light fields
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