Marine Chemistry, 30 (1990) 1-29 1 Elsevier Science Publishers B.V., Amsterdam Ocean-atmosphere interactions in the global biogeochemical sulfur cycle* Meinrat O. Andreae Biogeochemistry Department, Max Planck Institute for Chemistry, P.O. Box 3060, D-6500 Mainz (F.R.G.) (Received December 5, 1989; accepted December 15, 1989) ABSTRACT Andreae, M.O., 1990. Ocean-atmosphere interactions in the global biogeochemicai sulfur cycle. Mar. Chem., 30: 1-29. Sulfate is taken up by algae and plants and then reduced and incorporated into organosulfur com- pounds. Marine algae produce dimethylsuifonium propionate (DMSP), which has an osmoregulating function but may also be enzymatically cleaved to yield the volatile dimethylsulfide (DMS). At- tempts to identify the variables which control the oceanic production of DMS have shown that there are no simple relationships with algal biomass or primary productivity, but suggest that the concen- tration of DMS in the ocean is regulated by a complicated interplay of algal speciation and trophic interactions. Part of the biogenically produced DMS diffuses into the atmosphere, where it is oxi- dized, mostly to aerosol sulfate. The ability of these aerosol particles to nucleate cloud droplets, and thereby influence the reflectivity and stability of clouds, forms the basis of a proposed geophysiologi- cal feedback loop involving phytoplankton, atmospheric sulfur, and climate. Carbonylsulfde (COS) is produced photochemically from dissolved organic matter in seawater. The mechanism of this reaction is still unknown. Diffusion of COS from the ocean to the atmosphere is a globally signifcant source of this gas, which participates in the stratospheric ozone cycle. Hydro- gen sulfide and carbon disulfide are produced in the surface ocean by still unidentified processes, which appear to be related to biogenic activity. For these gases, the oceans are a minor source to the troposphere. SOURCES OF SULFUR TO THE ATMOSPHERE: AN OVERVIEW Recent aircraft measurements of atmospheric sulfur species show that an- thropogenic emissions are influencing the global atmospheric sulfur cycle even over remote ocean regions (e.g. Andreae et al., 1988 ). The human perturba- tion of the atmospheric sulfur cycle results largely from the emission of sulfur dioxide (SO2) from fossil fuel burning. A number of recent papers have re- viewed these emissions and presented a detailed source allocation (e.g. Cullis *Presented at the section on Atmospheric and Marine Chemistry of the 32nd IUPAC Congress in Stockholm, Sweden, August 2-7, 1989. 0304-4203/90/$03.50 © 1990 -- Elsevier Science Publishers B.V. 2 M.O. ANDREAE and Hirschler, 1980; M6ller, 1984). The estimates for man-made sulfur emis- sions fall into a relatively narrow range: about 2.5 ___0.3 Tmol yr- 1 (Tmol: 1 Teramol = 1012 mol = 32 × 1012 g). The characteristics of the natural biogeo- chemical sulfur cycle in the atmosphere-biosphere-ocean system are much less well known, but are currently receiving intense interest because of their potential involvement in the regulation of global climate (Charlson et al., 1987). A summary of natural sulfur emissions from all sources is given in Table 1. This table presents the best estimates of these fluxes based on current infor- mation; it must be emphasized that most of these estimates are rather uncer- tain. This applies especially to the emissions of particulate sulfur in the form of dust and seaspray and to the emissions from soil and plants on the conti- nents. The main reasons for the uncertainty regarding continental emissions ofbiogenic sulfur compounds are ( 1 ) the difficulty of accurately determining the various biogenic sulfur spe- cies, particularly hydrogen sulfide (H2S), at the low levels found in unpol- luted environments, (2) the technical problems of measuring emission fluxes from forest and brush ecosystems, and (3) the inadequate geographical coverage of existing data. Recent measurements of biogenic sulfur fluxes from terrestrial ecosystems have shown much lower emission rates than had been assumed just a few years ago, leading to lower estimates of their contribution to the atmospheric sulfur cycle and consequently making the oceans and fossil fuel burning by TABLE 1 Estimates of natural sulfur emissions (in Tmol S year-1 ) SO2 H2S COS DMS CS2 Sulfate Other Total Seaspray 1.2-10 1.2-10 Dust 0.1-1 0.1-1 Total 1.3-11 1.3-11 particulates Volcanoes 0.23-0.29 0.03 0.0003 - 0.0003 <0.1 ? 0.3-0.4 Soils and - 0.1-0.3 +0.02 0.006-0.12 0.02-0.025 - 0.03 0.15-0.4 plants Coastal - 0.03 0.004 0.02 0.002 - 0.004 0.06 wetlands Biomass 0.08 ? 0.003 - ? ? ? t> 0.08 burning Oceans - 0.05-0.2 0.011 0.6-1.6 0.01 - ? 