Phycology or algology is the study of the algae. The word phycology is derived from the Greek word phykos, which means “seaweed.” The term algology, described in Webster's dictionary as the study of the algae, has fallen out of favor because it resembles the term algogenic, which means “producing pain.” The algae are thallophytes (plants lacking roots, stems, and leaves) that have chlorophyll a as their primary photosynthetic pigment and lack a sterile covering of cells around the reproductive cells. This definition encompasses a number of plant forms that are not necessarily closely related, for example, the cyanobacteria which are closer in evolution to the bacteria than to the rest of the algae.

Algae most commonly occur in water, be it freshwater, marine, or brackish. However, they can also be found in almost every other environment on Earth, from the algae growing in the snow of some American mountains to algae living in lichen associations on bare rocks, to unicellular algae in desert soils, to algae living in hot springs. In most habitats they function as the primary producers in the food chain, producing organic material from sunlight, carbon dioxide, and water. Besides forming the basic food source for these food chains, they also form the oxygen necessary for the metabolism of the consumer organisms. In such cases, humans rarely directly consume the algae as such, but harvest organisms higher up in the food chain (i.e., fish, crustaceans, shellfish). Some algae, particularly the reds and browns, are harvested and eaten as a vegetable, or the mucilages are extracted from the thallus for use as gelling and thickening agents.

Structure of the Algal Cell

There are two basic types of cells in the algae, prokaryotic and eukaryotic. Prokaryotic cells lack membrane-bounded organelles (plastids, mitochondria, nuclei, Golgi bodies, and flagella) and occur in the cyanobacteria (Fig. 2.8). The remainder of the algae are eukaryotic and have organelles.

A eukaryotic cell (Fig. 1.1) is often surrounded by a cell wall composed of polysaccharides that are partially produced and secreted by the Golgi body. The plasma membrane (plasmalemma) surrounds the remaining part of the cell; this membrane is a living structure responsible for controlling the influx and outflow of substances in the protoplasm. Locomotory organs, the flagella, propel the cell through the medium by their beating.

The cyanobacteria or blue-green algae form a natural group by virtue of being the only prokaryotic algae. Prokaryotic algae have an outer plasma membrane enclosing protoplasm containing photosynthetic thylakoids, 70S ribosomes, and DNA fibrils not enclosed within a separate membrane. Chlorophyll a is the main photosynthetic pigment, and oxygen is evolved during photosynthesis.

The Glaucophyta include those algae that have endosymbiotic cyanobacteria in the cytoplasm instead of chloroplasts. Because of the nature of their symbiotic association, they are thought to represent unsuccessful intermediates in the evolution of the chloroplast.

The pigments of the Glaucophyta are similar to those of the Cyanophyceae: both chlorophyll a and the phycobiliproteins are present; however, two of the cyanobacterial carotenoids, myxoxanthophyll and echinenone, are absent (Chapman, 1966).

Although similar to cyanobacteria, the cyanelles should be regarded as organelles rather than endosymbiotic cyanobacteria (Helmchen et al., 1995; McFadden, 2001). Cyanobacteria have over 3000 genes whereas cyanelles have about the same number of genes as plastids (about 200 genes). It is clear the cyanelle (and plastid) genomes have undergone substantial reduction during endosymbiosis. Many of the missing genes eventually relocated to the nucleus, while other genes were lost – made redundant in the cyanelles’ new role as an endosymbiont. For example, cyanobacteria have a respiratory electron-chain whereas plastids do not, the respiratory electronchain is coded by the nucleus in eukaryotic algae.

The organisms in the Glaucophyta are very old; McFadden (2001) calls them the coelocanths of endosymbiosis. The Glaucophyta probably branched off the evolutionary tree before the divergence of red and green algae from one another (Keeling, 2004).

The fact that in such syncyanoses one is dealing with composite organisms that exhibit features altogether new and no longer characteristic of either partner alone, led Skuja in 1954 to establish the phylum Glaucophyta. It must be appreciated that the organisms in the phylum represent a very old group, and that, when evolving, they were very plastic and undergoing a great deal of change in the attempt to reach the relatively stable level of a cell with a chloroplast. Such a dynamic group was formed consisting of a large number of organisms not well suited to compete with their more highly developed progeny. Such a situation led to the demise of many of the original members of the Glaucophyta, resulting in the existence today of few extant members of the group.

