This is a guest post by Sophie, written in response to a reader request I am unqualified to fulfil.
Picocyanobateria are cyanobacteria that are less than 3 µm in diameter. Their tiny size makes them significant parts of nutrient cycles: their surface area:volume ratios make them very efficient at nutrient uptake, much more so than larger cells. In lakes, they can contribute up to 80% of the primary production when conditions are suitable (Stockner et al., 2000). In ultraoligotrophic environments, picocyanobacteria dominate, as seen in Lake Superior where picocyanobacteria have radiated in the offshore part of the lake (Ivanikova et al., 2007). Their success in this type of environment is due to advantages in phosphorus use: they can make membranes with less phosphorus than other bacteria (Dyhrman et al., 2009) and they have higher uptake rates of phosphorus due to their ability to hydrolyse dissolved organic phosphorus (Moore et al., 2005).
Picocyanobacteria can be individuals (rods and coccoids) or they can be colonial. Colonies have more than 50 individuals; microcolonies of less than 50 individuals also form. Colonial picocyanobacteria are chroococcal ovoids, rods and cones. The colony can be loose, dense, stalked or filamentous. The arrangement is species-specific, used for identification.
Our knowledge of individual picocyanobacteria is limited. They are in a clade called the Syn/Pro clade with three genera, Synechococcus, Prochlorococcus and Cyanobium. They have higher diversity in marine environments: most strains belong to one lineage and are collected in two genera, Synechococcus and Prochlorococcus (Sánchez-Baracaldo et al., 2005). Freshwater strains are similar to Synechococcus (Weisse, 1993). This summary is not satisfactory because Synechoccus is polyphyletic (Robertson et al., 2001), but this is the state of the art at the moment.
Picocyanobacteria are classified in two types according to the light-harvesting pigment, visible with epifluorescence microscopy: yellow autofluorescing phycoerythrin or red autofluorescing phycocyanin (Wood et al., 1985). The first absorb green light (560 nm), the second absorb orange-red light (625 nm).
Their phylogeny is investigated only with molecular sequences (Steglich et al., 2003). Morphology is useless (Ernst et al., 2003) but molecular sequences are also problematic. Ssu rDNA is not reliable for broad phylogenetic study (Litvaitis, 2002) because it is too conserved. ITS-1 (the spacer between 16S and 23S rDNA) is useful for fingerprinting (Becker et al., 2002) because of its variability, but not good for phylogeny. Pigment genes are tricky because of lateral gene transfer (Dufresne et al., 2008).
Sánchez-Baracaldo et al. (2005) is the largest and most reliable study of cyanobacterial phylogeny, including picocyanobacteria. It is a total evidence analysis with phylogenomics, individual sequences and some morphology. Picocyanobacteria are the second most ancient lineage. By parsimony character reconstruction, the study concludes that the habitus of picocyanobacteria is representative of the first cyanobacteria. It also concludes that cyanobacteria evolved on land or in freshwater, not in the oceans. All together this means that studying freshwater picocyanobacteria is an exciting avenue for future research.
Picocyanobacteria can be health hazards. Their small size allows them to pass through all filters used in water treatment. Toxin-producing (microcystin) picocyanobacteria have been found (Bláha and Marsálek, 1999) and are a potential risk if they bloom in drinking water, as shown by deaths of people in Brazil getting treated with contaminated water (Domingos et al., 1999).
Becker S, Fahrbach M, Börger P & Ernst A. 2002. Quantitative tracing, by Taq nuclease assays, of a Synechococcus ecotype in a highly diversiﬁed natural population. Applied Environmental Microbiology 68, 4486-4494.
Bláha L & Marsálek B. 1999. Microcystin production and toxicity of picocyanobacteria as a risk factor for drinking water treatment plants. Algological Studies 92, 95-108.
Domingos P, Rubim RK, Molica RJR, Azevedo SMFO & Carmichael WW. 1999. First report of microcystin production by picoplanktic cyanobacteria isolated from a Northeast Brazilian drinking water supply. Environmental Toxicology 14, 13-35.
Dufresne A, Ostrowski M, Scanlan DJ, Garczarek L, Mazard S, Palenik BP, Paulsen IT, Tandeau de Marsac N, Wincker P, Dossat C, Ferriera S, Johnson J, Post AP, Hess WR, Partensky F. 2008. Unraveling the genomic mosaic of a ubiquitous genus of marine cyanobacteria. Genome Biology 9, R90.
Dyhrman ST, Ammerman JW & van Mooy BAS. 2009. Microbes and the Marine Phosphorus Cycle. Oceanography 20, 110-116.
Ernst A, Becker S, Wollenzien UIA & Postius C. 2003. Ecosystem-dependent adaptive radiations of picocyanobacteria inferred from 16S rRNA and ITS-1 sequence analysis. Microbiology 149, 217-228.
Ivanikova NV, Popels LC, McKay RML & Bullerjahn G. 2007. Lake Superior supports novel clusters of cyanobacterial picoplankton. Applied Environmental Microbiology 73, 4055-4065.
Litvaitis MK. 2002. A molecular test of cyanobacterial phylogeny: inferences from constraint analyses. Hydrobiologia 468, 135-145.
Moore LR, Ostrowski M, Scanlan DJ, Feren K & Sweetsir T. 2005. Ecotypic variation in phosphorus-acquisition mechanisms within marine picocyanobacteria. Aquatic Microbial Ecology 39, 257-269.
Robertson BR, Tezuka N & Watanabe MM. 2001. Phylogenetic analyses of Synechococcus strains (Cyanobacteria) using sequences of 16S rDNA and part of the phycocyanin operon reveal multiple evolutionary lines and reflect phycobilin content. IJSEM 51, 861-871.
Sánchez-Baracaldo P, Hayes PK & Blank CE. 2005. Morphological and habitat evolution in the Cyanobacteria using a compartmentalization approach. Geobiology 3, 145-165.
Steglich C, Post AF & Hess WR. 2003. Analysis of natural populations of Prochlorococcus spp. in the northern Red Sea using phycoerythrin gene sequences. Environmental Microbiology 5, 681-690.
Stockner J, Callieri C & Cronberg G. 2000. Picoplankton and Other Non-Bloom-Forming Cyanobacteria in Lakes. In: Whitton B & Potts M (eds.). The Ecology of Cyanobacteria: Their Diversity in Time and Space.
Weisse T. 1993. Dynamics of autotrophic picoplankton in marine and freshwater ecosystems. Advances in Microbial Ecology 13, 327-370.
Wood AM, Horan PK, Muirhead K, Phinnea DA, Yentsch CM & Waterbury JB. 1985. Discrimination between types of pigments in marine Synechococcus spp. by scanning spectroscopy, epiﬂuorescence microscopy and ﬂow cytometry. Limnology and Oceanography 30, 1303-1315.
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P. SÁNCHEZ-BARACALDO, P. K. HAYES, & C. E. BLANK (2005). Morphological and habitat evolution in the Cyanobacteria using a compartmentalization approach Geobiology DOI: 10.1111/j.1472-4669.2005.00050.x
Dufresne A, Ostrowski M, Scanlan DJ, Garczarek L, Mazard S, Palenik BP, Paulsen IT, de Marsac NT, Wincker P, Dossat C, Ferriera S, Johnson J, Post AF, Hess WR, & Partensky F (2008). Unraveling the genomic mosaic of a ubiquitous genus of marine cyanobacteria. Genome biology, 9 (5) PMID: 18507822