Interactive Effects of Vertical Mixing, Solar Radiation and Microbial Activity on Oceanic Dimethylated Sulfur Cycling
Total Page：16
File Type：pdf, Size：1020Kb
Interactive effects of vertical mixing, solar radiation and microbial activity on oceanic dimethylated sulfur cycling Martí Galí Tàpias 23 d©octubre de 2012 Tesi presentada per obtenir el títol de Doctor per la Universitat Politècnica de Catalunya, Programa de Doctorat en Ciències del Mar. Director: Rafel Simó Martorell Universitat Politècnica de Catalunya (UPC), Departament d©Enginyeria Hidràulica, Marítima i Ambiental Institut de Ciències del Mar (CSIC) Departament de Biologia Marina i Oceanografia El Doctorand, El Director Martí Galí Tàpias Rafel Simó Martorell Abstract Microscopic plankton thriving in the sunlit upper ocean play a key role in the bio- geochemical functioning of the biosphere. The production and subsequent emission of volatile compounds is one of the numerous ways by which they participate in the cycling of elements and influence the Earth's climate. Dimethylsulfide (DMS), pro- duced by enzymatic decomposition of the algal intracellular compound dimethylsul- foniopropionate (DMSP), is the more abundant organic volatile in the upper ocean. Its global emission amounts ca. 28 Tg S per year, and represents the main bio- genic source of sulfur to the troposphere and about 30% of the total S emission (anthropogenic, biogenic and volcanic). Atmospheric oxidation of DMS contributes to atmospheric acidity, and is believed to promote the formation and growth of aerosols. Furthermore, DMS-derived sulfate aerosols have been suggested to cool the climate by reducing the amount of shortwave solar radiation reaching the Earth's surface by scattering solar radiation and, more important, by acting as cloud con- densation nuclei. The `CLAW' hypothesis postulates that, if oceanic DMS emission was in turn stimulated by solar radiation, a regulatory feedback mechanism could operate between marine plankton and the radiative budget over the oceans. However, the relationship between DMS emission and solar radiation is not straight- forward, since a number of biochemical and photochemical transformations come into action from the moment DMSP is synthesized by phytoplankton until DMS is emitted. These transformations are intimately linked to the physical environ- ment, the ecological setting and the microbial interactions, rendering the picture of dimethylated sulfur cycling a lot more complicated. Surprisingly, though, the sea- sonal cycle of seawater DMS concentration seems to follow that of solar radiation in the majority of oceanic regions, regardless their productivity regimes. This charac- teristic feature of oceanic DMS, while broadly accepted, is not yet well understood. The premise of this thesis is that, to understand this emerging pattern, we need to understand what regulates the DMS production and consumption processes and their balance (that is, DMS budgets). To this end, we have studied the response of biotic and abiotic DMS cycling to solar radiation by means of incubation exper- iments. At another level, we have studied the response of ecosystem DMS budgets to different radiation climates, and from polar to subtropical areas. Since the depth of the upper mixed layer regulates the amount and spectral composition of the `light' seen by the cells and molecules, our studies have been backed by a careful characterization of underwater radiation fields and vertical mixing dynamics. Our results show that solar radiation has a stimulating effect on gross DMS pro- duction. Moreover, the stimulation is more effective at shorter and more energetic wavelengths, particularly in the ultraviolet (UV) region. Among the potential ex- planations, direct DMS production and/or increased DMSP release by UV-stressed phytoplankton is the most plausible mechanism, and fits with the putative antiox- idant function of DMS and its metabolites. In such stress conditions, the role of bacterial metabolism and microzooplankton grazing on DMS production seems to be secondary. At the ecosystem level, we have shown that vertical mixing-mediated solar exposure regulates whether DMS is preferentially oxidized by bacteria or by photochemical reactions. The outcome of this competition between DMS sinks is that total DMS loss rate constants vary little across oceanic biomes. As a result, the seasonal and also the short-term variability in DMS concentrations respond mainly to gross DMS production. The stress response occurs at different temporal scales: seasonally, through the succession of microbial communities towards stronger DMSP producers and higher DMS yields in summer stratified waters; and across day-night cycles, where short-term radiative stress modulates DMSP to DMS con- version yields. The thesis is closed by a literature meta-analysis, which places our results among the existing literature, and improves our current understanding of the distinct DMS(P) cycling regimes and their links with the ecological geography of the sea. Contents List of Figures vii List of Tables xi Introduction xiii 0.1 Climate, biogeochemical fluxes and feedbacks . xiii 0.2 Microbes in motion: pelagic ocean biogeochemistry . xiv 0.3 Vertical mixing, radiation climate, and microbial activities in the upper ocean . xvii 0.4 Dimethylated sulfur cycling in the ocean: more than climate feedbacks . xxviii Aims and outline of the thesis xxxv 1 Occurrence and cycling of dimethylated sulfur compounds in the Arctic during summer receding of the ice edge 1 1.1 Introduction . 