Abstract
Recent discoveries of minute coccoid cyanobacteria as ubiquitous and previously unrecognized components of the photosynthetic organisms of the oceans have raised important questions about their role in the primary productivity of the sea. We have identified photosynthetic characteristics of two strains of Synechococcus sp. which will determine their role in primary productivity. Photosynthesis and growth rate of these cyanobacteria saturate at very low irradiances. Their photosynthesis is sensitive to oxygen, suggesting a tendency for photorespiration. In natural phytoplankton populations, photosynthesis by cells µm contributed more to productivity at lower than at higher light intensities. Cyanobacteria, therefore, might be expected to make a significant contribution to phytoplankton photosynthesis at the bottom of the euphotic zone.