Nutritional characteristics of a mixotrophic nanoflagellate,Ochromonas sp.

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Abstract
Autotrophic and heterotrophic growth characteristics of a nano-flagellate were investigated. The flagellate,Ochromonas sp., was isolated from the northern Baltic Sea. Autotrophic growth was poor. Axenically pregrown flagellates did not increase significantly in cell number during incubation in different inorganic media. The number of flagellates remained constant 3–5 weeks in cultures kept in the light (100μmol m−2 sec−1), whereas in the dark, a high mortality rate was found. Uptake of inorganic14C into an acid-stable fraction indicated thatOchromonas had a functional photosynthetic apparatus. Heterotrophic growth in both liquid medium and medium containing bacteria was rapid. The maximum growth rate corresponded to a generation time of 5.3 hours. Light had no effect on heterotrophic growth. Cells pregrown onEscherichia coli minicells survived without additional bacteria as food when kept in the light, but rapid death occurred in darkness. In conclusion, heterotrophy is the major mechanism to support growth in this species ofOchromonas, but under poor environmental conditions photoautotrophy might be a strategy for survival rather than growth.