Synechococcus (from the Greek synechos, in succession, and the Greek kokkos, granule) is a unicellular cyanobacterium that is very widespread in the marine habitats. Its size varies from 0.8 to 1.5 micrometre. The photosynthetic coccus are preferentially found in well–lit where it can be very abundant (generally 1,000 to 200,000 cells per ml). Many fresh water species of Synechococcus have also been described.
The genome of S. elongatus strain PCC7002 has a size of 3.4 Mbase pair, whereas the oceanic strain WH8102 has a genome of size 2.4 Mbp.
Introduction
Synechococcus is one of the most important components of the
prokaryote picoplankton in the
temperate to
tropics 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
.
Cells are known to be Motility by a gliding method
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.
Pigments
The main photosynthetic pigment in
Synechococcus is
chlorophyll a, while its major accessory pigments are
phycobiliprotein.
The four commonly recognized
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
Chlorophyta and
. Similarly, phycoerythrin is also found in
Red algae and some
.
Phylogeny
Phylogenetics description of
Synechococcus is difficult. Isolates are morphologically very similar, yet exhibit a
GC-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.
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 RuBisCO) found in these organisms.
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.
As of 2020, the morphologically similar " Synechococcus collective" has been split into 15 genera under 5 different orders:
(gen. nov. means that the genus is newly created in 2020).
Ecology and distribution
Synechococcus has been observed to occur at concentrations ranging between a few cells to 10
6 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.
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 Ocean gyre, where cell abundances are often very low, population growth rates are often high and not drastically limited.
Prochlorococcus is thought to be at least 100 times more abundant than Synechococcus in warm oligotrophic waters.
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.
DNA recombination, repair and replication
Marine
Synechococcus species possess a set of genes that function in DNA recombination,
DNA repair and
DNA 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.
Some
Synechococcus strains are naturally competent for genetic transformation, and thus can take up extracellular DNA and recombine it into their own genome.
[ 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.][
]
Species
-
Synechococcus ambiguus Skuja
-
Synechococcus arcuatus var. calcicolus Fjerdingstad
-
Synechococcus bigranulatus Skuja
-
Synechococcus brunneolus Rabenhorst
-
Synechococcus caldarius Okada
-
Synechococcus capitatus A. E. Bailey-Watts & J. Komárek
-
Synechococcus carcerarius Norris
-
Synechococcus elongatus (Nägeli) Nägeli
-
Synechococcus endogloeicus F. Hindák
-
Synechococcus epigloeicus F. Hindák
-
Synechococcus ferrunginosus Wawrik
-
Synechococcus intermedius Gardner
-
Synechococcus koidzumii Yoneda
-
Synechococcus lividus Copeland
-
Synechococcus marinus Jao
-
Synechococcus minutissimus Negoro
-
Synechococcus mundulus Skuja
-
Synechococcus nidulans (Pringsheim) Komárek
-
Synechococcus rayssae Dor
-
Synechococcus rhodobaktron Komárek & Anagnostidis
-
Synechococcus roseo-persicinus Grunow
-
Synechococcus roseo-purpureus G. S. West
-
Synechococcus salinarum Komárek
-
Synechococcus salinus Frémy
-
Synechococcus sciophilus Skuja
-
Synechococcus sigmoideus (Moore & Carter) Komárek
-
Synechococcus spongiarum Usher et al.
-
Synechococcus subsalsus Skuja
-
Synechococcus sulphuricus Dor
-
Synechococcus vantieghemii (Pringsheim) Bourrelly
-
Synechococcus violaceus Grunow
-
Synechococcus viridissimus Copeland
-
Synechococcus vulcanus Copeland
See also
Further reading
External links
