Synechococcus

Synechococcus
Synechococcus PCC 7002 cells in DIC microscopy
Scientific classification
Domain:	Bacteria
Phylum:	Cyanobacteria
Class:	Cyanophyceae
Order:	Synechococcales
Family:	Synechococcaceae
Genus:	Synechococcus Nägeli, 1849
Species
See text

Synechococcus (from the Greek synechos, in succession, and the Greek kokkos, granule) is a unicellular

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.^{[1][2][3][citation needed]}

Introduction

Synechococcus is one of the most important components of the

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).

Pigments

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]

Phylogeny

Phylogenetic description of Synechococcus is difficult. Isolates are morphologically very similar, yet exhibit a G+C content ranging from 39 to 71%,^[10] 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.^[14] 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.^[6] 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.^[15] 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 B₁₂ for growth. Cluster 4 contains a single isolate, PCC7335. This strain is obligate marine.^[16] This strain contains phycoerthrin and was first isolated from the intertidal zone in Puerto Peñasco, Mexico.^[6] 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.^[17]

More recently, Badger et al. (2002) proposed the division of the cyanobacteria into a α- and a β-subcluster based on the type of

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.^[19]

Ecology and distribution

Synechococcus has been observed to occur at concentrations ranging between a few cells to 10⁶ cells per ml in virtually all regions of oceanic

Prochlorococcus is thought to be at least 100 times more abundant than Synechococcus in warm oligotrophic waters.[20] 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.

Evolutionary history

Free floating viruses have been found carrying photosynthetic genes, and Synechococcus samples have been found to have viral proteins associated with photosynthesis. It is estimated 10% of all photosynthesis on earth is carried out with viral genes. Not all viruses immediately kill their hosts, 'temperate' viruses co-exist with their host until stresses or nearing end of natural life span make them switch their host to virus production; if a mutation occurs that stops this final step, the host can carry the virus genes with no ill effects. And if a healthy host reproduces while infectious, its offspring can be infectious as well. It is likely such a process gave Synechococcus photosynthesis.^[31]

DNA recombination, repair and replication

Marine Synechococcus species possess a set of genes that function in DNA recombination, repair and replication. This set of genes includes the recBCD gene complex whose product, exonuclease V, functions in recombinational repair of DNA, and the umuCD gene complex whose product, DNA polymerase V, functions in error-prone DNA replication.^[32] Some Synechococcus strains are naturally competent for genetic transformation, and thus can take up extracellular DNA and recombine it into their own genome.^[32] Synechococcus strains also encode the gene lexA that regulates an SOS response system, that is likely similar to the well-studied E. coli SOS system that is employed in the response to DNA damage.^[32]

Species

See also

• Gloeomargarita lithophora
• Photosynthetic picoplankton
• Prochlorococcus
• Synechocystis, another cyanobacterial model organism

References