The Bacillariophyceae or the diatoms probably evolved from a scaly member of the Chrysophyceae (similar to the organisms in the Parmales) or Bolidophyceae (Guillou et al., 1999 ; Medlin, 2016) around the Devonian–Carboniferous transition (360 Ma)(Brown, 2010). The diatoms are unicellular, sometimes colonial, algae found in almost every aquatic habitat as free-living photosynthetic autotrophs, colorless heterotrophs, or photosynthetic symbiotes (Schmaljohann and Rottger, 1978). They may occur as plankton or periphyton, with most brownish-green films on substrates such as rocks or aquatic plants being composed of attached diatoms. The cells are surrounded by a rigid two-part boxlike cell wall composed of silica, called the frustule. The chloroplasts contain chlorophylls a, c 1, and c 2 with the major carotenoid being the goldenbrown fucoxanthin, which gives the cells their characteristic color.

In discussing diatoms and silica, there is often confusion over terminology in regard to silicon. Silicon is the element. Silica is a short convenient designation for silicon dioxide (SiO 2) in all of its crystalline, amorphous, and hydrated or hydroxylated forms. Silicate is any of the ionized forms of monosilicic acid [Si(OH) 4] (Iler, 1979).

Cell Structure

The two-part frustule surrounds protoplasm that has a more or less central nucleus suspended in a system of protoplasmic threads. The chloroplasts occupy most of the cell usually as two parietal plastids although sometimes as numerous discoid plastids. The storage product, chrysolaminarin, occurs in vesicles in the protoplasm.

Cell Wall

The characteristic feature of the Bacillariophyceae is their ability to secrete an external wall composed of silica, the frustule. It is constructed of two almost equal halves, the smaller fitting into the larger like a Petri dish (Figs. 17.1, 17.8, 17.9, 17.34). The outer of the two half-walls is the epitheca and the inner the hypotheca. Each theca is composed of two parts, the valve, a more or less flattened plate, and the connecting band, attached to the edge of the valve. The two connecting bands, one attached to each valve, are called the girdle (von Stosch, 1975). Sometimes the connecting bands themselves are called girdle bands (Fig. 17.34).

Euglenoid flagellates occur in most freshwater habitats: puddles, ditches, ponds, streams, lakes, and rivers, particularly waters contaminated by animal pollution or decaying organic matter (Buetow, 1968). Usually larger bodies of purer water, such as rivers, lakes, and reservoirs, have sparser populations of less common euglenoids as planktonic organisms. Marine euglenoids are more common than supposed, with Eutreptia, Eutreptiella (Figs. 6.8, 6.11(c)), and Klebsiella occurring exclusively in marine or brackish water, and many other genera having one or a few marine species. These occur in the open sea, in tidal zones among seaweeds, and as sand inhabitants on beaches. Brackish species of Euglena (Figs. 6.1, 6.2, 6.4, 6.11(a)) often color estuarine mud flats green when light intensity is low, the green color disappearing in full sunlight as the euglenoids creep away from the surface. There are also several parasitic euglenoid flagellates, mostly species of Khawkinea, Euglenamorpha, and Hegneria.

Euglenoids are characterized by chlorophylls a and b, one membrane of chloroplast endoplasmic reticulum, a mesokaryotic nucleus, flagella with fibrillar hairs in one row, no sexual reproduction, and paramylon or chrysolaminarin as the storage product in the cytoplasm.

Euglenoid cells have two basal bodies and one or two emergent flagella (Fig. 6.2). The flagella are similar to those of trypanosomes in having a paraxonemal rod (paraxial rod) that runs the length of the flagellum inside the flagellar membrane (Ngo and Bouck, 1998 ; Bastin and Gull, 1999 ; Talke and Preisfeld, 2002). The paraflagellar rod is composed of two major proteins forming an elongated alpha-helical stalk that parallels the axoneme. The one emergent flagellum in Euglena has helically arranged fibrillar hairs (no microtubules) attached along the length of the flagellar membrane. The fibrillar hairs are of two lengths: there is a single helical row of long (3 μm) hairs and two helical rows of short (1.5 μm) hairs in Euglena (Bouck et al., 1978). Other genera have flagella similar to Euglena (Hilenski and Walne, 1985). There are two basic types of flagellar movement in the class.

