Cyanobacterial morphology
Cyanobacterial morphology refers to the form or shape of cyanobacteria. Cyanobacteria are a large and diverse phylum of bacteria defined by their unique combination of pigments and their ability to perform oxygenic photosynthesis.
Cyanobacteria often live in colonial aggregates that can take a multitude of forms. Of particular interest among the many species of cyanobacteria are those that live colonially in elongate hair-like structures, known as trichomes. These filamentous species can contain hundreds to thousands of cells. They often dominate the upper layers of microbial mats found in extreme environments such as hot springs, hypersaline water, deserts and polar regions, as well as being widely distributed in more mundane environments.
Many filamentous species are also motile, gliding along their long axis, and displaying photomovement by which a trichome modulates its gliding according to the incident light. The latter has been found to play an important role in guiding the trichomes to optimal lighting conditions, which can either inhibit the cells if the incident light is too weak, or damage the cells if too strong.
Overview
Cellular functions require a well-organized and coordinated internal structure to operate effectively. Cells need to build, sustain, and sometimes modify their shape, which allows them to rapidly change their behaviour in response to external factors. During different life cycle stages, such as cell growth, cell division or cell differentiation, internal structures must dynamically adapt to the current requirements. In eukaryotes, these manifold tasks are fulfilled by the cytoskeleton: proteinaceous polymers that assemble into stable or dynamic filaments or tubules in vivo and in vitro. The eukaryotic cytoskeleton is historically divided into three classes: the actin filaments, the microtubules and the intermediate filaments, although other cytoskeletal classes have been identified in recent years. Only the collaborative work of all three cytoskeletal systems enables proper cell mechanics.The long-lasting dogma that prokaryotes, based on their simple cell shapes, do not require cytoskeletal elements was finally abolished by the discovery of FtsZ, a prokaryotic tubulin homolog, and MreB, a bacterial actin homolog. These discoveries started an intense search for other cytoskeletal proteins in bacteria and archaea which finally led to the identification of bacterial IF-like proteins such as Crescentin from Caulobacter crescentus and even bacterial-specific cytoskeletal protein classes, including bactofilins. Constant influx of new findings finally established that numerous prokaryotic cellular functions, including cell division, cell elongation or bacterial microcompartment segregation are governed by the prokaryotic cytoskeleton.
Cyanobacteria are today's only known prokaryotes capable of performing oxygenic photosynthesis. Based on the presence of an outer membrane, cyanobacteria are generally considered Gram-negative bacteria. However, unlike other Gram-negative bacteria, cyanobacteria contain an unusually thick peptidoglycan layer between the inner and outer membrane, thus containing features of both Gram phenotypes. Additionally, the degree of PG crosslinking is much higher in cyanobacteria than in other Gram-negative bacteria, although teichoic acids, typically present in Gram-positive bacteria, are absent.
While Cyanobacteria are monophyletic, their cellular morphologies are extremely diverse and range from unicellular species to complex cell-differentiating, multicellular species. Based on this observation, cyanobacteria have been classically divided into five subsections. Subsection I cyanobacteria are unicellular and divide by binary fission or budding, whereas subsection II cyanobacteria are also unicellular but can undergo multiple fission events, giving rise to many small daughter cells termed baeocytes. Subsection III comprises multicellular, non-cell differentiating cyanobacteria and subsection IV and V cyanobacteria are multicellular, cell differentiating cyanobacteria that form specialized cell types in the absence of combined nitrogen, during unfavorable conditions or to spread and initiate symbiosis. Whereas subsections III and IV form linear cell filaments that are surrounded by a common sheath, subsection V can produce lateral branches and/or divide in multiple planes, establishing multiseriate trichomes. Considering this complex morphology, it was postulated that certain subsection V-specific proteins could be responsible for this phenotype. However, no specific gene was identified whose distribution was specifically correlated with the cell morphology among different cyanobacterial subsections. Therefore, it seems more likely that differential expression of cell growth and division genes rather than the presence or absence of a single gene is responsible for the cyanobacterial morphological diversity.
Cell walls
To protect bacteria from unpredictable and often hostile environments, their cells are enclosed by a complex multilayered structure. The structure of this layer is not similar for all bacteria and is divided into two major categories known as Gram-positive and Gram-negative. Gram-positive bacteria are surrounded by a thick layer of peptidoglycan and lack an outer membrane. By contrast, Gram-negative bacteria generally have a thin peptidoglycan cell wall, as well as an outer membrane containing lipopolysaccharide.All described species of cyanobacteria are Gram-negative. Gram-negative means the cell wall composition has specific characteristics involving a layer of peptidoglycan which is generally thin and a lipopolysaccharide outer membrane. However, cyanobacteria’s peptidoglycan layer is thicker compared to most Gram-negative bacteria. Unicellular strains such as Synechococcus have relatively thin peptidoglycan layers, while filamentous strains such as Oscillatoria princeps have relatively thick layers.
