Phycobilisome Antennae and Chromatic Acclimation

Abstract

Phycobilisomes are astonishing in their size, complexity, and function. Their phenotypic plasticity, however, is an often underappreciated trait that allows them to maintain optimal photon capture rates under variable environmental conditions. While changes in phycobilisome size, composition, and shape occur in response to most abiotic cues, perhaps the most dramatic adjustments in these structures occurs in response to changes in the spectral distribution, or color, of the ambient light. Called chromatic acclimation, this response is best characterized in the Cyanobacteria, where over a century ago, it was revealed by the dramatic changes in cell coloration that occurred in response to growth in green versus orange light. In the past decade, there has been an explosion in the identification of new and different forms of chromatic acclimation in cyanobacteria, many of these showing more subtle or no visible phenotypes, compared to the first form of chromatic acclimation described. There are currently seven known forms of chromatic acclimation, which in all cases studied thus far owe their light color sensing abilities to photoreceptors belonging to the phytochrome superfamily. The mechanisms through which chromatic acclimation occurs and the signal transduction pathways by which it is controlled are being rapidly defined. Perhaps the most interesting findings from these efforts are that the different types of chromatic acclimation discovered to date have the capacity to sense a wide range of light colors and that many individual cyanobacteria species have successfully integrated multiple chromatic acclimation systems to create complex light color sensing and response networks.