9630674 Castenholz The goals of this project are to evaluate the negative effects of ultra-violet (UV) radiation on photosynthesis, growth, and health of cyanobacteria and to determine the methods used by these microorganisms to survive UV stress. Cyanobacteria (also called blue-green algae) are the closest living relatives of those oxygen-producing microbes that built the Precambrian stromatolites (1 -3 billion yrs ago) and that accounted eventually for the high oxygen values in the Earth's atmosphere. The investigators are asking the question of what biological "strategies" these microorganisms use to tolerate current UV stress levels and the higher levels which would have been prevalent during this strictly microbial period of the Earth's history. They have been working on avoidance responses by motile cells and also on the effect of UV "sunscreen" pigments, one of which (scytonemin), is located in a sheath or capsule surrounding the cells of many species. The investigators have been able to characterize this yellow pigment and experimentally show its great effectiveness in allowing cells to tolerate high UV levels. This pigment enhances survival of cyanobacteria under the high solar radiation of their natural habitats and may have provided the protection for their growth, colonization, and evolution during the Precambrian. By leaming how specific organisms protect themselves from this harmful short-wave radiation, we may also better understand the nature of UV damage in other organisms. Basic research of the type we are doing may lead to applications in human health as well. For example, as a result of previous work by the principal invesigator, scytonemin is now undergoing clinical tests in another laboratory as a compound with highly effective anti-inflammatory properties. In the continuation of this work with cyanobacteria the investig ators hope to further understand the role of scytonemin in UV protection in both actively growing and dormant cells, and the role of various environmental factors in the regulation of scytonemin synthesis. In addition, the investigators intend to determine the extent of metabolic repair processes required for UV tolerance in cyanobacteria that do not possess UV screening pigments. In this line of experimentation the investigators will also assay the degree of damage to cells caused by "reactive oxygen" (e.g. singlet oxygen) which may also be produced in cells as a result of UV absorption. If this secondary effect of UV is substantial, we will determine the role of cell carotenoid pigments (e.g. beta-carotene) in quenching this form of damage.