Transmission electron micrograph showing a species of the cyanobacteriaSynechococcus. Thecarboxysomes appear as polyhedral dark structures.
Synechococcus (from the Greeksynechos, in succession, and the Greekkokkos, granule) is a unicellularcyanobacterium that is very widespread in themarine environment. Its size varies from 0.8 to 1.5 μm. Thephotosyntheticcoccoid cells are preferentially found in well–litsurface waters where it can be very abundant (generally 1,000 to 200,000 cells per ml). Manyfreshwater species ofSynechococcus have also been described.
Thegenome ofS. elongatus strain PCC7002 has a size of 3.4Mbp, whereas the oceanic strain WH8102 has a genome of size 2.4 Mbp.[1][2][3][4]
Synechococcus is one of the most important components of theprokaryoticautotrophicpicoplankton in thetemperate totropical oceans. The genus was first described in 1979,[5][6] 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".[7] This definition of the genusSynechococcus contained organisms of considerable genetic diversity and was later subdivided into subgroups based on the presence of the accessory pigmentphycoerythrin. The marine forms ofSynechococcus are coccoid cells between 0.6 and 1.6 μm in size. They areGram-negative cells with highly structured cell walls that may contain projections on their surface.[8] Electron microscopy frequently reveals the presence of phosphate inclusions,glycogen granules, and more importantly, highly structuredcarboxysomes.
Cells are known to bemotile by a gliding method[9] and a novel uncharacterized, nonphototactic swimming method[10] that does not involveflagellar motion. While some cyanobacteria are capable ofphotoheterotrophic or evenchemoheterotrophic growth, all marineSynechococcus strains appear to be obligate photoautotrophs[11] that are capable of supporting their nitrogen requirements using nitrate, ammonia, or in some casesurea as a sole nitrogen source. MarineSynechococcus species are traditionally not thought to fix nitrogen.
In the last decade, several strains ofSynechococcus 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.[12] 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”.[13]
Much like other cyanobacterial taxa, the taxonomic description ofSynechococcus was originally based onmorphology alone. It was greatly reshaped, repeatedly, with the advance of DNA-based methods, to better reflect what new information indicates is the likelyphylogeny of these bacteria.
GC content data for many strains became available in the 1980s. It was found that so-calledSynechococcus had a GC content ranging from 39 to 71%, an indicator of large genetic diversity.[11] Accordingly, attempts were made to divide the group into three subclusters, each with a specific range of genomic G+C content.[15] The observation that open-ocean isolates alone nearly span the complete G+C spectrum, however, indicates thatSynechococcus is composed of at least several species.
Bergey's Manual of 2001[16] now dividesSynechococcus 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 (formerlyAnacycstis nidulans) and PCC6312, which were isolated from fresh water inTexas andCalifornia, respectively.[7]
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 inYellowstone National Park.[17]
Cluster 3 includes phycoerythrin-lacking marineSynechococcus species that areeuryhaline, i.e. capable of growth in both marine and freshwater environments. Several strains, including the reference strain PCC7003, are facultative heterotrophs and requirevitamin B12 for growth.
Cluster 4 contains a single isolate, PCC7335. This strain is obligate marine.[18] This strain contains phycoerythrin and was first isolated from theintertidal zone inPuerto Peñasco,Mexico.[7]
The last cluster contains what had previously been referred to as ‘marine A and B clusters’ ofSynechococcus. 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.[19]
More recently, Badgeret al. (2002) proposed the division of the cyanobacteria into a α- and a β-subcluster based on the type ofrbcL (large subunit ofribulose 1,5-bisphosphate carboxylase/oxygenase) found in these organisms.[20] 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 Badgeret al. analyze the phylogeny of carboxysomal proteins, which appear to support this division. Also, two particularbicarbonate transport systems appear to only be found in α-cyanobacteria, which lack carboxysomal carbonic anhydrases.
A completephylogenetic tree of 16S rRNA sequences ofSynechococcus-shaped bacteria was produced in 2014. It revealed at least 12 groups, which morphologically correspond toSynechococcus, but they have not derived from the common ancestor. Moreover, it has been estimated based on molecular dating that the firstSynechococcus lineage has appeared 3 billion years ago in thermal springs with subsequent radiation to marine and freshwater environments.[21]
Komáreket al. (2020) uses a combination of phylogenomics and 16S rRNA to divide theSynechococcus-shaped bacteria into a number of genera, families, and orders. Some of these are newly created (marked with "nov."):
All of these above names are validly published under the provivisions of theBotanical Code. They are accepted by AlgaeBase and LPSN.
