Academic literature on the topic 'Calcium carbonate (CaCO3) cycle'

The hydrolysis of urea by the widely distributed enzyme urease is special in that it is one of the few biologically occurring reactions that can generate carbonate ions without an associated production of protons. When this hydrolysis occurs in a calcium-rich environment, calcite (calcium carbonate) precipitates from solution forming a solid-crystalline material. The binding strength of the precipitated crystals is highly dependent on the rate of carbonate formation and under suitable conditions it is possible to control the reaction to generate hard binding calcite cement (or Biocement). The objective of this thesis was to develop an industrially suitable cost-effective microbial process for the production of urease active cells and investigate the potential for urease active cells to act as a catalyst for the production of Biocement. The biocementation capability of two suitable strains was compared. Sporosarcina pasteurii (formally Bacillus pasteurii) produced significantly higher levels of urease activity compared to Proteus vulgaris, however the level of urease activity was variable with respect to biomass suggesting that the enzyme was not constitutive as indicated by the literature, but subject to regulation. The environmental and physiological conditions for maximum urease activity in S. pasteurii were investigated and it was found that the potential urease capacity of the organism was very high (29 mM urea.min-1.OD-1) and sufficient for biocementation without additional processing (e.g. concentration, cell lysis). The regulation mechanism for S. pasteurii urease was not fully elucidated in this study, however it was shown that low specific urease activity was not due to depletion of urea nor due to the high concentrations of the main reaction product, ammonium. pH conditions were shown to have a regulatory effect on urease but it was evident that another co-regulating mechanism existed. Despite not fully exploiting the urease capability of S. pasteurii, sufficient urease activity to allow direct application of the enzyme without additional processing could still be achieved and the organism was considered suitable for biocementation. Urease was the most expensive component of the cementation process and cost-efficient production was desired, thus an economic growth procedure was developed for large-scale cultivation of S. pasteurii. The organism is a moderate alkaliphile (growth optimum pH 9.25) and it was shown that sufficient activity for biocementation could be cultivated in non-sterile conditions with a minimum of upstream and downstream processing. The cultivation medium was economised and expensive components were replace with a food-grade protein source and acetate, which lowered production costs by 95%. A high level of urease activity (21 mM urea hydrolysed.min-1) was produced in the new medium at a low cost ($0.20 (AUD) per L). The performance of urease in whole S. pasteurii cells was evaluated under biocementation conditions (i.e. presence of high concentrations of urea, Ca2+, NH4 +/NH3, NO3 - and Cl- ions). It was established that the rate of urea hydrolysis was not constant during cementation, but largely controlled by the external concentrations of urea and calcium, which constantly changed during cementation due to precipitation of solid calcium carbonate from the system. A simple model was generated that predicted the change in urea hydrolysis rate over the course of cementation. It was shown that whole cell S. pasteurii urease was tolerant to concentrations of up to 3 M urea and 2 M calcium, and the rate of urea hydrolysis was unaffected up to by 3 M ammonium. This allowed the controlled precipitation of up to 1.5 M CaCO3 within one treatment, and indicated that the enzyme was very stable inspite of extreme chemical conditions. A cost-efficient cementation procedure for the production of high cementation strength was developed. Several biocementation trials were conducted into order to optimise the imparted cementation strength by determining the effect of urea hydrolysis rate on the development of strength. It was shown that high cementation strength was produced at low urea hydrolysis rates and that the development of cementation strength was not linear over the course of the reaction but mostly occurred in the first few hours of the reaction. In addition, the whole cell bacterial enzyme had capacity to be immobilised in the cementation material and re-used to subsequent applications, offering a significant cost-saving to the process. An industry-sponsored trial was undertaken to investigate the effectiveness of Biocement for increasing in-situ strength and stiffness of two different sandy soils; (a) Koolschijn sand and (b) 90% Koolschijn sand mixed with 10% peat (Holland Veen). After biocementation treatment, Koolschijn sand indicated a shear strength of 1.8 MPa and a stiffness of 250 MPa, which represents an 8-fold and 3-fold respective improvement in strength compared to unconsolidated sand. Significantly lower strength improvements were observed in sand mixed with peat. In combination, trials of producing bacteria under economically acceptable conditions and cementation trials support the possibility of on-site production and in-situ application of large field applications.

