Externally, changes in pH as a result of photosynthesis and respiration 

 and the precipitation and solution of calcium carbonate may affect the 

 rate of abiotic redox reactions. 



Carbon, nitrogen, oxygen, sulfur, iron, and manganese are the main 

 elements that participate in oceanic reduction-oxidation reactions. 

 These "redox elements" and the principal redox reactions mediated 

 by plants and microbes are summarized in Table 7 in a form which 

 focuses on the chemical species associated with each biological process. 

 These reactions play an essential role in the vitality and chemistry of 

 the ocean and need to be investigated in more detail. 



Direct measurement of Eh values in the ocean is complicated and 

 limited by practical problems. Quantitative interpretation of the results 

 has been disappointing and has given only a general picture of the 

 redox level. A better understanding may be obtained by directing more 

 effort to biochemical studies of electron transport systems of the organ- 

 isms that mediate or catalyze the transfer of electrons between the 

 oxidizing and reducing substances. Once we understand the fundamental 

 roles of these processes and know their location and magnitude in the 

 ocean, we will better understand the essential chemical fluxes in the 

 marine ecosystem and the practical problems of corrosion and recycling 

 of pollutants. 



pH 



The chemistry of the oceans depends on the indirect influence of the 

 biological assimilation and sorption or secretion and excretion of com- 

 pounds that react with elements in the water. For example, pH changes 

 depend on the amount of carbon dioxide taken up or released during 

 photosynthesis and respiration. Also, pH is influenced by the secretion 

 and dissolution of calcium carbonate by marine plants and animals. 

 Diurnal variations in the euphotic zone change the pH of the surface 

 layers. These changes, although small in the water column, may be 

 significant in micro-environments close to biological and particulate 

 surfaces. They may be significant to the following: 



1. Degree of protolysis of acids in solution: The distributions of 

 carbonate species, borates, phosphates, sulfides, hydrous oxides of 

 iron and aluminum, and organic acids are determined by pH. 



2. Precipitation reactions: Biologically induced changes in the pH 

 of seawater may result in the precipitation of species that are at or 

 near saturation. The coprecipitation of other elements is an important 

 secondary effect, e.g., coprecipitation of Sr+^, Pb+^, and Zn+^ with 

 CaCOs. 



3. Complexation: Since many complexing agents are also conjugate 

 bases of weak acids, the degree and stability of complexes in solution 

 depends on pH. 



63 