Aside from protection and sensing environmental stress, transport of nutrients and metabolites into and out of the cell is one of the major roles of the cellular membrane. Marine systems teem with viruses that target bacteria. The outer membrane in cyanobacteria provides an extra layer of protection against phage attachment and injection. Many marine phages specifically recognize LPS or outer membrane proteins. Gram-positive bacteria lack the additional outer membrane, which means they can be more easily injected by phages, as illustrated in the diagram.
Cyanobacterial grazing by protists can be intense in the photic zone, and the outer membrane may reduce vulnerability to engulfment or enzymatic attack. The outer membrane can also confer resistance to toxins or osmotic fluctuations common in marine gradients.
Cyanobacteria were the endosymbionts that gave rise to plastids in eukaryotic algae and plants. The double-membrane structure of plastids mirrors the diderm Gram-negative envelope. This structural match facilitated successful endosymbiosis, which in turn amplified cyanobacterial-like photosynthesis in marine eukaryotic phytoplankton. Gram-positive bacteria would lack this compatible envelope for such integration.
The thinner peptidoglycan and outer membrane makes Gram-negative cells generally smaller and more streamlined than Gram-positive cells. This aids buoyancy regulation and diffusion in water columns. As a result, many cyanobacteria can use gas vesicles as buoyancy control for vertical positioning to optimise light. Some cyanobacteria have thick peptidoglycan. This can combine robustness with the protective outer membrane—ideal for withstanding shear forces or ultraviolet light in surface waters.
Morphogenesis
is the biological process that causes an organism to develop its shape. Cyanobacteria show a high degree of morphological diversity and can undergo a variety of cellular differentiation processes in order to adapt to certain environmental conditions. This helps them thrive in almost every habitat on Earth, ranging from freshwater to marine and terrestrial habitats, including even symbiotic interactions.One factor which can drive morphological changes in cyanobacteria is light. As cyanobacteria are bacteria that use light to fuel their energy-producing photosynthetic machinery they depend on perceiving light in order to optimize their response and to avoid harmful light that could result in the formation of reactive oxygen species and subsequently in their death. Optimal light conditions may be defined by quantity, duration and wavelength. The photosynthetically usable light range of the solar spectrum is generally referred to as PAR, but some cyanobacteria may expand on PAR by not only absorbing in the visible spectrum, but also the near-infrared light spectrum. This employs a variety of chlorophylls and allows phototrophic growth up to a wavelength of 750 nm. To sense the light across this range of wavelengths, cyanobacteria possess various photoreceptors of the phytochrome superfamily.
Morphological plasticity, or the ability of one cell to alternate between different shapes, is a common strategy of many bacteria in response to environmental changes or as part of their normal life cycle. Bacteria may alter their shape by simpler transitions from rod to coccoid as in Escherichia coli, by more complex transitions while establishing multicellularity or by the development of specialized cells, structures or appendages where the population presents a pleomorphic lifestyle. The precise molecular circuits that govern those morphological changes are yet to be identified, however, a so-far constant factor is that the cell shape is determined by the rigid PG sacculus which consists of glycan strands crosslinked by peptides. To grow, cells must synthesize new PG while breaking down the existent polymer to insert the newly synthesized material. How cells grow and elongate has been extensively reviewed in model organisms of both, rod-shaped and coccoid bacteria. The molecular basis for morphological plasticity and pleomorphism in more complex bacteria, however, is slowly being elucidated as well.
Despite their morphological complexity, cyanobacteria contain all conserved and so far known bacterial morphogens. Understanding cyanobacterial morphogenesis is challenging, as there are numerous morphotypes among cyanobacterial taxa, which can also vary within a given strain during its life cycle. Changes in cellular or even trichome morphologies are tasks that would require active cell wall remodelling and thus far no genes attributed to the different morphotypes have been identified in cyanobacteria. Therefore, the most likely scenario is that genes or their products are differentially regulated during these cell morphology transitions, as it has been hypothesized for most bacteria. In multicellular cyanobacteria, division of labor between cells within a trichome is achieved by different cell programing strategies. Thus, gene regulation occurs differentially in these specific cell types .