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 intermediates in the evolution of the chloroplast. The endosymbiotic theory of chloroplast evolution, first proposed by Mereschkowsky in 1905, is the one most widely accepted. According to this theory, a cyanobacterium was taken up by a phagocytic organism into a food vesicle. Normally the cyanobacterium would be digested by the flagellate, but by chance a mutation occurred, with the flagellate being unable to digest the cyanobacterium. This was probably a beneficial mutation because the cyanobacterium, by virtue of its lack of feedback inhibition, secreted considerable amounts of metabolites to the host flagellate. The flagellate in turn gave the cyanobacterium a protected environment, and the composite organism was probably able to live in an ecological niche where there were no photosynthetic organisms (i.e., a slightly acid body of water where free-living cyanobacteria do not grow; see Chapter 2). Pascher (1914) coined terms for this association; he called the endosymbiotic cyanobacteria cyanelles; the host, a cyanome; and the association between the two, a syncyanosis. In the original syncyanosis the cyanelle had a wall around it. Because the wall slowed the transfer of compounds from the cyanelle to the host and vice versa, any mutation that resulted in a loss of wall would have been beneficial and selected for in evolution.

PELAGOPHYCEAE

The Pelagophyceae are a group of basically unicellular algae that are cytologically similar to the Chrysophyceae, in which they were previously classified (Andersen et al., 1993). The cells are very small (3–5 μm) members of the ultraplankton and appear as small spheres with indistinct protoplasm under the light microscope. Recent studies on the sequences of small-subunit RNA nucleotides in these algae have shown them to be closely related to each other and distinct from other members of the Heterokontophyta (Saunders et al., 1997). While these algae have been shown to be distinct from other members of the Heterokontophyta based on molecular data, they do not have cytological or morphological characters which are very different from other members of the phylum.

Pelagomonas calceolata is a very small (1.5 μm × 3 μm) ultraplanktonic marine alga with a single tinsel flagellum and basal body, and a single chloroplast and mitochondrion (Fig. 15.1(c)) (Andersen et al., 1993). Another member of the marine ultraplankton is Pelagococcus subviridis, a green-gold spherical non-motile cell (2.5–3.0 μm) with a single chloroplast, mitochondrion, and nucleus (Fig. 15.1(a)) (Vesk and Jeffery, 1987).

Members of the class are economically important because some of the algae produce “brown tides.” Aureoumbra lagunensis (Fig. 15.1(b)) is the causative agent of brown tides in Texas (DeYoe et al., 1997), while Aureococcus anophagefferens forms brown tides along the coasts of New Jersey, New York, and Rhode Island.

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 photo synthetic 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 fresh water, 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.

The Chlorophyta, or green algae, have chlorophylls a and b, and form starch with the chloroplast, usually in association with a pyrenoid. The Chlorophyta thus differ from the rest of the eukaryotic algae in forming the storage product in the chloroplast instead of in the cytoplasm. No chloroplast endoplasmic reticulum occurs around the chloroplasts.

The Chlorophyta are primarily freshwater; only about 10% of the algae are marine, whereas 90% are freshwater (Smith, 1955). Some orders are predominantly marine (Caulerpales, Dasycladales, Siphonocladales), whereas others are predominantly freshwater (Ulotrichales, Coleochaetales) or exclusively freshwater (Oedogoniales, Zygnematales). The freshwater species have a cosmopolitan distribution, with few species endemic in a certain area. In the marine environment, the green algae in the warmer tropical and semitropical waters tend to be similar everywhere in the world. This is not true of the Chlorophyta in the colder marine waters; the waters of the Northern and Southern hemispheres have markedly different species. The warmer waters near the equator have acted as a geographical barrier for the evolution of new species and genera.

Cell structure

In the Chlorophyta, microtubular hairs do not occur on the flagella, although fibrillar hairs (Chlamydomonas, Fig. 1.7(b)) and Golgi-produced scales (Pyramimonas (Fig. 5.10), are present in some genera.

Cell walls usually have cellulose as the main structural polysaccharide, although xylans or mannans often replace cellulose in the Caulerpales (Huizing et al., 1979).

EUSTIGMATOPHYCEAE

Eustimatophytes are yellow-green unicells that occur in freshwater, brackish water, and seawater as well as in the soil. The cells are similar to those in the Xanthophyceae, but differ in having an eyespot outside the chloroplast (Fig. 12.1) (the eyespot in the Xanthophyceae is in the chloroplast) (Hibberd and Leedale, 1970). Other characteristics of the class include a basal swelling of the tinsel flagellum adjacent to the eyespot, only chlorophyll a, chloroplasts without girdle lamellae and no peripheral ring of DNA, and chloroplast endoplasmic reticulum not connected to the nuclear envelope (Schnepf et al., 1996).

