Increased growth and pigment content of Chromera velia in mixotrophic culture

The alveolate microalga Chromera velia is an evolutionarily significant organism, representing the closest photosynthetic relative of the parasitic Apicomplexa. Chromera velia has been detected in and isolated from several stony corals and can be readily cultured in vitro under strictly autotrophic conditions. However, little is known about the ecology of this organism in the coral holobiont, an environment in which it could potentially access abundant organic carbon sources. To understand the response of C. velia to ecologically relevant organic compounds in vitro, we tested a mixotrophic culture strategy by supplementing inorganic f-medium with sugars, sugar-alcohols, organic acids and amino acids. For 15 of the 18 tested growth media, culture growth rate was significantly higher than that of strictly autotrophic cultures, and in three of these, a significant increase in maximum culture density was observed. In cultures supplemented with glutamate or glycine, the chlorophyll content per cell was up to 11-fold higher than cultures grown in standard inorganic media. Together, the in vitro culture growth and pigment responses demonstrate an ability to respond to nutritional resources when available. We propose that C. velia is a facultative opportunist in environments similarly enriched in such organic compounds, such as the coral holobiont.