Introduction to Cyanobacteria

Leticia Vidal, Andreas Ballot,

Sandra M. F. 0. Azevedo, Judit Padisak and Martin Welker

Introduction

Cyanobacteria are a very diverse group of prokaryotic organisms that thrive in almost every ecosystem on earth. In contrast to other prokaryotes (bacteria and archaea), they perform oxygenic photosynthesis and possess chlorophyll-д. Their closest relatives are purple bacteria (Woese et al., 1990; Cavalier-Smith, 2002) - and chloroplasts in higher plants (Moore et al., 2019). Photosynthetic activity of cyanobacteria is assumed to have changed the earth’s atmosphere in the Proterozoic Era some 2.4 billion years ago during the so-called Great Oxygenation Event (Hamilton et al., 2016; Garcia-Pichel et al., 2019).

Historically, cyanobacteria were considered as plants or plant-like organisms and were termed “Schizophyceae”, “Cyanophyta”, “Cyanophyceae” or “blue-green algae”. Since their prokaryotic nature has unambiguously been proven, the term “cyanobacteria” (or occasionally “cyanoprokary- otes”) has been adopted in the scientific literature. A metagenomic study by Soo et al. (2017) revealed that cyanobacteria also comprise groups of nonphotosynthetic bacteria and the taxon Oxyphotobacteria is proposed for cyanobacteria in a strict sense. However, in this volume, the term “cyanobacteria” will be used for photosynthetic, oxygenic bacteria.

Cell Types and Cell Characteristics

As prokaryotes, cyanobacteria lack a cell nucleus and other cell organelles, allowing their microscopic distinction from most other microalgae. In particular, cyanobacteria lack chloroplasts, and instead, the chlorophyll for the photosynthesis is contained in simple thylakoids, the site of the light- dependent reactions of photosynthesis (exception: Gloeobacter spp. not possessing thylakoids). Cyanobacteria occur as unicellular, colonial or multicellular filamentous forms. Diverse forms populate all possible environments where light and at least some water and nutrients are available - even if only in very low quantities. Examples for extreme environments in which cyanobacteria can be encountered are caves or deserts (Whitton & Potts, 2000). This volume primarily considers cyanobacteria in the aquatic environments where they may grow suspended in water (i.e., as “plankton”), attached to hard surfaces (“benthos” or “benthic”, respectively), or to macrophytes or any other submerged surfaces (“periphytic” or “metaphytic”).

Sexual reproduction has not been observed for cyanobacteria; therefore, their only means of reproduction is asexual, through division of vegetative cells.

The morphology of cyanobacterial cells shows a number of characteristics that can be used for microscopic examination and identification: primarily, the shape and size of cells, subcellular structures and specialised cells (Figure 3.1-3.3). Cyanobacterial cells can be spherical, ellipsoid, barrelshaped, cylindrical, conical or disc-shaped. Some taxa include cells of different shapes. Cyanobacteria do not possess flagella, as are found in many other bacterial or phytoplankton taxa. Nevertheless, many cyanobacteria, in particular filamentous forms, show gliding motility, the mechanism of which is not yet fully understood (Hoiczyk, 2000; Read et ah, 2007).

The size of cyanobacteria varies considerably between taxa: more or less spherical cells of unicellular cyanobacteria range in diameter from about 0.2 pm to over 40 pm. In consequence, cell volume may vary by a factor of at least 300 000, making simple cell counts an unreliable parameter for the determination of biomass, especially when reported without differentiation between individual taxa (see Chapter 13). Some filamentous forms have been observed to have cell diameters of up to 100 pm, but as these coin-shaped cells are generally very short, their cell volume is not necessarily much larger than that of other species (Figure 3.2; Whitton & Potts, 2000). The length of filaments (or trichomes; see below) can reach a few millimetres in certain benthic forms. Very small cells of cyanobacteria (in the size range 0.2-2 pm) have been recognised as a significant fraction of

Figure 3.1 Characteristics of cyanobacteria filaments, (a) General shapes; (b) presence of sheaths; (c) branching types.

Figure 3.2 Characteristics of cyanobacteria filaments, (a) Cell shapes and arrangement in filaments; (b) cell length-to-width ratios; (c) filament terminal region.

