The electron transport system of respiring organisms reduces 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl tetrazolium chloride (INT) to INT-formazan. Respiring bacteria deposit accumulated INT-formazan intracellularly as dark red spots. Corresponding to electron transport system activity, these deposits attain a size and a degree of optical density which allows them to be examined by light microscopy. If polycarbonate filters and epifluorescence microscopy are applied to analyze an INT-treated water sample, it is possible to differentiate between respiring and apparently nonrespiring bacteria. This differentiation, which permits determinations of the total number of bacteria and the proportion thereof involved in respiration, is realized directly within one and the same microscopic image. Initial applications of the present method for hydrobiological purposes showed that the proportion of respiring aquatic bacteria ranged between 6 to 12% (samples taken from coastal areas of the Baltic Sea) and 5 to 36% (samples taken from freshwater lakes and ponds). Cells of 1.6 to 2.4 ym (freshwater) and 0.4 ttm (Baltic Sea) account for the highest proportion of respiring bacteria.

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During a cruise over the Atlantic from 40øS to 50øN in March-April 1987 the concentrations of dimethylsulfide (DMS) in the ocean and atmosphere were measured as well as the distribution of its precursor, dimethylsulfoniopropionate (DMSP), and of several biological parameters such as chlorophyll, phytoplankton species, and adenosine-5-triphosphate (ATP) in the surface water. The DMS concentration varied in the range 0.2-2 nmol DMS L -1 (surface water) and 0.05-3 nmol DMS m -3 (atmosphere) in the region of the remote tropical and subtropical Atlantic and increased to 2-10 nmol DMS L -1 (surface water) and 1-8 nmol DMS m -3 (atmosphere) north of 40øN and in the English Channel. Based on these results the mean flux of DMS from the Atlantic to the atmosphere is estimated to be 4-4.65 nmol DMS m -2 min -1 . A moderate diurnal variation of atmospheric DMS was found with a minimum during daytime. The DMS concentration in seawater correlated well with the concentration of DMSP and showed a similar trend to ATP, chlorophyll, and some phytoplankton species. Paper number 89JD03776. 0148-0227/90/89 JD-03776505.00 evidence for the role of DMSP as the biogenic precursor of DMS, the major volatile sulfur compound in seawater. EXPERIMENTAL METHODS Sampling of SeawaterSurface water samples were taken 3 times a day (08.30, 12.30, and 16.30 hours) with a 10-L bucket in front of or in the bow wave of the ship. They were used for the analysis of chlorophyll a, salinity, DMSP, and ATP as well as for phytoplankton counting. The seawater samples analyzed for DMS were obtained by means of the ship's pumping system from a depth of 10 m below the surface at 6-hour intervals (0100, 0700, 1300, and 1900).

Carbonyl sulfide (OCS) in the boundary layer of the marine atmosphere was measured by gas chromatography with flame photometric detection on board ship following a meridional transect in the Atlantic Ocean between 37øS and 5 IøN. Overall, OCS levels from 174 individual samples averaged 537 ppt with a standard deviation of 104 ppt. A pronounced northward increase in OCS of, on the average, 26 ppt per 10 ø latitude was found, together with a mean interhemispheric ratio of 1.25 between OCS in northern and southern hemisphere air masses. OCS correlated significantly with CH 4 and CO values obtained during the cruise by an independent investigator. I Now at Deutscher Bundestag, Secretariat, Enquete-Kommission Vorsorge zum Schutz der Erdatmosphiire, Bonn, Federal stratospheric aerosols, its atmospheric budget, including possible anthropogenic perturbations, is of special interest. However, many parameters in the global budget of OCS are poorly known. Nonetheless, its tropospheric mixing ratio of approximately 510 ppt, with a natural variability (lcr) of about 10% in the free troposphere [Torres et al., 1980; Carroll, 1985] and 30-40% in the boundary layer of the marine atmosphere [Johnson and Harrison, 1986] has been established. The global atmospheric burden is about 4.6 Tg OCS. It is currently believed that a major source of OCS to the global atmosphere is evasion from seawaters [WMO, 1986]. In seawater, OCS is produced through the photolysis of dissolved sulfur compounds, which are likely of biogenic origin, but as yet unidentified [Ferek and Andreae, 1984]. The supersaturation of surface waters with respect to the atmosphere has been demonstrated for OCS both for coastal waters [Turner and Liss, 1985; Ferek and Andreae, 1983, 1984] and for the open ocean [Johnson and Harrison, 1986]. Microbial production in soils (0.40 Tg/yr), biomass burning (0.2 Tg/yr), and the photolysis of CS2 and subsequent oxidation to OCS (0.60 Tg/yr) are considered further sources of OCS to the atmosphere [Khalil and Rasmussen, 1984]. Carroll et al. [1986] measured fluxes of OCS and CS2 from a saltwater marsh to the atmosphere and concluded that this source is an insignificant contributor (<1%) to the global OCS cycle. Since most of the source estimates are extrapolations which are based on very few field data, their uncertainty is about a factor of 2 or larger [Turco et al., 1980; Khalil and Rasmussen, 1984]. Turco et al. [1980] estimate anthropogenic OCS emissions of 1-5 Tg/yr. Khalil and Rasmussen (1984) consider anthropogenic sources, both directly through fossil fuel combustion (.08 Tg/yr) and industrial release (0.06 Tg/yr) and indirectly through oxidation of anthropogenic CS 2 (0.6 Tg/yr), to contribute only minor amounts to atmospheric OCS. Information on the processes removing OCS from the atmosphere is also incomplete. The stratospheric sink of OCS is estimated to remove approximately 1 Tg OCS/yr. The remaining sink of 1 Tg OCS/yr, which is necessary to balance the budget by Khalil and Rasmussen [1984], is postulated to be the upt...

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