Les chirurgiens sont confrontés à des pertes de substance osseuse qui impliquent l'utilisation de biomatériaux synthétiques de comblement. Leur utilisation permet de limiter les réponses immunitaires et leur disponibilité est quasi illimitée. Notre travail a porté sur des géopolymères (Si/Al=21). Ces aluminosilicates synthétisés ont été associés à des phosphates de calcium. Chauffés à 500ʿC, les géopolymères présentent des pH de 7 et un bon compromis propriétés mécaniques/ porosité. Ces matériaux ont été étudiés in vitro et in vivo pour évaluer leur potentiel pour une utilisation comme biomatériaux. Les cinétiques de minéralisation des géopolymères et du biomatériau CaCO3 ont été étudiées. Les résultats obtenus par PIXE et NAA pour le CaCO3 montrent un comportement in vivo similaire à celui d'un TCP. Les études in vivo réalisées sur les géopolymères ont confirmé l’ostéointégration des implants. Les analyses par PIXE des implants confirment la consolidation de l'interface dès 1 mois.

Le développement considérable réalisé ces dix dernières années dans les techniques de diffraction sur des échantillons sous forme de poudre même très texturés (temps d’enregistrement raisonnable et très bonne résolution) et les progrès fascinants des moyens de calcul sur des machines individuelles rendent possible l’accès à des informations précises et avec des temps de calcul raisonnablement courts. Dans ce travail, nous avons appliqué l’analyse combinée basée sur la méthode de Rietveld pour étudier des échantillons de carbonates de calcium naturels de coquilles de certaines espèces de mollusques (structure, texture, tailles anisotropes…etc. ) et synthétiques par biomimétisme dans le but de comprendre l’influence des différents facteurs intervenants dans la croissance et la forme des grains des différents polymorphes de CaCO3. Dans les échantillons naturels, nous avons clairement mis en évidence l’existence de déformations de mailles cristallines dues à la présence des macromolécules intra-cristallines et extra-cristallines, en complément d’études du même genre réalisées sur des échantillons broyés. Dans les échantillons synthétiques obtenus par une méthode de synthèse simple, nous avons analysé l’effet de l’ajout d’acide polyacrylique, en termes de modulateur de croissance des polymorphes de CaCO3. La simulation des digrammes de diffraction anisotropes réalisée sur ces deux types d’échantillons nous a permis d’accéder aux informations pertinentes de façon précise (tailles anisotropes des cristallites, structure, texture, proportion volumique des polymorphes)<br>The tremendous development of powder diffraction techniques achieved over the last ten years even operational on strongly textured samples within good resolutions, together with the fascinating development of individual calculating capabilities, make it possible to access precise information in reasonably short times. In this work, we apply the combined analysis based on the Rietveld method to study samples of natural calcium carbonates from two living species of molluscan shells and of synthetic CaCO3 in order to understand the influence of the different parameters in the growth and shape of the crystallites of the different polymorphs of CaCO3. In natural species we clearly put into evidence the existence of distortions of crystalline cells due to the presence of the intra- and extra-crystalline macromolecules, in opposition with similar studies operated on powderised samples. In synthetic samples we studied, using a simple synthesis method and taking as a model the polyacrylic acid, the role of the soluble macromolecules in the growth of the CaCO3 polymorphs. The simulation of the anisotropic diffraction diagrams on both sample types allowed us to reach pertaining information within good precision (anisotropic sizes of the crystallites, structure, texture, volume fraction of the polymorphs

Muitos organismos vivos, tal como seus constituintes, são formados por sistemas químicos complexos que envolvem a interação entre compostos orgânicos e inorgânicos ligados química e/ou fisicamente. Nestes sistemas as matrizes orgânicas são geralmente compostas por macromoléculas como polissacarídeos e proteínas. Essas moléculas têm o papel de direcionar a nucleação e o crescimento da porção inorgânica. O uso de superfícies metálicas adequadas recobertas por este tipo de filme híbrido tem potencial aplicação em implantes de substituição óssea, no qual são requeridas superfícies quimicamente inertes, mas que ao mesmo tempo estimulem processos de calcificação. Nesta tese estudou-se o crescimento de CaCO3 sobre superfícies metálicas de alumínio e aço inox recobertas por matrizes orgânicas compostas por diferentes poliânions e pelo policátion quitosana, na forma de filmes montados camada-a-camada (do inglês LbL), na presença ou não de fosfolipídeos (filmes Langmuir-Blodgett), formando um meio confinado para o crescimento do mineral. Diferentes técnicas foram utilizadas: microscopia eletrônica, microscopia de força atômica, espectroscopias de reflexão nas regiões do Uv-vis, e do infravermelho, Raman, espalhamento e difração de raios-X. Estudou-se a influência de diferentes grupos carregados dos fosfolipídeos e dos poliânions, tal como sua conformação, no crescimento de CaCO3. O tipo de interação