Strunecký, Otakar, and Mareš (2023) revised the classification of cyanobacteria, including those traditionally called "Synechococcus", at the order and family levels. The work combines a phylogenomic tree with a 16S ribosomal tree. This forms the currently-accepted backbone of taxonomy.[22]
Salazaret al. (2020) split theSynechococcus-shaped bacteria into 15 genera under 5 different orders using phylogenomics, specifically the methods ofGTDB:[23]
Cyanobacteriales ord. nov.: Limnotrichaceae fam. nov. ("Limnothrix", which is not correctly represented in GTDB due to a lack of type species genome, but more properlyPicosynechococcus);
The GTDB framework is not accepted as legitimate by LPSN and AlgaeBase because the authors have neglected tovalidly publish the new taxa by providing explicit, individual descriptions for them.[24] It is possible to map the GTDB names to the accepted framework of 2023 by comparing the phylogenetic trees used in the two works, made easier by the fact that the trees do assemble each other due to both being based on phylogenomics (most synonymy from LPSN):[25]
Due to the GTDB and the 2020s accepted frameworks now agreeing on using [[
Synechococcus has been observed to occur at concentrations ranging between a few cells to 106 cells per ml in virtually all regions of oceaniceuphotic zone except in samples from theMcMurdo Sound andRoss Ice Shelf inAntarctica.[11] 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.[26]Synechococcus has also been observed to occur at high abundances in environments with low salinities and/or low temperatures. It is usually far outnumbered byProchlorococcus in all environments where they co-occur. Exceptions to this rule are areas of permanently enriched nutrients such asupwelling areas andcoastal watersheds.[26] 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.[27][28][29][30][31] VerticallySynechococcus is usually relatively equitably distributed throughout themixed 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 ofProchlorococcus in thewater column. In the Pacifichigh-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.[27][28][32]
The factors controlling the abundance ofSynechococcus still remain poorly understood, especially considering that even in the most nutrient-depleted regions of thecentral gyres, where cell abundances are often very low, population growth rates are often high and not drastically limited.[26] 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 andSynechococcus abundance,[26][29] with an inverse relationship toProchlorococcus[30] in the uppereuphotic zone, where light is not limiting. One environment whereSynechococcus thrives particularly well is coastal plumes of major rivers.[33][34][35][36] Suchplumes are coastally enriched with nutrients such as nitrate and phosphate, which drives largephytoplankton blooms. High productivity in coastal riverplumes is often associated with large populations ofSynechococcus and elevated form IA (cyanobacterial)rbcL mRNA.
Prochlorococcus is thought to be at least 100 times more abundant thanSynechococcus in warm oligotrophic waters.[26] Assuming average cellular carbon concentrations, it has thus been estimated thatProchlorococcus accounts for at least 22 times more carbon in these waters, thus may be of much greater significance to the globalcarbon cycle thanSynechococcus.
MarineSynechococcus species possess a set of genes that function in DNArecombination,repair andreplication. This set of genes includes therecBCD gene complex whose product,exonuclease V, functions in recombinational repair of DNA, and theumuCD gene complex whose product,DNA polymerase V, functions in error-prone DNA replication.[37] SomeSynechococcus strains are naturally competent forgenetic transformation, and thus can take up extracellular DNA and recombine it into their own genome.[37]Synechococcus strains also encode the genelexA that regulates anSOS response system, that is likely similar to the well-studiedE. coli SOS system that is employed in the response toDNA damage.[37]
^Castenholz, R.W. (1982). "Motility and taxes". In N. G. Carr; B. A. Whitton (eds.).The biology of cyanobacteria. University of California Press. pp. 413–439.ISBN978-0-520-04717-4.OCLC8169723.
^Herdmanet al. (2001)Synechococcus in Garrity, G.M., Boone, D.R. & Castenholz, R.W. (eds.).Bergey's Manual of Systematic Bacteriology, 2nd ed., vol. 1, Springer-Verlag, New York, NY
^Waterbury, J.B.; Stanier, R.Y. (1981). "Isolation and growth of cyanobacteria from marine and hypersaline environments". In Starr; Stulp; Truper; Balows; Schleeper (eds.).The prokaryotes: a handbook on habitats, isolation, and identification of bacteria, Vol 1.Springer-Verlag, Berlin. pp. 221–3.ISBN978-0-387-08871-6.
^Strunecký, Otakar; Ivanova, Anna Pavlovna; Mareš, Jan (February 2023). "An updated classification of cyanobacterial orders and families based on phylogenomic and polyphasic analysis".Journal of Phycology.59 (1):12–51.doi:10.1111/jpy.13304.
^Salazar, Vinícius W.; Tschoeke, Diogo A.; Swings, Jean; Cosenza, Carlos A.; Mattoso, Marta; Thompson, Cristiane C.; Thompson, Fabiano L. (November 2020). "A new genomic taxonomy system for the Synechococcus collective".Environmental Microbiology.22 (11):4557–4570.doi:10.1111/1462-2920.15173.
^"Order "Leptococcales"".LPSN.🧍 While the authors did not explicitly propose this taxon name, they did not provide a proper citation of the authority of the name either.
^abcdePartensky F, Blanchot J, Vaulot D (1999). "Differential distribution and ecology ofProchlorococcus andSynechococcus in oceanic waters: a review". In Charpy L, Larkum AW (eds.).Marine cyanobacteria. Bulletin de l'Institut Océanographique Monaco. Vol. NS 19. Monaco: Musée océanographique. pp. 457–475.ISBN2-7260-0210-2.OCLC606510917.
Waterbury, J.B.; Watson, S.W.; Valois, F.W.; Franks, D.G. (1986). "Biological and ecological characterization of the marine unicellular cyanobacteriumSynechococcus". In W.K.W. Li (ed.).Photosynthetic Picoplankton. Ottawa, Canada: Department of Fisheries and Oceans. pp. 71–120.ISBN0-660-12243-X.OCLC16576851.
