Research Summary

Since joining the faculty of Biology at the University of Oregon, Dr. Castenholz has been active in research with support mainly from the NSF and NASA. After working on the seasonal ecology of freshwater and marine diatoms, he switched in the middle 1960's, almost completely, to the community ecology and physiological ecology of photosynthetic bacteria of hot springs, cyanobacteria primarily, but also non-oxygen producing phototrophs with the discovery in his lab of the new genera, Chloroflexus and Heliothrix which became the basis for a new phylum, the Chloroflexi.

Later research developed into a study of phototrophic prokaryotes in microbial mats of hypersaline marine and freshwater habitats, including Antarctic mats dominated by cyanobacteria. The research questions asked include: How do various microorganisms tolerate and adapt to environmental extremes? These extremes include high and low temperature, low pH, high salinity and desiccation, toxic levels of sulfide, and high solar irradiance. A long-term project has been an evaluation of the adaptive value of responses by cyanobacteria to high UV-radiation, in the context of natural habitats. In these studies, the Castenholz lab team has identified and characterized scytonemin, a UV-sunscreen pigment in sheaths surrounding many highly exposed cyanobacteria, and they have demonstrated the importance of this compound in increasing fitness. They have also shown a vital, vertical escape response of motile cyanobacteria to UV radiation in microbial mats, both in temperate and polar environments.

Throughout the last 25 years, Dr. Castenholz has also been involved in trying to unravel the confusion in cyanobacterial taxonomy and classification. He served as co-editor and coordinator for the phototrophic sections and a co-author of portions of this subject in Volume One of the second edition of the Bergey's Manual of Systematic Bacteriology (2001). In addition, he has established a culture collection of cyanobacteria from many locations and habitats, including a large number of isolates from hot springs (alkaline and acidic) over much of the globe, from hypersaline waters, and from polar freshwater habitats. The culture collection now contains over 1,200 strains and is named the Culture Collection of Microorganisms from Extreme Environments (CCMEE).

Research in the Castenholz Lab is currently focused on three areas:

1. The ecological distribution and ecotypic diversity of strains of the thermo-acidophilic unicellular red algae of the Order Cyanidiales within diverse habitats in Yellowstone National Park, contrasting these with those strains we have isolated from Japan, New Zealand, and Iceland. The differentiation of ecotypes is now based on sequences of a few genes and on several physiological characteristics. The main objective is to correlate ecotypes of this group and their heterotrophic associates with specific types of acidic waters and habitats that differ greatly in the content of various metals (e.g. Hg, Fe, Cu, Zn, Al) and metalloids (As). The chemistry of many of these springs have been analyzed in detail by another study and the data are available
2. The distribution and physiological properties of endolithic cyanobacteria and microalgae from within travertine rocks deposited by former springs within Yellowstone National Park and adjacent lands, ranging in extinction age from less than 10 to over 350,000 years with comparisons of phototrophic community members from ancient travertine deposits in California, Nevada, and northern Mexico.
3. The tolerance of the sheathed, scytonemin-rich cyanobacterium, Lyngbya aestaurii to desiccation under extreme conditions of hypersalinity and high solar irradiance and the rapidity of resuming photosynthetic capacity after rehydration under various light intensities and salinities.
4. The enhancement of scytonemin synthesis in selected cyanobacteria by various stress factors in addition to UV radiation, one of the principal being enforced nitrogen-fixation that requires a large energy expenditure.