Chlamydomonas: Anoxic Acclimation and Signaling

Chlamydomonas is a soil-dwelling freshwater alga that probably encounters hypoxic or anoxic conditions frequently in its natural habitat. Its repertoire of fermentative enzymes is remarkably complex and flexible. Despite this adaptability, at least under laboratory conditions, Chlamydomonas does not tolerate dark anoxia very well. It appears to respond to this condition by a state of quiescence, but still does not survive without light and molecular oxygen (O2) longer than a couple of days. The amounts of hundreds of transcripts change in anoxic algal cultures, but not much is known about the regulation of the anoxic response in Chlamydomonas. The phenotypes of mutants deficient for one or the other fermentative enzyme indicate a significant level of control by metabolic signals, and a subset of genes is coordinately regulated in copper and O2 deficiency by the copper response regulator1 (CRR1). However, the majority of genes that are differentially expressed in anaerobic Chlamydomonas cells are regulated CRR1 independently. While details on O2 sensing in Chlamydomonas remain elusive, principles and patterns begin to emerge, such as a possible influence of light and the involvement of nitric oxide (NO) as a signaling molecule. This chapter summarizes knowledge on the anoxic metabolism and its regulation in Chlamydomonas and hypothesizes on putative signaling pathways based on existing reports and observations.