Newly discovered specie provides probable link between cyanobacteria and chloroplast!

This is my another blog on cyanobacteria. It is not only about a newly discovered specie but also a newly discovered symbiotic relationship. This cyanobacterium has not been cultured yet and so has been given a provisional name Candidatus Atelocyanobacterium thalassa. This cyanobacterium is unusual in the sense that it lacks some v.basic components without which it is not able to perform photosynthesis and thus unable to fix carbon for sustaining itself; these components are RuBisCo, photosystem II, tricarboxylic acid cycle.

Despite the absence of these enzymes and pathways, this cyanobacteria survive; this makes us think how? The answer to this lie in a phenomenon that could provide us insights about the very birth of chloroplast!

This cyanobacterium is found in a symbiotic association with an alga; a single celled,free living, photosynthetic picoeukaryote prymnesiophyte. The cyanobacterium gets its carbon requirements from this alga and in return it provides the alga fixed nitrogen which it efficiently fixes from marine environment. There is an imbalance in synergy because the cyanobacterium gives around 95% of fixed nitrogen to the alga whereas the alga gives a v.little amount of carbon(1-17%) but this imbalance can be attributed to the difference in their sizes.

This discovery does not only gives an insight to the planktonic symbiotic world but also gives some proof for the much sought out answer about the link between  ancient cyanobacteria and chloroplast. Chloroplasts are organelles present inside photosynthetic cells and are responsible for fixing carbon and thus supporting all life forms. This cyanobacterium lives on the surface of the alga in some groove like structure. This type of association might have preceded the event of the alga engulfing the cyanobacterium and thus giving rise to the present chloroplast.

                                                Microscopy showing the symbiotic partners

Reference: Thompson AW, Foster RA, Krupke A, Carter BJ, Musat N, Vaulot D, Kuypers MM, Zehr JP. Unicellular cyanobacterium symbiotic with a single-celled eukaryotic alga. Science. 2012 Sep 21;337

Cyanobacteria which sequesters carbonate granules internally

Cyanobacterial metabolism is quite interesting leading to different kinds of products ranging from biofuels, sugars, isoprene, carbonates. Until now, what have been found with cyanobacteria is extracellular carbonate deposits but it is recently found that a specie of cyanobacteria produce intracellular carbonate deposits. This specie has been given the nomenclature as order Gloeobacterales, Candidatus Gloeomargarita lithophora.

During photosynthesis,CO₂ is fixed leading to a more alkaline environment inside the cell. This makes the cells excrete alkalinity outside and if calcium is present in the environment, carbonate precipitates and form crystals with it leading to calcium carbonate deposits.

But, in these cyanobacteria the mechanism for excreting carbonates is either not present because of their ancient origin or the reason could be something that makes this phenomenon advantageous for them. The probable advantage of these inclusions is that they raise the density of cells to ~12% which has been predicted to be useful for these ancient benthic organisms to stay grounded.

The intracellular granules are found to be ~270nm and average number of inclusions per cell are ~21. These inclusions are rich in Ca, Ba, Sr,Mg and their ratios in the granules indicates their abundance in the local environment.

Reference: Couradeau E, Benzerara K, Gérard E, Moreira D, Bernard S, Brown GE Jr, López-García P. An early-branching microbialite cyanobacterium forms intracellular carbonates. Science. 2012 Apr 27;336(6080):459-62.