SUMMARY: requires light intensities < 300 foot-candles and grows at 10°-33°. The medium developed for is representative of the early types of artificial media having a relatively high concentration of chelator and, necessarily, of trace metals. Other media, varying widely in trace metal: chelator ratios and concentrations allow growth of . Tris 2-amino-2-hydroxymethylpropane-1:3-diol is a suitable pH buffer; its antagonism of potassium utilization was demonstrated and was amply overcome by the addition of 200 mg. K/l. to the medium. Inorganic and organic phosphates were utilized and were required at relatively low concentrations. utilized NO and NH but not atmospheric nitrogen. NaNO was the most satisfactory inorganic nitrogen source but the cultures bleached after prolonged growth. Asparagine (0·2-0·8 g./l.) maintained the normal purple pigmentation over a longer period; it served as a non-toxic slowly-utilized reservoir of available N. Vitamin B is the only essential vitamin and is needed at a concentration (100 mμg./l.) which is higher than for most algae. All the cobalamins including factor B, but not methionine or deoxyribosides, can substitute for vitamin B. The physiological and ecological resemblances of to red or violet blue-green algae growing in deep water are discussed.


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