These golden-brown algae are characterized by tentacles or rhizopodia on basically amoeboid vegetative cells (Moestrup, 1995 ; Preisig, 1995). Amoeboid cells are relatively rare among the algae, being mostly restricted to the Dictyochophyceae and the Xanthophyceae (Hibberd and Chretiennot-Dinet, 1979). The algae in the Dictyochophyceae have been previously classified in the Chrysophyceae, although molecular evidence shows them to be most closely related to the Pelagophyceae (Cavalier-Smith et al., 1995) or Eustigmatophyceae (Daugbjerg and Andersen, 1997).

Classification

The Dictyochophyceae can be divided into three orders (Preisig, 1995).

Rhizochromulinales

This order contains the more primitive organisms in the order (O'Kelly and Wujek, 1995). Rhizochromulina (Fig. 14.1(a), (b)) has amoeboid non-flagellated vegetative cells with many finebeaded filipodia and a single golden-brown chloroplast (Hibberd and Chretiennot-Dinet, 1979). The fusiform zoospore has a single tinsel flagellum with a second basal body in the protoplasm (Fig. 14.1(b)). Chrysoamoeba (Fig. 14.1(d)) lives as a solitary amoeba for the greater part of its life cycle, transforming into swimming cells with a single long flagellum only for short periods. In Phaeaster (Fig. 14.1(c)), the anterior portion of the cell is drawn out into rhizopodia.

Pedinellales

The pedinellids are unique in containing genera that are phototrophic (Apedinella (Fig. 14.2(c)), Pseudopedinella), mixotrophic (able to photosynthesize and take up organic compounds) (Pedinella) (Fig. 14.2(a), (b)), and phagotrophic (Pteridomonas, Ciliophrys). The phagotrophic genera have vestigial chloroplasts and evolved from genera with chloroplasts (Sekiguchi et al., 2002). The organisms in this order have three interconnected microtubules (triads) that course from the nuclear envelope through tentacles (if they are present) to the plasma membrane (Fig. 14.2(a)) (Daugbjerg, 1996).

Algae are organisms that have plastids, or organisms that are derived from cells whose ancestors possessed plastids. Until 1994, it was thought that the apicomplexa did not have plastids (and consequently were not covered in phycology textbooks). Then it was shown that a known organelle in many apicomplexa was actually a reduced colorless plastid called an apicoplast with a genome reduced to 35 kb (Fig. 8.1) (Wilson, 1993 ; Wilson et al., 1994 ; Parsons et al., 2009 ; McFadden, 2011). Molecular studies have shown that the apicoplast and dinoflagellate plastids are related and originated from red algae by a single endosymbiotic event that occurred relatively early in eukaryotic evolution (Fast et al., 2001 ; Janouskovec et al., 2010). Chromera velia has chlorophyll a, is photosynthetic, and is a relative of the apicomplexa with similar cell structure (Janouskovec et al., 2010 ; Obornek et al., 2011).

The discovery of the apicoplast generated considerable interest since most apicomplexans are unicellular endoparasites that cause some of the most significant tropical diseases (Foth and McFadden, 2003). Malaria in humans is produced by the apicomplexan Plasmodium. About 300 million people are infected with malaria, leading to one million deaths annually (Ralph et al., 2004). Apicomplexans cause other serious diseases in livestock and humans, such as cryptosporidiosis, babesiosis (Texas cattle fever), theileriosis (East Coast fever), and toxoplasmosis. The realization that these endoparasites were once algae raised hopes that the apicoplast might be a drug target for two reasons (Goodman and McFadden, 2014). The first is that the apicoplast is essential for the survival of Plasmodium and Toxoplasma. The second is that drugs effective against prokaryotic organisms might be effective against the apicoplast since all plastids originally evolved from endosymbiotic prokaryotic cyanobacteria. Apicomplexans are absolutely dependent on the apicoplast, which has led to speculation that this curious organelle is a potential “Achilles heel” of parasites, such as Plasmodium.

The apicomplexan have functional mitochondria. However, the apicomplexan mitochondria are unconventional in that the majority of cellular ATP is synthesized through glycolysis and lactic acid production and there is no evience of mitochondrial acetyl CoA (Butterfield et al., 2013).