A strain of C. pulsatilla isolated from a supralittoral rock pool near St. Andrews, New Brunswick, Canada, grows over a wide range of salinities. Glycerol is the main osmoregulatory solute. It grows heterotrophically on acetate, and has a high affinity for this substrate (Km .apprx. 40 .mu.M). Acetate strongly enhances growth at low light intensities, but the flagellate can also grow photoautotrophically. Nitrate and urea cannot be used as sources of N for growth, while ammonium and several amino acids are excellent N sources. The alga is well adapted to withstand changes in salinity and to take advantage of reduced inorganic and organic N sources present in its rook pool habitat.