ATMOSPHERIC SULFUR AND ITS LINKS TO THE BIOTA 175 



proteins is a widespread property among bacteria. 2 7 Organisms that perform this 

 function may be found in both aerobic and anaerobic environments, especially 

 those containing large amounts of readily decomposable organic matter. In 

 aerobic environments, protein sulfur is usually released as sulfate but sometimes 

 as H 2 S. In anaerobic environments, sulfide is the released form. 



Regardless of type of bacterial mediation, the locale of generation is 

 probably in an aqueous medium, whether in a film of moisture in soil or within 

 the sediments or bulk water of lakes or oceans. The chemical form of the 

 generated compound will vary with pH. 2 ' Ionization constants for H 2 S in 

 solution at 18 C are 



[H 3 + ] [SH _ ] - 7 



k ' = [H 2 S] =°- 9X1Q7 



[H3O-][S 2 -]^ 10 - 15 

 2 [SH ] 



From these expressions it can be seen that undissociated H 2 S will be the more 

 abundant form in acid waters, whereas SH will predominate in basic waters. 

 Dissociation to sulfide ion will always be very small. We will henceforth refer to 

 the three species H 2 S, SH - , and S 2 ~, collectively, as sulfide except when it is 

 necessary to make a distinction among them. 



The sulfide generated as indicated above may be transformed chemically or 

 biologically to other forms, or it may be transported to the air— water interface 

 and there be released to the atmosphere as H 2 S. Some of the features of these 

 processes are described in the following paragraphs. 



One of the possible chemical transformations involves precipitation as a 

 metallic sulfide. 29 The presence of ferrous iron or other heavy metals in solution 

 may thus limit the total sulfide concentration in solution by precipitate 

 formation. The limitation will be a function of pH. Very small amounts of total 

 sulfide will coexist with ferrous iron in solution in neutral or basic waters, 

 whereas appreciable quantities may coexist in acid waters. Sulfide generated in 

 excess of that required to remove metal as insoluble sulfides will remain in 

 solution in basic waters. 



In a second possible transformation, the sulfide will be subject to oxidation 

 by dissolved oxygen. Rates of this reaction have been studied in laboratory and 

 field experiments by Skopintsev, Karpov, and Vershinina, 30 Ostlund and 

 Alexander, 31 and Cline and Richards. 32 Apparent rate constants are available, 

 but reaction details are not well known. Reaction products include thiosulfate, 

 sulfite, and sulfate. Ostlund and Alexander show that in seawater containing 

 5 ml 2 per liter the mean lifetime of H 2 S is of the order of 20 min. 



Several kinds of bacteria are capable of oxidizing sulfide. Chemolithotropic 

 sulfur bacteria are capable of utilizing reducing power obtained from the 

 oxidation of reduced sulfur compounds in order to reduce C0 2 to form cellular 



