CARBON-ISOTOPE DISCRIMINATION IN CYANOBACTERIA OF ROCKS OF INSELBERGSAND SOILS OF SAVANNAS IN THE NEOTROPICS

An extensive set of data on carbon isotope ratios in samples of terres trial cyanobacteria from the surface of rocks of inselbergs and soils of savannas in the neotropics is presented. A comparison of the data s hows two surprising features. First, all values deviate from the C-13 discrimination of carboxylation by ribulose-bis-phosphate carboxylase/ oxygenase (RuBISCO) which is generally about -27 parts per thousand. T hey are less negative than that by between 0.62 parts per thousand and 10.46 parts per thousand. Second, the values show a considerable vari ation, minima and maxima being -26.38 parts per thousand, and -16.54 p arts per thousand, respectively. This variation cannot be explained by differences in dominant taxa in the samples. it must be related to mi crosite characteristics, most probably the extent in time of coverage by liquid water, where diffusion of CO2 constitutes a limiting step wi th a lower discrimination against C-13 than that of carboxylation via RuBISCO.