HETEROTROPHIC BACTERIAL ALKALINE PHOSPHATASES DETERMINED C/N/P RATIOS IN DEVONIAN BLACK SHALES

Ratios of organic carbon to total nitrogen and total phosphorus are frequently reported in studies of ancient organic-rich shales where they are used to infer paleoenvironmental conditions, macronutrient availability, and organic matter sources. Although these ratios represent fundamental properties of an economically important resource, the biogeochemical processes that underlie them are not well understood. We propose a mechanism based upon the observed ecological stoichiometry of modern anaerobic sediments to account for the pattern of C/N and C/P in Late Devonian black shales. Across a variety of Late Devonian black shale sites sampled at centimeter to meter scales, the ratio of organic C to organic N is highly consistent within sites regardless of total organic C content and varies across sites in inverse proportion to median total phosphorus abundance. In contrast, the ratio of organic C to total P varies in direct proportion to the total organic carbon content of the individual sample.

Bacterial heterotrophy in organic-rich anaerobic sediments is frequently limited by the availability of labile carbon. In response, bacterial heterotrophs invest scarce C and N to produce alkaline phosphatases in order to alleviate C limitation by hydrolysis of organophosphates. This suggests that sediment organic C/N may represent a threshold beyond which the investment of intracellular C and N in the production of alkaline phosphatase no longer results in a stoichiometrically favorable return on the investment. If this is the case, then C/P represents the point in the diagenesis of organic matter at which the effectiveness of alkaline phosphatase in procuring labile organic matter by remineralization of P is constrained by the lability of the organic matter itself. Because alkaline phosphatase activity is expressed in inverse proportion to the porewater concentration of phosphate, at a given location an equilibrium determined by total phosphate influx results in consistent total phosphorus burial rates and organic C/N values independent of the total organic C content of the sediment.