The eyespot (Figs. 12.1, 12.2) is a large orangered body at the anterior of the motile cell and is completely independent of the chloroplast. It consists of an irregular group of droplets with no membrane around the whole complex of droplets. The flagellar sheath is extended to form a T-shaped flagellar swelling at the base of the tinsel flagellum (Figs. 12.1, 12.2). This swelling is always closely appressed to the plasmalemma in the region of the eyespot. In turn, in the eyespot there is a large droplet closely applied to the plasmalemma in the area of the flagellar swelling.

The chloroplasts of the Eustigmatophyceae have chlorophyll a and β-carotene, with the two major xanthophylls being violaxanthin and vaucheriaxanthin (Whittle and Casselton, 1969; Antia and Cheng, 1982), the only difference in pigments compared to the Xanthophyceae being the presence of violaxanthin and the absence of antheraxanthin. Violaxanthin is the major light-harvesting pigment in the Eustigmato-phyceae (Owens et al., 1987).

RHODOPHYCEAE

The Rhodophyceae, or red algae, comprise the only class in the division Rhodophyta. The Rhodophyceae are probably one of the oldest groups of eukaryotic algae. The red algae are most likely directly descended from a cyanome in the Glaucophyta (see Chapter 3). It is likely that the first red alga evolved into an ecological niche that was unoccupied by cyanobacteria, the only extant photosynthetic alga that evolved oxygen. This ecological niche would have been in waters with a pH less than 5, which, for some unknown reason, cyanobacteria are not able to inhabit (Brock, 1973). Indeed, modern phylogenetic studies utilizing nucleic-acid sequencing have shown that Cyanidium, an alga that lives in acidic waters, is probably the oldest extant red alga (Oliveira and Bhattacharya, 2000).

The Rhodophyceae lack flagellated cells, have chlorophyll a, phycobiliproteins, floridean starch as a storage product, and thylakoids occurring singly in the chloroplast.

A majority of the sea weeds are red algae, and there are more Rhodophyceae (about 4000 species) than all of the other major seaweed groups combined. Although marine red algae occur at all latitudes, there is a marked shift in their abundance from the equator to colder seas. There are few species in polar and subpolar regions where brown and green algae predominate, but in temperate and tropical regions they far outnumber these groups. The average size of the plants also differs according to geographical region.

CRYPTOPHYCEAE

This group is composed primarily of flagellates that occur in both marine and freshwater environments. The cells contain chlorophylls a and c2 and phycobiliproteins that occur inside the thylakoids of the chloroplast. The cell body is asymmetric with a clearly defined dorsi-ventral/right-left sides (Figs. 9.1, 9.9, 9.10). The asymmetric cell shape results in a peculiar swaying motion during swimming. Most cryptophytes have a single lobed chloroplast with a central pyrenoid.

Cell structure

There are two apically or laterally attached flagella at the base of a depression. Each flagellum is approximately the same length as the body of the cell (Figs. 9.1, 9.8, 9.9, 9.10). Depending on the species, there are one or two rows of microtubular hairs attached to the flagellum. In Cryptomonas sp., the hairs on one flagellum are 2.5 µm long and in two rows whereas the hairs on the other flagellum are only 1 µm long and arranged in a single row (Heath et al., 1970; Kugrens et al., 1987). Small, 150-nm-diameter organic scales (Fig. 9.2) are common on the flagellar surface and sometimes on the cell body (Lee and Kugrens, 1986).

The outer portion of the cell, or periplast (Gantt, 1971), is composed of the plasma membrane and a plate, or series of plates, directly under the plasma membrane (Figs. 9.1, 9.10) (Kugrens and Lee, 1987). The number and shape of these plates are used to characterize genera taking into consideration that the haploid and diploid phases of a single genus can have different plates (Hoef-Emden and Melkonian, 2003).

PHAEOTHAMNIOPHYCEAE

Recent nucleotide sequencing has uncovered an evolutionary line of golden-brown algae not related to other golden-brown algae (Bailey et al., 1998). These algae have been placed in the class Phaeothamniophyceae, a class that is most closely related to the Xanthophyceae and Phaeophyceae. The cytology of these three classes is similar (Fig. 20.1). The cells have two membranes of chloroplast endoplasmic reticulum with the outer membrane of chloroplast E.R. continuous with the outer membrane of the nuclear envelope. The chloroplasts have a ring-shaped genophore and girdle lamellae. The flagella are inserted laterally into the motile cells. The anterior tinsel flagellum has tripartite hairs that lack lateral filaments. The posterior flagellum lacks hairs. New daughter cells are formed by eleutheroschisis (parent cell wall is completely cast off and new daughter cell walls are formed). Vesicles under the plasma membrane appear similar to the physodes that occur in the Phaeophyceae. The Phaeothamniophyceae is the only class of algae where fucoxanthin and heteroxanthin occur together. Endogenous siliceous cysts (statospores) are not produced by these algae.

