Proteorhodopsin photosystem gene expression enables photophosphorylation in a heterologous host.

Proteorhodopsins (PRs) are retinal-containing proteins that catalyze light-activated proton efflux across the cell membrane. These photoproteins are known to be globally distributed in the ocean's photic zone, and they are found in a diverse array of Bacteria and Archaea. Recently, light-enhanced growth rates and yields have been reported in at least one PR-containing marine bacterium, but the physiological basis of light-activated growth stimulation has not yet been determined. To describe more fully PR photosystem genetics and biochemistry, we functionally surveyed a marine picoplankton large-insert genomic library for recombinant clones expressing PR photosystems in vivo. Our screening approach exploited transient increases in vector copy number that significantly enhanced the sensitivity of phenotypic detection. Two genetically distinct recombinants, initially identified by their orange pigmentation, expressed a small cluster of genes encoding a complete PR-based photosystem. Genetic and biochemical analyses of transposon mutants verified the function of gene products in the photopigment and opsin biosynthetic pathways. Heterologous expression of six genes, five encoding photopigment biosynthetic proteins and one encoding a PR, generated a fully functional PR photosystem that enabled photophosphorylation in recombinant Escherichia coli cells exposed to light. Our results demonstrate that a single genetic event can result in the acquisition of phototrophic capabilities in an otherwise chemoorganotrophic microorganism, and they explain in part the ubiquity of PR photosystems among diverse microbial taxa.

Pleistocene to recent dietary shifts in California condors.

We used carbon and nitrogen isotopes to investigate changes in the diet of California condors from the Pleistocene to the recent. During the Pleistocene, condors from California fed on both terrestrial megafauna and marine mammals. Early accounts reported condors feeding on the carcasses of marine mammals, but by the late 1700s, condor diets had shifted predominantly to terrestrial animals, following the commercial harvesting of marine mammals and the development of cattle ranching on land. At present, dairy calves provided by humans significantly augment condor diet, constituting an artificial support of the current population. Reestablishing a marine mammal component in the condor diet may be an effective strategy for fostering viable condor populations independent of direct human subsidies.

Linking breeding and wintering ranges of a migratory songbird using stable isotopes.

We used the natural abundance of stable isotopes (carbon and hydrogen) in the feathers of a neotropical migrant songbird to determine where birds from particular breeding areas spend the winter and the extent to which breeding populations mix in winter quarters. We show that most birds wintering on western Caribbean islands come from the northern portion of the species' North American breeding range, whereas those on more easterly islands are primarily from southern breeding areas. Although segregated by breeding latitude, birds within local wintering areas derive from a wide range of breeding longitudes, indicating considerable population mixing with respect to breeding longitude. These results are useful for assessing the effects of wintering habitat loss on breeding population abundances and for predicting whether the demographic consequences will be concentrated or diffuse.