BACKGROUND: Astaxanthin is a chemical used in aquaculture, pharmaceuticals, foods, and cosmetics. Although Haematococcus pluvialis has been studied intensively as an astaxanthin producer, its cultivation requires stringent contamination control and nutrition levels, thus rendering astaxanthin production energy-consuming. In the present study, the medium composition for cultivation of Monoraphidium sp. GK12, a newly isolated astaxanthin-producing microalga, was improved and photoautotrophic cultivation was attempted to examine its potential for application to energy-saving astaxanthin production. RESULTS: A cost-saving medium composed only of mineral salts was found to be suitable for photoautotrophic GK12 cultivation. A specific growth rate of 0.57 day(-1) was obtained, which is higher than that of H. pluvialis. A high cell density of around 2.0 x 10(7) cells mL(-1) was accomplished in outdoor batch cultivation, while outgrowth of contaminants was not observed. GK12 was also cultivated in a continuous photobioreactor, and the introduction of 1% CO2 stimulated GK12 growth and astaxanthin productivity. CONCLUSION: Control of temperature, illumination, and culture pH is unnecessary for GK12 outdoor cultivation and no outgrowth of contaminants was observed, suggesting an advantage over H. pluvialis outdoor cultivation. Besides improving Haematococcus cultivation methods, research on GK12 as an alternative astaxanthin-producer may contribute to the establishment of energy-saving astaxanthin production. (C) 2008 Society of Chemical Industry