1.1-1.8 (gases) Totalgases 0.3-0.4 0.2-0.6 0.00-0.04 0.6-1.7 0.03-0.04 <0.1 0.03 1.2-2.8" "Equivalent to 38-89 Tg S year-~. GLOBAL B1OGEOCHEMICAL SULFUR CYCLE 3 far the most important sources of atmospheric sulfur. Among continental sources of sulfur gases, emissions from plants are now recognized as being at least as important as soil emissions. The results from recent work on the bio- genic sulfur cycle over the continents have been reviewed by Andreae (1990a); a more detailed discussion of sulfur fluxes over the tropical continents can be found in Andreae and Andreae (1988), Andreae et al. (1990), and Bingemer et al. (1990). On a global scale, biomass burning appears to be a minor source of atmospheric sulfur, with an annual sulfur release rate of ~ 0.08 Tmol year- 1. It is however, a regionally important source in the tropics, where other sulfur emissions are sparse (Andreae, 1990b). In the following sections, I will discuss the principles of biogenic sulfate reduction and synthesis of volatile species, the oceanic emission of dimethyl- sulfide (DMS), carbonyl sulfide (COS) and other volatile sulfur species, and the fate of these compounds in the atmosphere. Additional information on other aspects of the sulfur cycle can be found in recent reviews (Andreae, 1985a; Andreae, 1986, and references therein) and in the proceedings vol- ume from the Symposium on Biogenic Sulfur in the Environment (Saltzman and Cooper, 1989). SULFATE REDUCTION BY BIOLOGICAL PROCESSES In the + 6 oxidation state, the chemistry of sulfur is dominated by sulfuric acid and sulfate, which are rather involatile chemical species. As only this oxidation state is stable in the presence of oxygen, sulfate is the predominant form of sulfur in seawater, fresh waters and soils. Therefore, the reduction of sulfate to a more reduced sulfur species is a necessary prerequisite for the formation of volatile sulfur compounds and their emission to the atmosphere. In the global geochemical cycle, there are two types of biochemical pathways which lead to sulfate reduction: assimilatory and dissimilatory sulfate reduc- tion. Table 2 shows estimates of the rates of sulfate reduction by these pro- cesses and compares these rates with the flux of sulfur through the atmosphere. Biological sulfate reduction has two major objectives: ( 1 ) the biosynthesis of organic sulfur compounds which are used for various purposes by the cell, e.g. in amino acids, and (2) the use of sulfate as a terminal electron acceptor to support respiratory metabolism in the absence of molecular oxygen. The former process is called assimilatory sulfate reduction (sulfur is being 'assim- ilated' ), the latter dissimilatory sulfate reduction. It is important to under- stand the ecological and biogeochemical differences between these two mech- anisms: inadequate awareness of these differences between the two pathways of sulfate reduction has led to many of the misinterpretations and false as- sumptions found in the literature on the atmospheric sulfur cycle, e.g. the assumption that H2S is the major reduced sulfur compound emitted from the oceans. 4 M.O.ANDREAE TABLE 2 Rates of sulfate reduction by major biogeochemical processes compared with anthropogenic and biogenic sulfur emissions to the atmosphere Process Tmol year- Bacterial, dissimilatory sulfate reduction Coastal zone 2.2 Shelf sediments 6 Slope sediments 9 Total 12-20 a Assimilatory sulfate reduction Land plants 3-6 Marine algae 10-20 Total 12-25 b Anthropogenic emission of SO 2 ~ 3 Total biogenic sulfur gas emissions ~ 1.5 Total natural sulfur emission ~ 2 alvanov and Freney (1983). bEhrlich et al. (1977). 3 Tmol SO2 yr -I . A~roposphe~ Assimtlatory sulfate reduction in the presence of 02 COS4 ~ (Plants and algae) (Land plants 3-6 Tmol yr -I) @/ o DMSj~ (Marine algae 10-20 Tmol yr "1) _~_~__oxic___mixing barrier (redoxcline) ~ HZs ~ onoxic FeS/ Dissimilatorysulfate reduction in the absence of 0 2 (Anaerobic bacteria) (12-20 Tmol yr -I) Fig. 1. Interactions in the global biogeochemical sulfur cycle. Figure 1 gives a simplified, conceptual overview of the biogeochemical sul- fur cycle. The global environment is subdivided into four compartments: at- mosphere, biosphere, hydrosphere and lithosphere (the last standing for the sediments and rocks of the Earth's crust). The major pathway for the produc- tion of H2S is dissimilatory sulfate reduction, which is used by microbes to obtain thermodynamic energy in an oxygen-depleted environment. The oxi- dation of organic matter by available electron acceptors is the energetic basis GLOBAL BIOGEOCHEMICALSULFUR CYCLE 5 for essentially all life processes. Molecular oxygen is the thermodynamically most favorable electron acceptor which, if available, will be used preferen- tially in any ecosystem. However, if the supply of organic compounds exceeds that of oxygen, other electron acceptors (e.g. nitrate or sulfate ) are used when oxygen has been depleted. Dissimilatory sulfate reduction is therefore most commonly observed in marine environments where water circulation, and consequently oxygen availability, is limited (e.g. in stratified basins or in sed- imentary pore waters) but where sulfate is easily available because of its rel- atively high concentration in seawater (28 mmol kg -~ ).