FACTORS AFFECTING THE DISTRIBUTION AND ABUNDANCE OF AN UNPIGMENTED HETEROTROPHIC ALGA PROTOTHECA-RICHARDSI

1. Numbers of the contramensal alga Prototheca richardsi were high in spring in two ponds used for breeding by anuran amphibians, but lower at other times of year and undetectable in two ponds not used by anura ns. 2. Prototheca richardsi became abundant in the silt of eight exper imental ponds which contained tadpoles, but remained undectable in fou r otherwise identical ponds lacking tadpoles. 3. Prototheca richardsi numbers in laboratory microcosms remained stable for many days in ster ile tap water, but declined with a half-life of about 6 days in pond w ater at 20-degrees-C. 4. Further studies with microcosms using antibio tics and electron microscopy indicated that mortality of P. richardsi was caused primarily by pathogenic bacteria.