Dynamics of Cyanobacterial and Ostracod Interactions in an Oregon Hot Spring

Abstract

Microbial mat communities are arrayed along a hot spring outflow as a result of biotic and abiotic requirements and/or antagonisms. It is generally assumed that temperature is the ultimate determined of species distributions and growth, hence of community composition and biomass. This assumption was tested through experimentation and detailed seasonal observations of mat communities in s southern Oregon hot spring. The microbial community most extensively studied was dominated by two species of cyanobacteria (blue—green algae), Calothrix thermalis and Pleurocapsa minor. These species comprised >70% of the community biomass and formed a distinctive crustose, nodular mat. This mat occupied the 35°—40° to 48°—51°C zone along the thermogradient of Fenceline Stream (Hunter's Hot Springs complex), as in many other thermal springs of Oregon, Nevada, and California. The geographical distribution of the nondular mat closely paralleled that of the thermophilic ostracod Potamocypris sp. Contrary to predictions, the lower border of the Fenceline nodular mat extended farther downstream in winter than in summer, and standing crop biomass was greater in winter. During all seasons the distribution of the nodular species was truncated with respect to their potential temperature ranges. Observations of ostracod behavior and population dispersion patterns, as well as results from temperature—effect and grazing experiments, indicated that the ostracods were responsible for the observed mat anomalies. The dynamics of the ostracod population and the grazing pressure exerted by this population were influenced by diel and cumulative (i.e., seasonal) thermogradient extensions and retractions. During diel cooling periods, ostracods controlled the upstream border of the Pleurocapsa Calothrix mat by grazing back the faster growing but less resistant Synechococcus—Chloroflexus mat. During autumnal cooling, the herbivore population dispersed upstream as food resources became available, thereby decreasing the grazing pressure over the lower portions of the nodular mat. This resulted in increased biomass and downstream growth of the nodular mat until it reached a winter maximum. Vernal thermogradient lengthening again changed the grazing range of the ostracod populations, which subsequently increased the grazing pressur over the Pleurocapsa—Calothrix mat. The coalesced nodular mat was reduced to patchy nodules and the lower border gradually shifted upstream, thus completing the atypical seasonal cycle. A grazer—induced dynamic equilibrium prevented exclusion of Pleurocapsa by the faster growing Calothrix, as the latter organism was easily grazed yet gained refuge within the partially refractory masses of Pleurocapsa cells. The persistence of the Pleurocapsa Calothrix mat community in hot springs of the area is grazer dependent, since without Potamocypris the temperature zone inhabited by the nodule species becomes overgrown by Synechococcus or other cyanobacteria