[1] Methanol is a biogeochemically active compound and a significant component of the volatile organic carbon in the atmosphere. It influences background tropospheric photochemistry and may serve as a tracer for biogenic emissions. The mass of methanol in the atmospheric reservoir, the annual mass flux of methanol from sources to sinks, and the estimated atmospheric lifetime of methanol in the free troposphere, marine boundary layer, continental boundary layer, and in-cloud, are evaluated. The atmosphere contains approximately 4 Tg (terragrams, 10 12 g) of methanol. Estimates of global methanol sources and sinks total 340 and 270 Tg methanol yr À1, respectively, and are in balance given their estimated precision. Sink terms were evaluated using observed methanol distributions; the total loss is approximately a factor of 5 larger than prior estimates. The adopted source is a factor of 3 larger than its prior estimate. Recent net flux observations and the magnitude of the estimated sink suggest biogenic methanol emissions to be near their current estimated upper limit, >280 Tg methanol yr À1 , and this value was adopted. The methanol source will be larger with the inclusion of an argued for oceanic gross emission of 30 Tg methanol yr À1, but a major uncertainty concerns whether the oceans are a major net sink or source of methanol, an issue which will not be resolved without new measurements. Other large uncertainties are the estimates of primary biogenic emissions and gas surface deposition. The first loss estimates of methanol by in-cloud chemistry and precipitation are presented. They are approximately equal at 10 Tg methanol yr À1 , each. These are small in comparison to the surface loss and gas phase photochemical loss estimated here but would be significant additional losses in earlier budgets. Surface exchange processes dominate the atmospheric budget of methanol and its distribution. The atmospheric deposition of methanol and the argued for methanol produced in the upper ocean are ubiquitous sources of C 1 substrate capable of sustaining methylotrophic organisms throughout the surface ocean.

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The reproductive rates of 21 species of marine phytoplankton were measured in media in which free zinc, manganese, and iron ion activities were controlled at different levels using EDTA-trace metal ion buffer systems. In general, the reproductive rates of neritic species were limited by zinc activities below 1O-11.5 M, while those of oceanic species were either not limited or only slightly limited at the lowest zinc activity attained in the experiment, ca. lo-l3 M. The reproductive rates of oceanic coccolithophores were either not limited or only slightly limited by the lowest manganese ion activity attained, ca. 10-l' M, but those of a neritic coccolithophore and all diatoms, both neritic and oceanic, were limited below a manganese activity of 10-l" M. Neritic species had reduced reproductive rates in media containing clod7 M iron while oceanic species reproduced at maximal or close to maximal rates in the media with the lowest iron concentrations, ca. IO+' M. The habitat-related patterns in zinc, manganese, and iron requirements of oceanic and neritic species are consistent with the oceanic-neritic distributions of concentrations of these metals. This similarity in requirement and distributional patterns provides evidence that Zn, Mn, and Fe availability have been important selective forces on marine phytoplankton populations and communities. -The nutrients most frequently considered to limit the reproductive rates of phytoplankton in the ocean are the macronutrients nitrogen, phosphorus, and silicon. The possibility that trace metal micronutrients can be of significance in the ecology of phytoplankton has been considered occasionally (Harvey 1947 have for several reasons. Extensive contamination of water samples analyzed by trace metal chemists (Boyle and Edmond 1975; Bruland et al. 1978 Bruland et al. , 1979 and used for bioassays has, in the past, eliminated much of the evidence for trace metal limitation. It is only within the last few years that reasonably uncontaminated samples have been available for accurate measurement of trace metal levels in the ocean (Boyle et al. 1977; Bruland et al. 1978; Klinkhammer and Bender 1980;Bruland 1980). Also, only recently has methodology been developed to quantify relationships between phytoplankton reproductive rate and low free ion activities of trace metals. This methodology, proposed by Hutner et al. (1950), involves the use of trace metal ion activity buffers (combinations of synthetic chelators and trace metal salts), which permit quantification and control of extremely low activities of trace metal ions. These buffers also have 1182

Copper speciation in the upper marine water column is dominated b>, strong ligands thought to be of recent biological origin. Cultures of the marine cyanobacteria Synechococcu.? spp., a ubiquitous and important group of phytoplankton highly sensitive to Cu toxicity, were previously shown to produce chelators comparable in strength to those detected in the water column. Here we shoyw that cultures of Synechococcus exposed to toxic concentrations of Cu produce an extracellular ligand with a binding constant comparable to constants for ligands found in the water column. Coordination of Cu by this compound decreases the concentration of free cupric ion (the toxic form) in the culture media to le-rels that do not inhibit growth. A tight linear correlation between chelator and Cu concentration suggests l.hat production of this substance may be regulated by the concentration of free Cu in the media in a feeldback mechanism. Similarly, the concentrations of Cu and Cu-binding ligands in the water column are o?ten closely related. These results suggest that cyanobacteria modify Cu chemistry in seawater, creating conditions more favorable for growth.Considerable evidence suggests that the distribution and speciation of trace metals in the upper water column plays an important role in the species composition and physiology. of phytoplankton assemblages (Sunda 1994 and refs. therein). Speciation is important because only certain forms of a given metal are biologically available. In the upper water column, speciation of many biologically active trace metals is controlled by complexation with strong organic ligands (Bruland et al. 199 1;Sunda 1994). Complexation generally lowers the biological availability of a given metal because the free metal ions are the most biologically available forms (Sunda 1994). In addition, complexation may play an important role in the geochemical cycling of these elements in the upper ocean. For many elements, concentrations of these ligands are highest in the euphotic zone and decline to nondetectable values in deep waters (Bruland et al. 199 1;Rue and Bruland 1995). This distribution suggests that the compounds are of recent biological origin and are not refractory compounds. However, their biological function, if any, is unknown.Complexation is of particular importance for Cu, an essential micronutrient that is also toxic. Cu is complexed by low concentrations of a strong ligand having a con- AcknowledgmentsThis work was supported by Office of Naval Research contracts N00014-93-1-0833 to J. W. Moffett and NOOO14-93-1-0893 to L. E. Brand.Thanks to J. Waterbury and three anonymous reviewers for their comments on this manuscript.Contribution 9063 from the Woods Hole Oceanographic Institution.