Figure 3.3 Arrangement of heterocytes (a) and akinetes (b) in filamentous cyanobacteria.

the so-called picoplankton in various freshwater and marine environments, such as Prochlorococcus that is found in huge numbers in the world’s oceans (Flombaum et ah, 2013). The occurrence of picocyanobacteria in freshwaters is well established (Postius & Ernst, 1999; Stomp et ah, 2007) but possibly is underestimated, especially when biomass estimates are based on microscopy. With molecular tools such as metagenomics (section 13.4), our understanding of the role of picocyanobacteria in lake ecosystems may increase (Sliwiriska-Wilczewska et ah, 2018; Nakayama et ah, 2019).

A number of cyanobacterial taxa can (facultatively) produce so-called aerotopes that are clearly visible in microscopy as light-refracting structures. Aerotopes (sometimes incorrectly named “gas vacuoles” - they are not vacuoles in the cytological sense) are bundles of cylindrical protein microstructures that form the gas vesicles. These vesicles are filled with air entering the lumen by diffusion (see Walsby (1994) for an extensive review). Gas vesicles have a density of about one-tenth of that of water and thus render the entire cells less dense than water, providing buoyancy and making them float or emerge to the water surface (see Section 3.2). The gas vesicles measure some 75 nm in diameter and up to 1.0 pm in length. The cylinders, capped by conical ends, are formed by a single wall layer of 2 nm thickness. The distribution of aerotopes within the cells is characteristic for individual taxa and can be used for identification by microscopical examination, but they can disintegrate after fixation with Lugol’s solution (see Chapter 13).

Other subcellular (ultrastructural) characteristics such as the distribution of thylakoids are used in taxonomic studies (Hoffmann et al., 2005; Komarek et ah, 2014). As thylakoids are not visible using light microscopy with standard equipment, other methodologies are generally applied for their examination, such as transmission electron microscopy.

In some groups of cyanobacteria (see Table 3.1), specialised cells occur, which are morphologically different from vegetative cells and which can be

Table 3.1 Major groups of cyanobacteria in the taxonomic schemes proposed by Castenholz et al. (2001) and Cavalier-Smith (2002)

The morphological characteristics are based on microscopic observation. Exemplary genera are given for subsections.

generally easily recognised by light microscopy (see examples below), that is, heterocytes and akinetes.

Heterocytes are specialised cells that allow the fixation of atmospheric nitrogen, a process also called diazotrophy that involves nitrogenases, enzymes capable to reduce nitrogen to ammonium (Berman-Frank et al., 2003). Note that “heterocyte” is the more appropriate term than the traditionally used term “heterocyst” because a “cyst” has another, clearly defined meaning in cytology. Both terms may be seen as synonyms, while in this volume the term “heterocyte” is preferred.

Heterocytes lack the complete photosynthetic apparatus, thus avoiding the production of oxygen which would irreversibly damage nitrogenases (Bothe et al., 2010). Further, they possess a thickened cell wall, which further supports the anoxic intracellular milieu needed for diazotrophy.

Heterocytes often differ in size and shape from vegetative cells. In the microscope, they are generally easily recognised due to their different size and light refraction properties. Their number and the location of heterocytes in filaments can be used for taxonomic determination (Figure 3.3), although heterocyte formation depends on environmental and physiological conditions and may hence vary. They may be completely absent under conditions of ample availability of inorganic nitrogen. For example, Apbanizomenon spp. without heterocytes may be confused with Planktotbrix agardhii if the terminal cells of the filaments are not examined carefully. Some authors suggested that Rapbidiopsis spp. could be a nonheterocytous stage or type of Cylindrospermopsis spp. as recent studies showed both taxa to be phylogenetically very close (Moustaka-Gouni et al., 2009) and should hence be combined (Aguilera et ah, 2018).

Akinetes are resting stages that can be found in the same taxa that form heterocytes. They are characterised by a generally (much) larger size compared to vegetative cells and different light refraction in microscopic view. Their cell wall is multilayered, and they often contain granules of glycogen and cyano- phycin but generally no polyphosphate granules. Akinete formation and germination is triggered by environmental conditions (Adams & Duggan, 1999).

The position, number and distribution of the heterocytes and akinetes are important morphological characteristics of species and genera. Heterocytes can be in an intercalary position between vegetative cells, that is, in the middle of a trichome, or terminal or subterminal. Akinetes are in an intercalary or subterminal position but generally not terminal. Because the formation of heterocytes and akinetes is triggered by environmental conditions, individual species can appear variable in natural samples or strain cultures. The distribution of these specialised cells also determines the symmetry of the trichome.