The Thermophilic Cyanophytes of Iceland and the Upper Temperature Limit¹

The upper temperature limit for the growth of chlorophyllous cyanophytes was estimated at about 63-64 C by observations of field populations and by the growth of culture isolates at that temperature. Several isolates of Mastigocladus laminosus were made from different Iceland springs and temperatures. A few continued to grow at 62 C and one at 64 C. M. laminosus was the cyanophyte in Iceland tolerant of the highest temperature. The several species or races of narrow-cell-form Synechococcus, which almost exclusively inhabit North American and other springs above 62 C and up to about 74 C, were not found in Iceland. According to my observations and those of others, a maximum of 6-8 cyanophyte species occur in waters over 45 C in all of Iceland. Many times this number has been reported from continental regions of great thermal activity. The low number of species and the absence of particular thermophilic cyanophytes may result from inadequate dispersal from other thermal areas since the end of the last glaciation. Environmental inadequacies (e.g., winter darkness, or minimum-maximum concentrations of nutrients or other solutes) may also be factors in the absence of certain species.