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:
- 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
- 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.
- 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.
- 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.