CO 2 exchange between the atmosphere and soil algal crusts of the Trachypogon savannas of the Orinoco Llanos has been analyzed using an open gas exchange system. These savannas encompass a wide range of physiognomic types, from herbaceous communities to savanna woodlands. A maximum CO 2 -2 -1 . . . . flux of 0.207 mg m s was measured in the crusts of the Guampa savannas, while m the other examined crusts (0.035-0.105 mg m -2 s-1) the flux was similar to values reported for terrestrial algae. The CO: flux data were statistically fitted to the photosynthetically active radiation by a logarithmic relationship, and the photosynthetic efficiencies of the crusts were compared. The activation energy calculated for the CO 2 fixation indicates that limitations by diffusion and photochemical processes were excluded in the Guanipa crusts (above 12 kcal mole i), whereas they were evident in the other crust studied. An optimum CO 2 incorporation as a function of the crust water potential was established and carbon gain strategies were proposed on the basis of the results and characteristics of the habitats.
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