Selective Metal Ion Homeostasis in Cyanobacteria
Metal homeostasis systems are responsible for the uptake and efflux of both essential and non-essential metals. The capacity of these systems to acquire a particular metal, whilst excluding another is essential for the survival of not just cyanobacteria,
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Abstract Metal homeostasis systems are responsible for the uptake and efflux of both essential and non-essential metals. The capacity of these systems to acquire a particular metal, whilst excluding another is essential for the survival of not just cyanobacteria, but all organisms. The initial step in the acquisition of metal ions from the environment is the physiological binding, or adsorption, of metals to cells. The second step, often energy expensive, is the internalisation of metals, which is facilitated by uptake systems. Metal release from cells requires an efflux system. Both uptake and efflux systems may be controlled by their own regulatory elements. The effectiveness of these transport systems is dependent upon their ability to discriminate effectively between metals. This discrimination is achieved largely by the proteins involved comprising of different metal coordinating ligands strategically positioned in the tertiary structures. For cyanobacteria, arguably the most adept organisms at survival on earth, the information on metal coordination and binding is still limited. However, studies identifying and providing functional characterisation of metal transporters and metalloproteins in cyanobacteria are contributing new insights into metal homeostasis across all living organisms. Keywords Cyanobacteria transporters Ligands
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1 Introduction For cyanobacteria, the trace elements cobalt, copper, iron, manganese, magnesium, nickel and zinc are essential for cellular metabolism. Such metals are only required at trace (ppb-ppm) levels, with either deficiency or over-abundance of these metal ions reducing cell viability and potentially leading to death. Non-essential metals L. Hudek ⋅ M.L. Ackland (✉) Centre for Cellular and Molecular Biology, School of Life and Environmental Sciences, Deakin University, Burwood, Victoria, Australia e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2017 B.N. Tripathi and D. Kumar (eds.), Prospects and Challenges in Algal Biotechnology, DOI 10.1007/978-981-10-1950-0_7
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such as: arsenic, cadmium, chromium and lead are not required by cells, and whilst they may be tolerated at low concentrations, they have overall deleterious effects on cellular metabolic activity resulting in death. Metal levels in the environment fluctuate greatly. In soils, metal levels can range from less than 1 ppm (0.0001%), through to 100,000 ppm (10%) or higher in extreme examples, such as mineral ore deposits (Olajire et al. 2003; Wuana and Okieimen 2011; J. Halili et al. 2013; Nessa and Jewel 2014). Cyanobacteria (Cyanophyta) are arguably the most diverse phylum exemplified by Prochlorococcus marinus and Nostoc punctiforme. Prochlorococcus spp. typically represents 50% of total chlorophyll in sub-tropical marine waters, contributes 30–80% of the total photosynthetic activity in oligotrophic oceans, physically are the smallest known phototrophs (at 0.5–0.7 µM) and have a genome size of only ∼1.8 Mb (P
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