Synechococcus
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. Introduction. "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, 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". 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. 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 and a novel uncharacterized, nonphototactic swimming method 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 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 (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” (Racharaks et al, 2019). Pigments. The main photosynthetic pigment in "Synechococcus" is chlorophyll a, while its major accessory pigments are phycobiliprotein. The  four commonly recognized phycobilins are phycocyanin, allophycocyanin, allophycocyanin B and phycoerythrin. 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. Phylogeny. Phylogenetic description of "Synechococcus" is difficult. Isolates are morphologically very similar, yet exhibit a G+C content ranging from 39 to 71%, illustrating the large genetic diversity of this provisional taxon. Initially, attempts were made to divide the group into three subclusters, each with a specific  range of genomic G+C content. The observation that open-ocean isolates alone nearly span the complete G+C spectrum, however, indicates that "Synechococcus" is composed of at least several species. "Bergey's Manual" (Herdman "et al." 2001) now divides "Synechococcus" into five clusters (equivalent to genera) based on morphology, physiology, and genetic traits. Cluster 1 includes relatively large (1–1.5 µm) nonmotile obligate photoautotrophs that exhibit low salt tolerance. Reference strains for this cluster are PCC6301 (formerly "Anacycstis nidulans") and PCC6312, which were isolated from fresh water in Texas and California, respectively. Cluster 2 also is characterized by low salt tolerance. Cells  are obligate photoautrotrophs, lack phycoerythrin, and are thermophilic. The reference strain PCC6715 was isolated from a hot spring in Yellowstone National Park. Cluster 3 includes phycoerythrin-lacking marine "Synechococcus" species that are euryhaline, i.e. capable of growth in both marine and freshwater environments. Several strains, including the reference strain PCC7003, are facultative heterotrophs and require vitamin B for growth. Cluster 4 contains a single isolate, PCC7335. This strain is obligate marine. This strain contains phycoerthrin and was first isolated from the intertidal zone in Puerto Peñasco, Mexico. The last cluster contains what had previously been referred to as ‘marine A  and B clusters’ of "Synechococcus". These cells are truly marine and have been isolated from both the coastal and the open ocean. All strains are obligate photoautrophs and are around 0.6–1.7 µm in diameter. This cluster is, however, further divided into a population that either contains (cluster 5.1) or does not contain (cluster 5.2) phycoerythrin. The reference strains are WH8103 for the phycoerythrin-containing strains and WH5701 for those strains that lack this pigment. More recently, Badger "et al." (2002) proposed the division of the cyanobacteria into a α- and a β-subcluster based on the type of rbcL (large  subunit of ribulose 1,5-bisphosphate carboxylase/oxygenase) found in these organisms. α-cyanobacteria were defined to contain a form IA, while β-cyanobacteria were defined to contain a form IB of this gene. In support for this division Badger "et al." analyze the phylogeny of carboxysomal proteins, which appear to support this division. Also, two particular bicarbonate transport systems appear to only be found in α-cyanobacteria, which lack carboxysomal carbonic anhydrases. The complete phylogenetic tree of 16S rRNA sequences of "Synechococcus" revealed at least 12 groups, which morphologically correspond to "Synechococcus", but they have not derived from the common  ancestor. Moreover, it has been estimated based on molecular dating that the first "Synechococcus" lineage has appeared 3 billion years ago in thermal springs with subsequent radiation to marine and freshwater environments. Ecology and distribution. "Synechococcus" has been observed to occur at concentrations ranging between a few cells to 10 cells per ml in virtually all regions of oceanic euphotic zone except in samples from the McMurdo Sound and Ross Ice Shelf in Antarctica. Cells are generally much more abundant in nutrient-rich environments than in the oligotrophic ocean and prefer the upper, well-lit portion of the euphotic zone . "Synechococcus" has also been observed to occur at high abundances in environments with low salinities and/or low temperatures. It is usually far outnumbered by "Prochlorococcus" in all environments where they co-occur. Exceptions to this rule are areas of permanently enriched nutrients such as upwelling areas and coastal watersheds. In the nutrient-depleted areas of the oceans, such as the central gyres, "Synechococcus" is apparently always present, although only at low concentrations, ranging from a few to 4×10³ cells per ml. Vertically "Synechococcus" is usually relatively equitably distributed throughout the mixed layer and exhibits an affinity for the higher-light  areas. Below the mixed layer, cell concentrations rapidly decline. Vertical profiles are strongly influenced by hydrologic conditions and can be very variable both seasonally and spatially. Overall, "Synechococcus" abundance often parallels that of "Prochlorococcus" in the water column. In the Pacific high-nutrient, low-chlorophyll zone and in temperate open seas where stratification was recently established both profiles parallel each other and exhibit abundance maxima just about the subsurface chlorophyll maximum. The factors controlling the abundance of "Synechococcus" still remain poorly understood, especially considering that even in the most nutrient-depleted regions of the central gyres, where cell abundances are  often very low, population growth rates are often high and not drastically limited. Factors such as grazing, viral mortality, genetic variability, light adaptation, and temperature, as well as nutrients are certainly involved, but remain to be investigated on a rigorous and global scale. Despite the uncertainties, a relationship probably exists between ambient nitrogen concentrations and "Synechococcus" abundance, with an inverse relationship to "Prochlorococcus" in the upper euphotic zone, where light is not limiting. One environment where "Synechococcus" thrives particularly well is coastal plumes of major rivers. Such plumes are coastally enriched with nutrients such as nitrate and phosphate, which drives  large phytoplankton blooms. High productivity in coastal river plumes is often associated with large populations of "Synechococcus" and elevated form IA (cyanobacterial) "rbcL" mRNA. "Prochlorococcus" is thought to be at least 100 times more abundant than "Synechococcus" in warm oligotrophic waters. Assuming average cellular carbon concentrations, it has thus been estimated that "Prochlorococcus" accounts for at least 22 times more carbon in these waters, thus may be of much greater significance to the global carbon cycle than "Synechococcus".