entre o poliânion e a quitosana leva ao crescimento de matrizes poliméricas com diferenças em suas espessuras e capacidade de retenção de líquido, modificando as condições de supersaturação local e influenciando no tipo de estrutura de CaCO3. Puderam ser identificados dois polimorfos formados sobre os filmes orgânicos de poli(ácido acrílico) e quitosana, sugerindo que existem dois diferentes sítios onde a nucleação pode ser iniciada: a partir da solução de CaCl2 aprisionada na matriz polimérica e o outro a partir dos íons cálcio ligados como contra-íons aos grupos negativamente carregados do poliânion. Na presença do pré-recobrimento LB, a natureza da cabeça polar do fosfolipídeo direciona o tipo de ligação e crescimento da matriz polimérica, que levam ao crescimento de partículas de CaCO3 com morfologia e tamanho variados, explicados em termos da presença de ambientes com diferenças de concentrações locais de Ca2+. Além disso, verificou-se que a rugosidade superficial dos suportes metálicos pode favorecer a formação do polimorfo de CaCO3 cineticamente mais estável, mostrando que o processo de cristalização sobre estes suportes é um processo governado por difusão. A hidrofilicidade dos suportes é aumentada pela presença da matriz orgânica e pela presença de CaCO3 sobre as matrizes. O crescimento de CaCO3 em meios confinados tridimensionais, formados por membranas de policarbonato modificadas com filme finos de polieletrólitos, também foi estudado. Este tipo de molde leva à formação de estruturas cilíndricas que seguem a morfologia dos poros da membrana. A presença de poli(ácido acrílico) leva a formação de estruturas cilíndricas ocas, enquanto que cilindros completamente preenchidos foram formados nos poros contendo quitosana na última camada. Estes resultados foram explicados com base em diferenças na etapa de nucleação: na presença de PAA a nucleação de CaCO3 deve iniciar-se a partir dos íons Ca2+ ligados ao poliânion que, por sua vez, está ligado diretamente às paredes do molde; já na presença que quitosana, com maior capacidade de retenção de liquido e sem interação específica com Ca2+ a nucleação e seqüente cristalização devem ocorrer por todo o poro da membrana. As estruturas formadas são em sua maioria monocristais de calcita hexagonal orientadas na direção cristalográfica <2 -2 1>.<br>Some living organisms as well as their constituents are formed by complex chemical systems which involves the interaction among organic and inorganic compounds bounded physically or chemically. In these systems the organic matrices are usually composed by macromolecules like polysaccharides and proteins. These molecules have an important hole in tailoring the nucleation and the sequent growth of the inorganic portion. Metallic surfaces coated with these hybrid films have potential application as implants for bone substitution for which the surfaces must be chemically inert but at the same time they should stimulate calcification processes. In this present thesis we studied the growth of CaCO3 over aluminium and stainless steal surfaces coated with layer-by-layer films composed by different polyanions and chitosan as polycation, in the presence or not of phospholipids (Langmuir-Blodgett films). These organic matrices formed a confined medium within which CaCO3 particles were growth. Different techniques were applied in order to understand these systems: electronic microscopy, atomic force microscopy, UV-Vis and infrared reflection spectroscopy, Raman, and X-ray scattering and diffraction. We studied the influence of the different charged groups of the phospholipids and the polyanion as well as their conformation on CaCO3 growth. The type of interaction between the polycation and the polyanions tailors the growth of the organic matrices, forming films with different thickness and different water retention abilities which change the local supersaturation conditions changing the structure of the CaCO3 formed. Two types of CaCO3 polymorphs were growth over poly(acrylic acid) (PAA) and chitosan films suggesting that there are two sites where the nucleation can be started: the CaCl2 solution retained in the gel-like organic films and the Ca2+ ions bounded to the negative groups of the polyanion. In the presence of the LB pre-coating, the nature of the phospholipid polar head tailors the binding and the growth of the polymeric matrices leading to the formation of CaCO3 particles with difference in their sizes and morphologies. This result was explained in basis of the differences in the Ca2+ local concentrations in each situation. Moreover, it was observed that the surface roughness of the supports can favour the formation of vaterite, the kinetically most stable CaCO3 polymorph, showing that the crystallization may be guided by diffusion processes. The hidrophilicity of the supports was improved by the presence of both organic and hybrid films. The growth of CaCO3 in tridimentional confined mediums was done using LbL modified polycarbonate membranes as template. This template leads to the formation of cylindrical CaCO3 particles following the morphology of the membrane pores. CaCO3 tube-like structures were formed in presence of PAA, while rod-like structures were formed in presence of chitosan in the top LbL layer. These results were explained on basis of the difference in the nucleation stages: in the presence of PAA the nucleation starts on the Ca2+ ions bounded to the polyanion that is linked to the walls of the template; in the presence of chitosan that presents higher water retention ability and has no specific interaction with Ca2+ ions, the nucleation and sequent crystallization should occur through the entire pore of the membrane. The electron diffraction patterns showed that the CaCO3 structures are single crystals of the calcite polymorph oriented in < 2 -2 1> crystallographic direction.

