Differential distribution and ecology of Prochlorococcus and Synechococcus in oceanic waters: a review

Photosynthetic prokaryotes evolving oxygen are a major component of oceanic ecosystems. The diversity of these micro-organisms in the picoplanktonic fraction of open oceans appears to be very limited, since they are represented almost exclusively by two genera: Synechococcus and Prochlorococcus. Synechococcus is virtually ubiquitous in all marine environments with concentrations ranging from 5 × 10 2 to 1.5 × 10 6 cell ml - (or 0.01 to > 1.3 × 10 9 cells per cm -2 ). It is much more abundant in nutrient-rich than in oligotrophic areas and its distribution is generally restricted to the upper well-lit layer. Synechococcus has also been reported at fairly high abundances from environments with low salinities and (or) low temperatures. In contrast, Prochlorococcus appears to be less ubiquitous. Although it is by far the most abundant group in the central oligotrophic part of oceans, with concentrations of 1-4 10 5 cell ml -1 (or 1 to 4 10 9 cells per cm -2 ), cell numbers drop dramatically at latitudes higher than 45°N. Moreover, this organism is generally absent from brackish or well-mixed waters. Nevertheless, it is not strictly restricted to oligotrophic areas, but can be found in mesotrophic conditions as well. In the Equatorial Pacific, for instance, there are no drastic changes of Prochlorococcus abundance between the oligotrophic warm pool and the high nutrient-low chlorophyll (HNLC) areas. Another peculiarity of this organism is its ability to colonize the water column down to depths of 150-200 m, which are reached by less than 0.1% of the surface irradiance. Thus, although Synechococcus and Prochlorococcus often co-occur, they have different types of adaptation with regard to biogeochemical conditions. Recent literature provides some clues about the biological and genetic bases of these differential behaviours. In terms of carbon biomass, Prochlorococcus, despite its narrower geographical distribution, seems to be more important than Synechococcus on a global scale since it is ca. 100 times more abundant (i.e. account for about 22 times more C) in warm oligotrophic areas, which correspond to a major part of the world ocean. An estimation of their relative contributions to the carbon biomass in the northern Atlantic ocean is proposed.