Synechococcus
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Synechococcus is a unicellular cyanobacterium that is very wide spread in the marine environment. Its size varies between 0.8 and 1.5 µm. The photosynthetic coccoid cells are preferentially found in surface well lit waters where this bacterium can be very abundant (generally from 1,000 to 200,000 of cells by milliliter). Many freshwater genera of Synechococcus have also been described.
The genome ofSynechococcus elongatus strain PCC7002 has a size of 2,7 Mpb, that of the oceanic strain WH8102 is 2.4 Mbp.
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[edit] Introduction
Synechococcus is one of the most important component of the prokaryotic autotrophic picoplankton in the temperate to tropical oceans. The organisms was first described 1979 (Johnson & Sieburth 1979, Waterbury et al. 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” (Rippka et al. 1979). 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 (Perkins et al. 1981). 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 type method (Castenholz 1982) and a novel uncharacterized, non-phototactic swimming method (Waterbury et al. 1985) 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 (Waterbury et al. 1986b) that are capable of supporting their nitrogen requirements using nitrate, ammonia or in some cases urea as a sole nitrogen source. Marine Synechococcus are traditionally not thought to fix nitrogen (This perception may be changing).
[edit] Pigments
The main photosynthetic pigment in Synechococcus in chlorophyll a, while its major accessory pigments are phycobilliproteins (Waterbury et al. 1979). The four commonly recognized phycobillins are phycocyanin, allophycocyanin, allophycocyanin B and phytoerythrin (Stanier & Cohen-Bazire 1977). In addition Synechococcus also contains zeaxanthin but no diagnostic pigment for this organism is known. Zeaxanthin is also found in Prochlorococcus, rhodophytes and as a minor pigment in some chlorophytes and eustigmatophytes. Similarly phycoerythrin is also found in rhodophytes and some cryptomonads (Waterbury et al. 1986b).
[edit] Phylogeny
Phylogenetic description of Synechococcus is difficult. Isolates are morphologically very similar, yet exhibited a G+C content ranging between 39 and 71% (Waterbury et al. 1986b) illustrating the large genetic diversity of this provisional taxon. Initially attempts were made to divide the group into three sub-clusters, each with a specific range of genomic G+C content (Rippka & Cohen-Bazire 1983). 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 one includes relatively large (1-1.5µm) non-motile obligate photoautotrophs that exhibit low salt tolerance. Reference strains for this cluster are PCC6301 (formerly Anacycstis nidulans) and PCC6312, which were isolated from freshwater in Texas and California respectively (Rippka et al. 1979). 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 (Dyer & Gafford 1961). Cluster 3 includes phycoerythrin lacking marine Synechococcus that are euryhaline i.e. capable of growth in both marine and fresh water 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 (Waterbury & Stanier 1981). This strain contains phycoerthrin and was first isolated from the intertidal zone in Perto Penasco, Mexico (Rippka et al. 1979). 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 ca. 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 (Waterbury et al. 1986b). More recently Badger (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 analyzes 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.
[edit] Ecology and distribution
Synechococcus has been observed to occur at concentrations ranging between a few cells per ml to 106 cells per ml in virtually all regions of the oceanic euphotic zone except in samples from the McMurdo Sound and Ross Ice Shelf in Antarctica (Waterbury et al. 1986b). 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 (Partensky et al. 1999a). Synechococcus has also been observed to occur at high abundances in environments with low salinities and/or low temperatures. Synechococcus 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 (Partensky et al. 1999a). In the nutrient deplete areas of the oceans, such as the central gyres, Synechococcus is apparently always present, although only at low concentrations ranging from a few to 4x103 cells ml-1 (Olson et al. 1990b, Blanchot et al. 1992, Campbell & Vaulot 1993, Li 1995, Blanchot & Rodier 1996). Vertically Synechococcus is usually relatively equitably distributed throughout the mixed layer and exhibits an affinity for the higher light regime. Below the mixed layer cell
concentrations rapidly decline. Vertical profiles are however 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 HNLC (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 SCM (Olson et al. 1990b, Li 1995, Landry et al. 1996).
The factors controlling the abundance of Synechococcus still remain poorly understood, especially considering that even in the most nutrient deplete regions of the central gyres, where cell abundances are often very low, population growth rates are often high and not very drastically limited (Partensky et al. 1999a). Factors such as grazing, viral mortality, genetic variability, grazing, light adaptation, temperature as well as nutrients are certainly involved, but remain to be investigated on a rigorous and global scale. Despite the uncertainties it has been suggested that there is at least a relationship between ambient nitrogen concentrations and Synechococcus abundance (Blanchot et al. 1992, Partensky et al. 1999a) and an inverse relationship to Prochlorococcus (Campbell & Vaulot 1993) in the upper euphotic zone, where light is not limiting. One environment where Synechococcus thrives particularly well are coastal plumes of major rivers (Paul et al. 2000, Wawrik et al. 2002, 2003, 2004a, b). Such plumes are coastally enriched with nutriets 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.
