Massive accumulation of ketocarotenoids in zygospores of Chlamydomonas reinhardtii

Sonja Werner, Matthias Bauch, Armin Hallmann1, Judith Köhler, Georg Schmidt, and Martin Lohr

Institute of General Botany, Johannes Gutenberg-University Mainz, Germany; and 1Dpt. of Cellular and Developmental Biology of Plants, University of Bielefeld, Germany

The carotenoid pattern of vegetative cells of C. reinhardtii comprising lutein, loroxanthin, violaxanthin, neoxanthin, and beta-carotene, has been known for decades. Mature zygospores of C. reinhardtii display a bright orange color which is likely due to a massive accumulation of carotenoids. Yet, there have been no reports on the pigment composition of zygotes. The recent discovery of a gene in the C. reinhardtii genome encoding a protein with homology to the beta-carotene ketolase (BKT) from the well-known astaxanthin producer Haematococcus pluvialis suggested that C. reinhardtii may be able to synthesize astaxanthin and other ketocarotenoids and that these pigments might accumulate specifically in the mature zygospore to protect this resting stage from (photo-)oxidative damage. Here we present experimental proof of this conjecture and show that, contrary to H. pluvialis, the major ketocarotenoid in C. reinhardtii is not astaxanthin but fritschiellaxanthin which is derived from lutein. Furthermore, we demonstrate ketolase activity of the putative BKT from C. reinhardtii by heterologous expression in E. coli and show data on the influence of abiotic stress on ketocarotenoid accumulation in the zygospores. We also investigated the pigment pattern of zygospores from two other green algae, Volvox carteri and Hydrodictyon reticulatum, and found that they accumulate massive amounts of ketocarotenoids as well. This suggests that the accumulation of secondary carotenoids in green algae as a response to adverse environmental conditions is a widespread phenomenon and may even be a general feature of chlorophytes.

e-mail address of presenting author: lohr@uni-mainz.de