Cyanobacteria are complicated prokaryotes. They have many distinctive features including a series of internal thylakoid membranes, the site of photosynthesis, and carboxysomes, where carbon fixation occurs.
Understanding the biochemistry and physiology of cyanobacteria
Schematic indicating the different cellular features of Synechocystis sp. PCC 6803
Electron micrograph of Synechococcus sp. PCC 7002
We have excellent genetic tools for generating cyanobacterial mutants. These are outlined in a paper and video in the Journal of Visualised Experiments:
BG11 agar plates of different Synechocystis sp. PCC 6803 mutants
We then analyse these mutants using a range of biochemical, microscopic and physiological techniques. A major area of interest in the lab is photosynthesis and respiration, specifically electron transport. Cyanobacteria are the simplest organisms capable of oxygenic photosynthesis. This occurs in the thylakoid membranes. In photosystem II, light energy is used to split water into oxygen, protons (H+) and electrons. The electrons are transferred to plastoquinone, then cytochrome b6f, plastocyanin, photosystem I, ferredoxin, before terminating at FNR:ferredoxin-NADP+-reductase, where NADPH is produced. Further proteins are pumped into the middle of the thylakoid membrane (lumen) throughout this process. These protons are used to generate ATP as they pass through another protein complex, ATP synthase. ATP and NADPH fuel various processes in the cell, most notably carbon fixation.
The electron transport chain in the thylakoid membranes of Synechocystis sp. PCC 6803. PSII: Photosystem II, Flv2/4: Flavodiiron 2/4, PQ: plastoquinone, PQH2: plastoquinol, cyt b6f: cytochrome b6f, Pc: plastocyanin, Cyt c6: cytochrome c6, PSI: Photosystem I, Fd: ferredoxin, FNR: ferredoxin-NADP+-reductase, NDH-1: NAD(P)H dehydrogenase 1, SDH: succinate dehydrogenase, NDH-2: NAD(P)H dehydrogenase 2, Cyd: bd-quinol oxidase, COX: cytochrome-c oxidase (Adapted from Lea-Smith et al, 2016)
However, other protein complexes such as the terminal oxidases and flavodiiron complexes, also interact with the electron transport chain. These complexes allow the cells to respond to a range of environmental conditions. For example, terminal oxidases play a role in respiration, which provides energy for cyanobacterial cells under dark conditions. However, they also protect the cell from excess light, by safely transferring electrons to oxygen to generate water. In their absence, electrons react randomly with oxygen, resulting in production of reactive oxygen species, which damage and eventually kill the cells.
Cultures of Synechocystis sp. PCC 6803 mutants deficient in various combinations of the terminal oxidases. Deletion of cytochrome-c oxidase (COX) and bd-quinol oxidase, results in a mutant (COX/Cyd) unable to survive sudden high light/dark changes, as shown by the loss of chlorophyll (Adapted from Lea-Smith et al, 2013)
Ermakova M, Huokko T, Richaud P, Bersanini L, Howe CJ, Lea-Smith DJ, Peltier G, Allahverdiyeva (2016). Distinguishing the alternate roles of thylakoid respiratory terminal oxidases in the cyanobacterium Synechocystis sp. PCC 6803. Plant Physiology 171 (2), 1307-1319.
Lea-Smith DJ, Bombelli P, Vasudevan AG, Howe CJ (2016).
Photosynthetic, respiratory and extracellular electron transport pathways in cyanobacteria. Biochemica Biophysica Acta-Bioenergetics 1857 (3), 247-255.
Photosynthetic, respiratory and extracellular electron transport pathways in cyanobacteria
Lea-Smith DJ, Ross N, Zori M, Bendall DS, Dennis JS, Scott SA, Smith AG, Howe CJ (2013). Thylakoid terminal oxidases are essential for the cyanobacterium Synechocystis sp. PCC 6803 to survive rapidly changing light intensities. Plant physiology 162 (1), 484-49.
Professor Christopher Howe, Department of Biochemistry, University of Cambridge
Dr Yagut Allahverdiyeva-Rinne, Faculty of Mathematics and Natural Science, University of Turku, Finland
We are also investigating membrane dynamics in cyanobacteria, specifically the role of hydrocarbons in inducing curvature and flexibility. Cyanobacterial cells lacking hydrocarbons demonstrate larger cell size, slower growth and division defects. Preliminary data suggests that membranes lacking hydrocarbons are less curved but further validation of this hypothesis is required.
Wild-type (left) and hydrocarbon deficient (right) Synechococcus sp. PCC 7002 cells.
Modelling of cyanobacterial membranes suggest that hydrocarbons (red compounds) accumulate within the middle of the membrane, resulting in swelling and curvature.
Lea-Smith DJ, Ortiz-Suarez ML, Lenn T, Nuernberg DJ, Baers LL, Davey MP, Cotton CAR, Mastroianni M, Bombelli B, Ungerer P, Stevens TJ, Smith AG, Bond PJ, Mullineaux CW, Howe CJ (2016). Hydrocarbons are essential for optimal cell size, division and growth of cyanobacteria. Plant Physiology 172 (3), 1928-1940.