Cyanobacteria in Geothermal Habitats

Summary

Recent advances in molecular biology enabled a much more detailed view of cyanobacteria that inhabit well-studied hot spring habitats. What seemed on the basis of microscopy and culture methods to be a relatively simple story became one which ismore complex. yet more ordered. For example. 16S rRNA studies of one well-studied Yellowstonehot spring cyanobacterial mat. often thought to be constructed by a single cosmopolitan Synechococcus species. revealed many cyanobacterial populations. most probably Synechococcus spp., whose genetic diversity is considerable.Some closely related populations exhibited orderly distributions along thermal and vertical gradients. and provided evidence that “speciation” of thermophilic cyanobacteria may have resulted. in part, from adaptive radiation of specialized ecotypes. These results correspond with previous descriptions of Synechococcus temperature “strains” cultivated from a well-studied Oregon hot spring mat, raising interesting questions which may be answered through molecular analysis. For example, are the same cyanobacteria found in geographically isolated springs, or does limited dispersal also cause divergence ofcyanobacteria with subsequent adaptive radiations in independent lineages? Advances in microelectrode technology provided detailed views of the vertical distribution and dynamics of light, oxygen and sulfide within the photic zone in such mats, which may range from ~0.5 mm to > 1 cm in thickness. These methods increased our knowledge of diel chemical changes within the mats, including the basis for dynamic migrations of motile cyanobacteria. Microelectrode studies ofone Yellowstone mat provided additional evidence for specialization of Synechococcus populations, and led to anew view of oxygenic photosynthesis in such habitats. Intensive localized photosynthesis strongly influences microenvironmental chemistry, whichin turnenhances photorespiration. While very active, these Synechococcus populations partition the majority ofphotosynthate into polyglucose, as opposed to macromolecules needed for growth. Photoexcretion of glycolate and dark fermentation of polyglucose result in a diel cross feeding of most of the fixed carbon to heterotrophs. Thorough investigation of a few hot spring systems, using both contemporary and traditional methods, is providing a more sophisticated view of the biodiversity, ecology and evolution of thermophilic cyanobacteria.