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 3,008,047 bp, whereas the oceanic strain WH8102 has a genome of size 2.4 Mbp.[1][2][3][citation needed]
Synechococcus is one of the most important components of the prokaryotic autotrophic picoplankton in the temperate to tropical oceans. The genus was first described in 1979,[4][5] and was originally defined to include "small unicellular cyanobacteria with ovoid to cylindrical cells that reproduce by binary traverse fission in a single plane and lack sheaths".[6] This definition of the genus Synechococcus contained organisms of considerable genetic diversity and was later subdivided into subgroups based on the presence of the accessory pigment phycoerythrin. The marine forms of Synechococcus are coccoid cells between 0.6 and 1.6 µm in size. They are Gram-negative cells with highly structured cell walls that may contain projections on their surface.[7] Electron microscopy frequently reveals the presence of phosphate inclusions, glycogen granules, and more importantly, highly structured carboxysomes.
Cells are known to be motile by a gliding method[8] and a novel uncharacterized, nonphototactic swimming method[9] that does not involve flagellar motion. While some cyanobacteria are capable of photoheterotrophic or even chemoheterotrophic growth, all marine Synechococcus strains appear to be obligate photoautotrophs[10] that are capable of supporting their nitrogen requirements using nitrate, ammonia, or in some cases urea as a sole nitrogen source. Marine Synechococcus species are traditionally not thought to fix nitrogen.
In the last decade, several strains of Synechococcus elongatus have been produced in laboratory environments to include the fastest growing cyanobacteria to date, Synechococcus elongatus UTEX 2973. S. elongatus UTEX 2973 is a mutant hybrid from UTEX 625 and is most closely related to S. elongatus PCC 7942 with 99.8% similarity[11] (Yu et al, 2015). It has the shortest doubling time at “1.9 hours in a BG11 medium at 41°C under continuous 500 μmoles photons·m−2·s−1 white light with 3% CO2”[12] (Racharaks et al, 2019).
The main photosynthetic pigment in Synechococcus is chlorophyll a, while its major accessory pigments are phycobiliprotein.[5] The four commonly recognized phycobilins are phycocyanin, allophycocyanin, allophycocyanin B and phycoerythrin.[13] In addition Synechococcus also contains zeaxanthin but no diagnostic pigment for this organism is known. Zeaxanthin is also found in Prochlorococcus, red algae and as a minor pigment in some chlorophytes and eustigmatophytes. Similarly, phycoerythrin is also found in rhodophytes and some cryptomonads.[10]