INHIBITION OF PHOTOSYNTHESIS IN PHYTOPLANKTON BY THE SUBMICRON SIZE FRACTION CONCENTRATED FROM SEAWATER

Ultrafiltration was used to concentrate the 2-200 nm size fraction from seawater samples 100- to 1000-fold. Electron microscopy indicated that these concentrates were heavily enriched with virus-like particles. When aliquots from these concentrates were added to natural seawater samples, primary productivity (C-14-bicarbonate uptake) was either little affected or suppressed within minutes by up to 78%. In some instances as little as a 20% increase in the concentration of the 2-200 nm size fraction in seawater reduced relative carbon fixation rates by nearly 50 %. Autoclaving the concentrates before addition reduced or eliminated the bioactive effect. Larger additions caused greater inhibition, although a point was reached where further additions of concentrate did not result in greater photosynthetic suppression, relative to samples to which autoclaved concentrate was added. Microauto-radiography indicated that there were fewer photosynthetically-active cells in seawater samples to which untreated concentrates were added, when compared to samples receiving autoclaved concentrate. Phytoplankton > 3 mum were affected the most. This indicated that specific organisms were affected as opposed to photosynthesis being reduced throughout the community. In vivo chlorophyll fluorescence paralleled changes in carbon incorporation, and increased most slowly when the greatest additions of concentrate were made. Nonetheless, growth of phytoplankton recovered several days after the concentrates were added, suggesting either that some cells were unaffected by the addition or that resistance to the bioactive agent(s) developed over time. The data provide further evidence for the presence of a strongly bioactive component that inhibits primary productivity, which is associated with the virus-rich 2-200 nm size fraction of seawater.