Ce mémoire décrit des essais de la micro et l'ultrafiltration dynamique effectués avec trois pilotes de laboratoire, un pilote MSD à disque membranaire rotatif sur deux arbres, un pilote à disque rotatif avec une membrane organique fixe, et un pilote à cylindre tournant avec une membrane concentrique. Le module MSD a été testé en microfiltration de suspensions de CaCO3 où nous avons comparé des membranes céramiques et organiques. Les essais ont confirmé l'importance du recouvrement des disques. Ensuite, nous avons appliqué le système de MSD aux traitements des effluents modèles de surfactants anioniques (solutions de SDBS et SDS), avec une comparaison au système à disque rotatif, qui permettait une pression plus élevée de fonctionnement. En terme de flux de perméat et de taux de rétention, ces deux systèmes possèdent des avantages par rapport aux systèmes de filtration tangentielle classique. Puis, les modules à disque rotatif et à cylindre tournant ont été employés à l'ultrafiltration des solutions d'albumine bovine (BSA) pour simuler l'hémofiltration (traitement de l'insuffisance rénale aiguë)<br>This thesis focuses on dynamic micro and ultrafiltration conducted on three laboratory pilots, one with rotating membrane disks mounted on two shafts (MSD), one with a disk rotating close to a static organic membrane, and one with a cylinder rotating inside a concentric membrane. The MSD module was first used for ultrafiltration of CaCO3 suspensions, and in these tests ceramic and organic membranes were compared. The results confirmed the importance of disk overlapping in this module. Then, the MSD system was applied to the treatment of model wastewater containing anionic surfactants (SDBS and SDS solutions), and compared with the rotating disk system, which permitted a higher operating pressure. These two dynamic filtration systems present advantages compared to crossflow filtration systems, in terms of the permeate flux and membrane retention. In addition, the rotating disk and the rotating cylinder modules were used for ultrafiltration of bovine serum albumin (BSA) solutions to simulate hemofiltration in the treatment of acute renal failure

A indústria da construção civil é conhecida como umas das atividades econômicas que causam os maiores impactos ambientais desde o processo de extração da matéria prima até a produção dos produtos, incluindo o transporte e manutenção do ambiente construído. A produção de um dos seus principais componentes, o cimento, é o maior contribuinte para a emissão de gases de efeito estufa, principalmente devido a queima de combustíveis fósseis. Por este motivo, pesquisas na área de biotecnologia sustentável são conduzidas para diminuir e até mitigar os efeitos danosos provocados pelos fatores que compõem a construção civil. Dentre estas pesquisas destacam-se as que se baseiam na Biomimética, que é uma ciência que busca na Natureza as soluções tecnológicas para os problemas que os desenvolvimentos humanos geralmente apresentam: a geração de resíduos poluentes, uso de produtos químicos tóxicos e processos que operam com energia e pressão elevadas. Com base nos conceitos biomiméticos, este trabalho se propôs a estudar a biomineralização, que é um processo que ocorre na Natureza a milhares de anos e é responsável pela formação de muitas estruturas biomineralizadas tanto no ambiente terrestre como aquático. A biomineralização é um fenômeno provocado pela ação de diversas espécies de microrganismos que durante o processo de obtenção de energia reciclam minerais presentes no solo e na água e os precipitam na forma de sais inorgânicos. Este material precipitado age como agente ligante de partículas como no caso de formações geológicas (estromatólitos) ou exoesqueletos de animais marinhos, por exemplo. Neste estudo foi avaliado a biomineralização por biodeposição de carbonato de cálcio precipitado na presença da espécie de bactéria ureolítica (Bacillus subtilis) em ensaios em escala laboratorial utilizando corpos de prova de areia, argamassa e concreto. Os corpos de prova em areia e argamassa foram observados em MEV e EDS permitindo a identificação de células de microrganismos, formação de biofilme e provável formação de cristais de carbonato de cálcio na região de biofilme. Os corpos de prova de concreto foram utilizados para avaliar as consequências da biodeposição na absorção de água por capilaridade do material. Resultados indicam redução de 20% na absorção de água por capilaridade. Com os resultados obtidos é possível concluir que a técnica de biodeposição pode ser uma alternativa ao tratamento superficial de estruturas de concreto, contudo requer estudos posteriores de aplicação técnica e viabilidade econômica.