Synechococcus (from the Greek synechos, in succession, and the Greek kokkos, granule) is a unicellular cyanobacterium that is very widespread in the marine environment. Its size varies from 0.8 to 1.5 μm. The photosynthetic coccoid cells are preferentially found in well–lit surface waters where it can be very abundant (generally 1,000 to 200,000 cells per ml). Many freshwater species of Synechococcus have also been described.

The genome of S. elongatus strain PCC7002 has a size of 3.4 Mbp, whereas the oceanic strain WH8102 has a genome of size 2.4 Mbp.

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