The small
coccoid genus Synechococcus (Figure 2d) is present in all
marine systems from the tropics to the poles, from
eutrophic to oligotrophic waters, while the atypical genus
Prochlorococcus (Figure 2e), which uses a modified chlorophyll
b as its main light-collecting pigment instead of
phycoerythrin, is probably the most numerous photosynthetic
organism on Earth (Partensky et al., 1999),
colonizing at high densities (105 cellsmL21) the entire
euphotic zone between 408N and 408S.