<br>The construction industry has been known as one of the economic activities that cause the major environment impacts since the process of raw material extraction until the products manufacturing including transport and maintenance of the built environment. The production of one of the main compounds, the cement, is the largest contributor to the greenhouse gas emissions, mainly due to burn fossil fuels. For this reason, researches in sustainable biotechnological area are conducted to minimize and even mitigate the damaging effects either promoted by construction industry factors. Among these ones, it stands out researches based on Biomimetic, which is a science that seeks in Nature the technological solutions for problems that human’s development usually presents: the generation of pollutant residues, the use of toxic chemicals and process that operates in high pressure and energy. Based on biomimetic concepts this research proposes to study the biomineralization, which is a process that has occurred in the Nature for thousands of years and it is responsible for the formation of many structures either in soil and water environments. The biomineralization is a phenomenon caused by several specimens of microorganisms that during the process of obtaining energy, they recycle minerals presents at soil and water inducing precipitation as inorganic salts. This precipitated material works as a binder of particles similar to geologic formations (stromatolites) or exoskeleton of sea animal for example. In this study the biomineralization was evaluated through biodeposition of precipitated calcium carbonate by specimen of ureolytic bacteria (Bacillus subtilis). Essays were held using samples made by sand, mortar and concrete. The samples made by sand and mortar were observed at MEV and EDS, allowing the identification of microorganism cells, biofilm formation and probable formation of calcium carbonate crystals at biofilm region. The concrete samples were used to evaluate the consequences of biodeposition on water absorption by capillarity of the material. The results show reduction of 20% on water absorption by capillarity. According the results achieved it possible to conclude that the biodeposition technique can be an alternative to superficial treatment for concrete structures. However, it will be required more studies to evaluate technical application and economical availability.

Thesis (PhD (Chemistry and Polymer Science))--Stellenbosch University, 2008.<br>Polysaccharides were modified with selected polymers via the grafting technique. Both anionic and cationic polysaccharides were prepared. Random and crosslinked graft copolymers were also prepared. The percentage grafting was determined by gravimetric analysis and results were confirmed by cross-polarization magic angle spinning carbon-13 nuclear magnetic resonance microscopy (CP/MAS 13C NMR). These modified biodegradable polymers were then used to flocculate precipitated calcium carbonate (PCC). The effects of pH, percentage grafting, crosslinker concentration and polysaccharide concentration on PCC flocculation were evaluated. Furthermore, the effects of anionic and cationic starch, either added to PCC sequentially or simultaneously, on PCC flocculation were also investigated. Generally, anionically modified starch showed excellent flocculation properties, which are desirable for the end application of PCC retention. The effect of polyacrylic acid (PAA) and polyacrylamide (PAM) modified cellulose fibers on calcium carbonate crystal nucleation and growth modification was investigated. When the heterogeneous crystallization of CaCO3 was carried out in the presence of modified cellulose fibers the CaCO3 crystals were found to be residing on the surface of the fibers. The morphologies of the crystallized CaCO3, polymorph and fiber surface coverage were different for cellulose materials grafted with polymers of different functionalities, meaning that there is interaction between the crystal growth modifier and the growing nuclei. The effect of the modified starch on the crystallization of calcium carbonate gave useful insight into designing CaCO3 filler morphologies. It was found that the filler size, morphology and surface properties of fillers can be tailor-made by choosing suitable CaCO3 crystallization conditions as well as a suitable crystal growth modifier. The crystallized CaCO3 had a negatively charged surface. Results of fluorescence studies showed that the PAA modified starch (polymeric additive used) resided on the surface of the crystals. Thus the presence of the polysaccharide on the surface of a filler could be advantageous for strengthening fiber–filler bonding in paper applications. Anionic starch materials were also used to prepare anionic-starch-coated starch particles. Both the anionic starch and anionic-starch-coated starch particles were evaluated for PCC retention and other properties of hand sheets. When anionic-starch-coated starch particles were used there was generally an improvement in the PCC retention, while the other paper properties remained desirable. The success achieved with the use of anionic-starch-coated starch particles now opens the way for the further preparation and testing of various modified starch particles, for optimization of filler retention.

