Tracing food webs with stable hydrogen isotopes.

The hydrogen isotopic content of an animal's food, not water, determines that animal's hydrogen isotopic content. Liver and muscle tissue from mice reared on a diet such that the ratio of deuterium to hydrogen (DIH) of their food and water was kept constant, have the same average D/H ratio as the food source. In a simple, natural population of snails and their possible algal diets, Littorina obtusata (northern Atlantic intertidal snails that feed almost exclusively on the brown alga Fucus vesiculosus) has the same D/H ratio as Fucus vesiculosis and not that of the other algae available to the snails.

Stable Hydrogen Isotope Fractionations during Autotrophic and Mixotrophic Growth of Microalgae.

Isotope effects, studied with precision isotope ratio mass spectrometry, have been used to locate critical steps in the H metabolism of plants. By manipulating the growth conditions of versatile microalgae, the discrimination of H isotopes between water in the growth medium and the organically bonded H in carbohydrates from these microalgae was -100 to -120 per thousand and was regulated by both the light and the dark reactions of photosynthesis. Photosynthetic electron transport discriminated against the heavy isotope of H and formed a pool of reductant available for biosynthesis that was enriched in the light isotope. Growth in red or white light activated phosphoglyceric acid reduction and H isotope discrimination, when H was fixed into organic matter. An additional fractionation of -30 to -60 per thousand occurred during the biosynthesis of proteins and lipids and was associated with glycolysis. This fractionation paralleled the isotope effect seen in carbohydrate metabolism, indicating that H metabolism in photosynthesis was coupled with that in dark biosynthetic reactions via the pool of reductant, probably NADPH.

Carbon isotope fractionation by ribulose-1,5-bisophosphate carboxylase from various organisms.

Carbon isotope fractionation by structurally and catalytically distinct ribulose-1,5-bisphosphate carboxylases from one eucaryotic and four procaryotic organisms has been measured under nitrogen. The average fractionation for 40 experiments was -34.1 per thousand with respect to the delta(13)C of the dissolved CO(2) used, although average fractionations for each enzyme varied slightly: spinach carboxylase, -36.5 per thousand; Hydrogenomonas eutropha, -38.7 per thousand; Agmenellum quadruplicatum, -32.2 per thousand; Rhodospirillum rubrum, -32.1 per thousand; Rhodopseudomonas sphaeroides peak I carboxylase, -31.4 per thousand; and R. sphaeroides peak II carboxylase, -28.3 per thousand. The carbon isotope fractionation value was largely independent of method of enzyme preparation, purity, or reaction temperature, but in the case of spinach ribulose-1,5-bisphosphate carboxylase fractionation, changing the metal cofactor used for enzyme activation had a distinct effect on the fractionation value. The fractionation value of -36.5 per thousand with Mg(2+) as activator shifted to -29.9 per thousand with Ni(2+) as activator and to -41.7 per thousand with Mn(2+) as activator. These dramatic metal effects on carbon isotope fractionation may be useful in examining the catalytic site of the enzyme.