RESPONSES OF THE ACIDOPHILIC ALGA EUGLENA MUTABILIS (EUGLENOPHYCEAE) TO CARBON ENRICHMENT AT pH 3 1

A defined medium (MAM) simulating acid mine drainage waters was developed which supported reproducible growth rates of three axenic strains of Euglena mutabilis Schmitz. Growth responses to various pHs and carbon sources were examined under defined culture conditions. A lab strain and two 5eld isolates, tested over pH range 1.5‐9.0, grew best under acidic conditions (pH < 5.5) with highest growth rates at pH 3‐4. Photoauxotrophic growth rates of all strains at pH 3 were improved significantly over unstirred batch controls by bubbling with air and even more by enrichment with 5% CO 2 in air. These results confirmed inorganic carbon limitation in batch culture. Organic carbon substrates were tested as possible carbon supplements in batch culture at pH 3. None of the strains survived in the dark on any of the twenty organic sources added. In the light, the lab strain exhibited some photoheterotrophic growth potential on glucose, sucrose, ethanol, and amino acids but growth was inhibited by acetate. Field strains showed little or no growth improvement with any organic substrate addition. Under simultaneous enrichment with acetate and 5% CO 2 acetate continued to be inhibitory. Simultaneous enrichment with glucose and 5% CO 2 gave higher yields of the lab strain than with CO 2 alone but did not enhance growth of the field strain. We conclude that E. mutabilis is an acidophilic photoauxotroph which appears unable to use organic carbon supplements for growth even under conditions of carbon limitation.