Viability and niche segregation of Prochlorococcus and Synechococcus cells across the Central Atlantic Ocean

A membrane permeability test (cell digestion assay) was used to quantify the proportion of living cells (%LC) in the natural populations of Synechococcus and Prochlorococcus across the Central Atlantic Ocean. %LC varied greatly from 48 to 100% for Prochlorococcus and from 15 to 100% for Synechococcus. The %LC was smaller in surface (5 to 30 m) waters of the South Atlantic subtropical gyre, where lower values were observed for Prochlorococcus (64 +/- 4 %, mean +/- SE) than for Synechococcus (74 6 %). The %LC increased in the North Atlantic. This is probably associated with the nutrient enrichment by the equatorial upwelling, with Synechococcus showing a more significant response than Prochlorococcus. The vertical distribution of %LC differed between the 2 picocyanobacteria groups, with the maximal %LC of Synechococcus found at shallower depths (i.e. those receiving 25 % light) than that of Prochlorococcus. Prochlorococcus showed increased viability as light decreased, suggesting a lower resistance to high light and probably a greater capacity to survive at low irradiance than Synechococcus. These contrasting vertical and spatial distributions of picocyanobacteria %LC suggest that these organisms have differing optimal niches in the Central Atlantic, segregated by their different sensitivity to stress conditions. The results indicate that Prochlorococcus experiences higher cell death than Synechococcus in the Central Atlantic Ocean. This study identifies cell death as a process structuring phytoplankton communities, which is likely to affect the carbon flow in the Central Atlantic Ocean.