3 1.2 Methods . 5 1.3 Results . 11 1.4 Discussion . 22 1.5 Conclusions . 32 2 Stimulation of gross dimethylsulfide (DMS) production by solar radiation 35 2.1 Introduction . 37 2.2 Methods . 38 2.3 Results and discussion . 42 2.4 Supplementary Information (SI) . 45 3 Sunlight-stimulated gross dimethylsulfide (DMS) production: Interactive ef- fects of spectral irradiance, the microbial community and its light history 53 3.1 Introduction . 55 v CONTENTS 3.2 Methods . 58 3.3 Results . 70 3.4 Discussion . 76 4 Differential response of planktonic primary production, bacterial production, and dimethylsulfide production to vertically-moving and static incubations in UV-transparent summer stratified waters 85 4.1 Introduction . 87 4.2 Methods . 89 4.3 Results and discussion . 91 4.4 Supplementary information . 96 5 Diel cycles of oceanic dimethylsulfide (DMS) cycling: microbial and physical drivers 99 5.1 Introduction . 101 5.2 Methods . 102 5.3 Results . 110 5.4 Discussion . 118 5.5 Supplementary information . 127 6 General discussion: A meta-analysis of oceanic DMS cycling 131 6.1 Overview and rationale . 132 6.2 Constructing a literature data compilation on oceanic DMS cycling . 132 6.3 Characterization of the dataset . 135 6.4 Preprocessing of the data and statistical analysis work flow . 136 6.5 Emergent properties: Multivariate analyses and PFT classification by clustering 139 6.6 Factors regulating DMSP production, food web transformations, and DMS pro- duction yields . 145 6.7 Factors regulating DMS cycling . 155 6.8 Recommendations for future oceanic DMS(P,O) cycling studies . 170 6.9 Supplementary information . 174 7 Conclusions 179 Bibliography 181 vi List of Figures 1 Hydrodynamics drive phytoplankton productivity . xv 2 The key role of the upper mixed layer . xvii 3 Spectra of incoming and outgoing solar radiation . xix 4 Diurnal cycles of vertical mixing . xx 5 Global mixed layer depth climatology . xxi 6 Absorption of solar radiation in the ocean . xxiv 7 The CLAW hypothesis as proposed by Charlson et al. (1987) . xxvii 8 Global sulfur emissions by latitude bands . xxix 9 Current view of DMS(P,O) cycling processes . xxxi 10 Global DMS climatology: the seasonal beat . xxxiii 1.1 Biogeochemical setting in the East Greenland current and Fram Strait . 13 1.2 Vertical profiles showing distinct biomass and dimethylated sulfur distributions with depth . 16 1.3 Influence of stratification on surface DMS(P,O) concentrations . 17 1.4 Measuring gross DMS production with the inhibitor technique . 18 1.5 Indirect evidence of the impact of photochemistry on surface DMS concentrations 26 1.6 Factors controlling bacterial DMS consumption in the UML . 27 1.7 DMS budgets in the UML . 29 1.8 Conceptual scheme linking dimethylated sulfur dynamics with surface stratifica- tion and ice melt dynamics during the ice-edge Phaeocystis bloom . 31 2.1 Effects of DMDS additions on DMS photolysis. 41 2.2 Dark vs. light gross DMS production in 24 h incubations . 43 3.1 Comparison between in situ and experimental exposure . 64 vii LIST OF FIGURES 3.2 Graphical example of how biologically or photochemicallyweighted irradiance was calculated . 65 3.3 Composition of the phytoplankton and bacterioplankton communities . 71 3.4 UV irradiance and photosynthetic performance of phytoplankton . 72 3.5 DMS cycling rates under different spectral irradiance conditions . 74 3.6 Stimulation factor of gross DMS production in sunlit vs. dark incubations in each experiment . 75 3.7 Graphical representation of the correlation between DMS production in the light treatments and the corresponding weighted irradiance dose . 79 3.8 `Irradiance responsiveness' of gross DMS production . 80 3.9 Conceptual synthesis of the interaction between solar exposure and the stimula- tion of gross DMS production . 83 4.1 Cartoon of the experimental design . 88 4.2 Response of microbial plankton and biogeochemical process rates to irradiance gradients in static and vertically moving incubations . 92 4.3 Phytoplankton absorption spectra at the end of experiment O2 (SI) . 96 4.4 Bacterial production measured by inoculating 40 mL Teflon bottles incubated `in situ' (SI) . 97 4.5 DMS photolysis dose-response in fixed and vertically-moving incubations (SI) . 97 5.1 Physical setting in the summert diel cycle experiments . 104 5.2 Diel changes in particulate primary production and leucine incorporation rates . 110 5.3 Diel changes in total DMSP (DMSPt) and DMS pools . 112 5.4 Diel changes in gross DMS production . 113 5.5 Diel changes in total DMSP consumption, DMS yield and radiolabelled DMSP assimilation . 115 5.6 Diel changes in microbial DMS consumption . 116 5.7 Vertical DMS profiles in the Mediterranean and Sargasso Sea oceanic experiments 117 5.8 Day versus night budgets of DMS cycling in the upper mixed layer .