It should also be noted that Prochlorococcus in thought to be at least 100 times more abundant than Synechococcus in warm oligotrophic waters (Partensky et al. 1999a). Assuming average cellular carbon concentrations it has thus been estimated that Prochlorococcus accounts for at least 22 times more carbon in these waters and may thus be of much greater significance to the global carbon cycle than Synechococcus.
 Typical vertical distribution of Synechococcus in the Gulf of Mexico. Profile was obtained during a cruise on the R/V Pelican during July of 1999 at an oligotrophic site west of Tampa Bay and outside the Florida Shelf. Cells are abundant throughout the mixed layer (here ca. 40 m thick) yet concentrations rapidly decline below. |
 Electron migrograph of Synechococcus WH7803 grown under 12:12 light:dark cycles at 23ºC. Cells were fixed in 2% gluteraldehyde and 1% OsO4. Grids were stained with 5% alcoholic uranyl acetate and sTable lead. Magnification = 12000X. Cell wall (PM), DNA (N), thylacoids (T) and a number of carboxysomes (V) are seen. |
[edit] References
- Badger MR, Hanson D, Price GD (2002) Evolution and diversity of CO2 concentrating mechanism in cyanobacteria. Funct. Plant Biol. 29:161-175.
- Blanchot J, Rodier M (1996) Picophytoplankton abundance and biomass in the western tropical Pacific Ocean during the 1992 El Nino year: results from flow cytometry. Deep Sea Research I 43:877-895.
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- Campbell L, Liu H, Nolla HA, Vaulot D (1997) Annual variability of phytoplankton and bacteria in the subtropical North Pacific Ocean at Station ALOHA during the 1991-1994 ENSO event. Deep Sea Research I 44:167-192.
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- Castenholz RW (1982) Motility and taxes. In: Carr NG, Whitton BA (eds) The biology of cyanobacteria. University of California Press, Berkeley and Los Angeles, p 413-439.
- Dyer DL, Gafford RD (1961) Some characteristics of a thermophilic blue-green alga. Science 134:616-617.
- Johnson, P. W. & Sieburth, J. M. 1979. Chroococcoid cyanobacteria in the sea: a ubiquitous and diverse phototrophic biomass. Limnol. Oceanogr. 24:928-35.
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- Partensky F, Guillou L, Simon N, Vaulot D (1997) Recent advances in the use of molecular techniques to assess the genetic diversity of marine photosynthetic microorganisms. Vie Milieu 47:367-374.
- Partensky F, Hess WR, Vaulot D (1999b) Prochlorococcus, a Marine Photosynthetic Prokaryote of Global Significance. Microbiology and Molecular Biology Reviews Mar:106-127.
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- Stanier RY, Cohen-Bazire G (1977) Phototrophic prokaryotes: The Cyanobacteria. Ann. Rev. Microbiol. 31:255-274.
- Waterbury JB, Stanier RY (1981) Isolation and growth of cyanobacteria from marine and hypersaline environments. In: Starr, Stulp, Truper, Balows, Schleeper (eds) Theprokaryotes: A Handbook on habitats, isolation, and identification of bacteria, Vol 1. Springer-Verlag, Berlin, p 221-223.
- Waterbury, J. B., Watson, S. W., Guillard, R. R. L. & Brand, L. E. 1979. Wide-spread occurrence of a unicellular, marine planktonic, cyanobacterium. Nature 277:293-4.
- Waterbury JB, Watson SW, Valois FW, Franks DG (1986a) Biological and ecological characterization of the marine unicellular cyanobacterium Synechococcus. In: Li WKW (ed) Photoynthetic Picoplankton. Department of Fisheries and Oceans, Ottawa, Canada, p 71-120.
- Waterbury JB, Watson SW, Valois FW, Franks DG (1986b) Biological and ecological characterization of the marine unicellular cyanobacterium Synechococcus. Can. Bull. Fish. Aquat. Sci. 214:71-120.
- Waterbury JB, Willey JM (1988) Isolation and growth of marine planktonic cyanobacteria. Methods Enzymol. 167:100-105 Waterbury JB, Willey JM, Franks DG, Valois FW, Watson SW (1985) A cyanobacterium capable of swimming motility. Science 30:74-76.
- Wawrik, B., D. John, M. Gray, D. A. Bronk, and J.H. Paul. 2004. Preferential Uptake of Ammonium in the Presence of Elevated Nitrate Concentrations by Phytoplankton in the Offshore Mississippi Plume. Aquatic Microbial Ecology 35:185-196.
- Wawrik, B., and J.H. Paul. 2004. Phytoplankton Community Structure and Productivity Along the Axis of the Mississippi Plume. Aquatic Microbial Ecology 35:175-184.
- Wawrik, B., J.H. Paul, L. Campbell, D. Griffin, L. Houchin, A. Fuentes-Ortega, and F. Müller-Karger, 2003. Vertical Structure of the Phytoplankton Community Associated with a Coastal Plume in the Gulf of Mexico. Marine Ecology Progress Series 251:87-101.
[edit] External links
- Reference algaeBase : genus 43582 Synechococcus
