Spectacular abundance of ciliates in anoxic pond water: contribution of symbiont photosynthesis to host respiratory oxygen requirements

Abstract Very large numbers (3466 ml−1) of ciliated protozoa were found living beneath the oxic-anoxic boundary in a stratified freshwater pond. Most ciliates (96%) contained symbiotic algae (Chlorella spp.). Peak abundance was in anoxic water with almost 1 mol free CO2 m−3 and a midday irradiance of 6 μmol photon m−2 s−1. Photosynthetic rate measurements of metalimnetic water indicated a light compensation point of 1.7 μmol photon m−2 s−1 which represents 0.6% of sub-surface light. We calculate that photosynthetic evolution of O2 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.