SPECTACULAR ABUNDANCE OF CILIATES IN ANOXIC POND WATER - CONTRIBUTIONOF SYMBIONT PHOTOSYNTHESIS TO HOST RESPIRATORY OXYGEN REQUIREMENTS

Very large numbers (3466 ml(-1)) of ciliated protozoa were found livin g beneath the oxic-anoxic boundary in a stratified freshwater pond. Mo st ciliates (96%) contained symbiotic algae (Chlorella spp.). Peak abu ndance was in anoxic water with almost 1 mol free CO2 m(-3) and a midd ay irradiance of 6 mu mol photon m(-2) s(-1). Photosynthetic rate meas urements of metalimnetic water indicated a light compensation point of 1.7 mu mol photon m(-2) s(-1) which represents 0.6% of sub-surface li ght. We calculate that photosynthetic evolution of O-2 by symbionts is sufficient to meet the demand of the host ciliates for 13 to 14 hours each day. Each photosynthetic ciliate may therefore become an aerobic island surrounded by anoxic water.