OXYGENIC PHOTOSYNTHESIS IS PERHAPS THE MOST TRANSFORMATIVE BIOENERGETIC INNOVATION IN THE HISTORY OF LIFE AS THE ACTIVITY THAT FUNDAMENTALLY CHANGED THE PLANET. THE CARBON CYCLE WAS STIMULATED BY AN INCREASE IN PRIMARY PRODUCTIVITY RESULTING IN CASCADES OF EVOLUTIONARY INNOVATIONS. O2 BECAME WIDELY AVAILABLE FOR UTILIZATION AS AN OXIDANT IN ENERGY METABOLISM AND IN DIVERSE ANABOLIC AND CATABOLIC PATHWAYS. GEOCHEMICAL CYCLES WERE ALSO GREATLY ALTERED LEADING TO THE ACCUMULATION OF OTHER OXIDANTS IN THE ATMOSPHERE AND OCEANS. THE AVAILABILITY OF O2 ULTIMATELY LED TO AEROBIC RESPIRATION EUKARYOTIC ENDOSYMBIOSIS AND COMPLEX MULTICELLULARITY. NOT SURPRISINGLY THE PRESENCE OF ABUNDANT ATMOSPHERIC O2 FORMS PART OF OUR SEARCH IMAGE FOR LIFE ON PLANETS OUTSIDE OF OUR SOLAR SYSTEM.ALL EVIDENCE SUPPORTS THE THEORY THAT OXYGENIC PHOTOSYNTHESIS EVOLVED BUT ONCE IN AN ANOXYGENIC ANCESTOR OF MODERN CYANOBACTERIA. HOWEVER DETAILS OF THE EVOLUTIONARY STEPS TAKEN DURING THE RISE OF OXYGENIC PHOTOSYNTHESIS REMAIN A MYSTERY. A BETTER UNDERSTANDING OF THE NATURAL HISTORY OF THE PHYLUM MARKS A PATH FORWARD. ARE LINEAGES OF BACTERIA THAT SHARE THE SAME PRE-PHOTOTROPHIC ANCESTOR AS THE CYANOBACTERIA EXTANT TODAY? MIGHT BEHAVIOR OF THESE CELLS INFORM THE PHYSIOLOGICAL PLATFORM UPON WHICH OXYGENIC PHOTOTROPHY EVOLVED?ENVIRONMENTAL SEQUENCING EFFORTS ILLUSTRATE THAT CYANOBACTERIA ARE MORE DIVERSE THAN HISTORICALLY APPRECIATED. SEVERAL UNCHARACTERIZED CLADES OF DEEP BRANCHING FORMS EXIST; MOREOVER THESE OCCUR IN PERENNIALLY DARK AND OFTEN ANOXIC ENVIRONMENTS WHICH SUGGESTS THAT THE CORRESPONDING ORGANISMS ARE NOT OXYPHOTOTROPHS (1). RECENT GENOME ANALYSES ON UNCULTIVATED REPRESENTATIVES OF ONE OF THESE 'DARK CYANOBACTERIA' CLADES (TERMED MELAINABACTERIA) REVEALED NO GENES ASSOCIATED WITH PHOTOSYNTHESIS (2 3); NEARLY ALL LACKED GENES FOR AEROBIC OR ANAEROBIC RESPIRATION. THUS MELAINABACTERIA AND OTHER NEWLY IDENTIFIED CLADES OF 'DARK CYANOBACTERIA' PROVIDE IMPORTANT INSIGHTS INTO THE ORGANISMS FROM WHICH OXYGENIC PHOTOSYNTHESIS AROSE.PILOT RESEARCH AT CALTECH IDENTIFIED AN ENVIRONMENT WHEREIN SEVERAL DIFFERENT LINEAGES OF DARK CYANOBACTERIA ARE RELIABLY AND REPRODUCIBLY RESIDING: THE COMPLEX FERMENTATION-DRIVEN GUT MICROBIAL COMMUNITIES OF TERMITES. THIS MODEL ENVIRONMENT IS PARTICULARLY ATTRACTIVE FOR LABORATORY-BASED STUDIES BECAUSE IT IS A MICROLITER IN VOLUME WELL-BOUNDED AVAILABLE IN LARGE NUMBERS OF REPLICATES AND STRAIGHTFORWARD TO MAINTAIN IN THE LABORATORY. PILOT EXPERIMENTS REVEAL THAT A SINGLE TERMITE HINDGUT CONTAINS AT LEAST FOUR INDEPENDENT CLADES OF THE DARK CYANOBACTERIA - THE HIGHEST AND DEEPEST DIVERSITY OBSERVED IN ANY ENVIRONMENT TO DATE. WE PROPOSE A DETAILED AND MULTIDISCIPLINARY STUDY OF THESE DARK CYANOBACTERIA USING ENVIRONMENTAL PERTURBATIONS COMPARATIVE METAGENOMICS AND TRANSCRIPTOMICS FISH AND OTHER MICROSCALE LOCALIZATION APPROACHES AS WELL AS PHYSIOLOGICAL LINES. COMPARATIVE GENEBASED ANALYSES OF DARK CYANOBACTERIAL DIVERSITY IN DIFFERENT TERMITE SPECIES REPRESENTING DISTINCT INSECT FAMILY LINEAGES WILL ALSO BE PERFORMED OFFERING A VIEW INTO EVOLUTIONARY RATES AND PATTERNS OF DIVERSIFICATION IN DARK CYANOBACTERIA [I.E. IN THE CONTEXT OF THE 100+ MILLION YEAR FOSSIL RECORD OF TERMITES AND OTHER METADATA ON THE HOSTS (4)].THE PROPOSED SCOPE FITS SEVERAL PROGRAM OBJECTIVES. FOR EXAMPLE EARLY EVOLUTION OF LIFE AND THE BIOSPHERE AIMS TO 'UNDERSTAND THE PHYLOGENY AND PHYSIOLOGY OF MICROORGANISM - WHOSE CHARACTERISTICS MAY REFLECT THE NATURE OF PRIMITIVE ENVIRONMENTS ' 'INVESTIGATE THE DEVELOPMENT OF KEY BIOLOGICAL PROCESSES AND THEIR ENVIRONMENTAL IMPACT ' AND 'INVESTIGATE THE EVOLUTION OF GENES PATHWAYS AND MICROBIAL SPECIES SUBJECT TO LONG-TERM ENVIRONMENTAL CHANGE.'1. F. WARNECKE ET AL. NATURE 450 560-565 (2007).2. S. C. DI RIENZI ET AL. ELIFE 2:E01102 (2013).3. R. M. SOO ET AL. GENOME BIOL EVOL 6 1031 (2014).4. X. ZHANG J.R. LEADBETTER (2012) MBIO 3:E00223-12 (2012).