Motility in Unicellular and Filamentous Cyanobacteria

Both unicellular and filamentous cyanobacteria exhibit diverse types of motility, including the ability to move on surfaces and to swim in liquids. Motile behaviour can be regulated such that cyanobacteria can respond to important environmental signals such as light or chemical gradients. In some cases, components of the motility machinery, such as Type IV pili that allow certain unicellular species to move on surfaces have been characterized. In others, as is the case for swimming in certain marine unicellular species, and gliding in filamentous genera, the motility apparatus has yet to be identified. We describe here recent advances that have led to the identification of multiple photoreceptors and novel proteins involved in motility. The complex signal transduction pathways governing motile behaviour and attempts to model this behaviour are discussed. We conclude with some of the major challenges that remain in our understanding of the complexities of motile behaviour. Introduction Ever since Leuwenhoek’s observations of ‘animalcules’ under the microscope, the motility of microorganisms has fascinated microbiologists. Many prokaryotes have been observed to move either in liquid or on moist surfaces, and this phenomenon was variously described as ‘swimming’, ‘twitching’, ‘swarming’ or ‘gliding’. Both unicellular and filamentous microorganisms exhibit motility and cells can move as individuals or in large coordinated groups. Furthermore, motile behaviour can be regulated which allows microorganisms to respond to various environmental signals such as light or to chemical gradients. In some cases, the motility apparatus required for movement and the signal transduction cascade that allows for a response to environmental cues are well understood, while in others they remain to be discovered. It is also apparent that motility is a complex and regulated phenomenon where more than one mechanism for movement can be used depending on the lifestyle and environmental conditions ( Jarrell and McBride, 2008). For instance, some bacteria encode both flagella and Type IV pili (TFP), while others can produce extracellular materials that aid or are required for motility. Unicellular and filamentous cyanobacteria as well as many photosynthetic microalgae exhibit motile behaviours, and their responses to light are particularly well studied (Gomelsky and Hoff, 2011; Hader and Lebert, 2009; Hoff et al., 2009; Jekely, 2009). The rich history of the study of their motility (initiated by Treviranus in 1817 and continued by Engelmann in Germany), stretches well over a hundred years and has been covered in many excellent and detailed reviews (see section 2). Nevertheless, we feel it is important to keep this history in perspective, particularly for younger researchers, because many fundamental issues that were addressed are still relevant today. Thus, we start with a brief overview of the major mileposts that describe the various motility phenomena in unicellular and filamentous cyanobacteria. These careful experiments and resultant hypotheses set the stage for molecular and mechanistic approaches, although in this chapter, we primarily focus on recent advances. We conclude [14]