La biominéralisation des carbonates de calcium est un processus répandu chez les bactéries. Cependant les mécanismes moléculaires sous-jacents restent mal compris. Récemment, la biominéralisation intracellulaire de phases amorphes de carbonate de calcium (iACC) a été découverte chez des cyanobactéries répandues environnementalement et phylogénétiquement. Un gène orphelin de fonction inconnue, nommé ccyA et codant pour une protéine nommée Calcyanine, a été identifié uniquement chez ces cyanobactéries. Ainsi mon travail de thèse a consisté à développer une approche génétique pour analyser la fonction in vivo du gène ccyA et déterminer si la protéine qu’il code est impliquée ou non dans la formation des iACC. L’échec des tentatives de délétion de ccyA chez Cyanothece sp. PCC 7425 et Synechococcus sp. PCC 6312 suggère que ce gène leur est essentiel. Nous avons donc construit une expression hétérologue de ccyA dans des cyanobactéries modèles ne formant pas naturellement des iACC. L’expression constitutive de diverses séquences du gène ccyA a montré que la croissance de Synechococcus elongatus PCC 7942 est ralentie par rapport à celle de la souche sauvage. En accord, les mutants exprimant le ccyA présentent plus de cellules mortes que la souche sauvage. Enfin, ces mutants possèdent plus de calcium intracellulaire que la souche sauvage, suggérant une modification de l’homéostasie du calcium. Enfin, j’ai introduit des plasmides par conjugaison chez Cyanothece sp. PCC 7425 et Synechococcus sp. PCC 6312 leur permettant de surproduire ccyA. La Calcyanine fusionnée avec la eGFP se localise à la membrane et aux pôles des cellules chez la souche Synechococcus sp. PCC 6312<br>The biomineralization of calcium carbonates is a common process in bacteria, however, the underlying molecular mechanisms remain poorly understood. Recently, intracellular biomineralization of amorphous calcium carbonate phases (iACC) has been discovered in environmentally and phylogenetically widespread cyanobacteria. An orphan gene of unknown function, named ccyA and coding for a protein named Calcyanin, has been identified only in these cyanobacteria. My thesis work consisted in developing a genetic approach to analyze the in vivo function of the ccyA gene and determine whether or not the encoded protein is involved in the formation of iACCs. The failure of ccyA deletion attempts in Cyanothece sp. PCC 7425 and Synechococcus sp. PCC 6312 suggests that this gene could be essential to cells viability. We have therefore introduced a plasmid allowing heterologous expression of ccyA in model cyanobacteria that do not naturally form iACC. The constitutive expression of various ccyA sequences in Synechococcus elongatus PCC 7942 is slowed down compared to that of the wild strain. In agreement, ccyA-expressing mutants have more dead cells than the wild-type strain. Finally, these mutants have more intracellular calcium than the wild strain, suggesting a modification of calcium homeostasis. Finally, I introduced plasmids by conjugation in Cyanothece sp. PCC 7425 and Synechococcus sp. PCC 6312 allowing them to overproduce ccyA. Calcyanin fused with eGFP is localized to the membrane and poles of cells in Synechococcus sp. PCC 6312