Unusual or Extreme Habitats - Freshwater Ecology: Concepts and Environmental Applications - page 300

celled Eukarya and the filamentous Fungi can withstand temperatures up
to 62°C. Photosynthetic bacterial primary producers can be found up
to 73°C.
Strains of individual microbial species can also be distributed along a
temperature gradient (Fig. 15.3). In this case, unicellular cyanobacteria that
appear identical under the microscope have adaptations to different tem-
perature optima and can dominate in a narrowly defined habitat. This
clearly illustrates that biochemical specialization is necessary for a strain to
compete successfully at individual temperatures. The extremely stable na-
ture of hot spring temperatures allows for these strains to dominate in the
narrow regions of their temperature optima.
An interesting case of organism distribution related to temperature has
been described for Hunter's Hot Spring in Oregon (Wickstrom and Cas-
tenholz, 1985). The spring leaves the ground at slightly less than boiling
and the water cools as it contacts the atmosphere and ground, creating a
gradient of decreasing temperature downstream. The cyanobacterium Syne-
chococcus dominates from 74 to 54°C because other primary producers
are unable to survive (Fig. 15.4). As the stream cools, the motile filamen-
tous cyanobacterium Oscillatoria terebriformis dominates, covering the
surface of the mat at moderate light levels and contracting to the margins
under very high light. As the stream cools further, the herbivorous ostra-
cod Potamocypris is able to survive. This algiverous thermophilic ostracod
can crop down Synechococcus and Oscillatoria allowing for the develop-
ment of a mixed leathery mat community of two cyanobacteria ( Pleuro-
capsa and Calothrix ) that are poorer competitors but are resistant to graz-
ing (Wickstrom and Castenholz, 1978). Such obvious effects of competition
and predation on community structure across a physical gradient are not
often observed in nature.

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30 40 50 60 70 80

Temperature ( ° C)

FIGURE 15.3 Growth curves of eight strains of Senecococcus isolated from different tem-
peratures in one hot spring [reprinted with permission from Nature (J. A. Peary and R. W.
Castenholz, Temperature strains of a thermophilic blue-green alga. Nature 5 (64), 720-721.
©1964 Macmillan Magazines Limited).