Phaeothamnion is a filamentous brown alga that produces zoospores that settle to produce new filaments (Fig. 20.1) (Andersen et al., 1998). Tetrachrysis occurs in environments such as peat ponds and has cells embedded in a common mucilage (Dop et al., 1980) (Fig. 20.2). Tetrasporopsis is a colonial freshwater alga that consists of a brown, gelatinous, bladdery sac (Entwisle and Andersen, 1990) (Fig. 20.2).

CYANOPHYCEAE

The Cyanophyceae or blue-green algae are, today, usually referred to as the cyanobacteria (blue-green bacteria). The term cyanobacteria acknowledges that these prokaryotic algae are more closely related to the prokaryotic bacteria than to eukaryotic algae. For the last quarter century, cyanobacteria were thought to have evolved about 3.5 billion years ago. These reports were based on interpretation of microfossils, difficult at best with such small organisms. It now appears that these investigators selected specimens that fit the assumptions of the authors, with most phycologists now rejecting their claims. Based on other reports, the actual time of evolution of cyanobacteria is thought to be closer to 2.7 billion years ago (Buick, 1992; Brasier et al., 2002; Dalton, 2002).

Cyanobacteria have chlorophyll a (some also have chlorophyll b or d), phycobiliproteins, glycogen as a storage product, and cell walls containing amino sugars and amino acids.

At one time, the occurrence of chlorophyll b in cyanobacteria was used as a criterion to place the organisms in a separate group, the Prochlorophyta. Modern nucleic-acid sequencing, however, has shown that chlorophyll b evolved a number of times within the cyanobacteria and the term Prochlorophyta has been discarded (Palenik and Haselkorn, 1992; Urback et al., 1992).

Morphology

The simplest morphology in the cyanobacteria is that of unicells, free-living (see Figs. 2.19(c), 2.20) or enclosed within a mucilaginous envelope (Figs. 2.48, 2.56(a), (b)). Subsequent evolution resulted in the formation of a row of cells called a trichome (Fig. 2.16).

PINGUIOPHYCEAE

The Pinguiophyceae is a class of marine planktonic algae that were previously classed as “chrysophytes.” Analysis of nuclear-encoded 18S rRNA and chloroplast-encoded rbcL gene sequences uncovered the close relationship of the organisms in this class (Kawachi et al., 2002). The class is characterized by unusually high concentrations of polyunsaturated acids, especially 20:5 (n-3) (EPA-eicosapentaenoic acid) in the cells (Kawachi et al., 1996) (Fig. 13.1). These fatty acids are the basis for choosing the latin noun “Pingue” (meaning fat, grease) as the root of the class name. The high percentage of unsaturated fatty acids, and the lack of a cell wall, make these algae desirable as a source of unsaturated fatty acids and of animal feed.

The cells (Figs. 13.2, 13.3) (Honda and Inouye, 2002) are derived from a typical heterokont ancestor with an anterior tinsel flagellum with tripartite hairs and a posterior smooth whiplash flagellum (although many of the genera have lost one or both flagella through evolution). The chloroplasts have two membranes of chloroplast endoplasmic reticulum, the outer membrane continuous with the nuclear envelope. A girdle band of thylakoids occurs under the chloroplast envelope. Pyrenoids occur in the chloroplast and the mitochondria have tubular cristae. Chlorophyll a and chlorophyll c-related pigments as well as fucoxanthin, violaxanthin, zeaxanthin, and β-carotene are present.

BACILLARIOPHYCEAE

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). 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 Röttger, 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 box-like cell wall composed of silica, called the frustule. The chloroplasts contain chlorophylls a, c1, and c2 with the major carotenoid being the golden-brown 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 (SiO2) 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 (Figs. 17.17, 17.46) 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.

PRYMNESIOPHYCEAE

The Prymnesiophyta are a group of uninucleate flagellates characterized by the presence of a haptonema between two smooth flagella. The Prymnesiophyta have two membranes of chloroplast endoplasmic reticulum, as do the Cryptophyta and the Heterokontophyta, but differ in having flagella without mastigonemes. Molecular data also show that the Prymnesiophyta are distinct from the Cryptophyta and Heterokontophyta (Bhattacharya and Ehlting, 1995; Medlin et al., 1994). Until 1962, the organisms were considered part of the Chrysophyceae, at which time Christensen split them off into a separate class, the Haptophyceae (named after the presence of the haptonema). The name Haptophyceae was a descriptive name and not based on a genus in the class; thus the name was later changed to Prymnesiophyceae, based on the genus Prymnesium (Fig. 22.7) (Hibberd, 1976). The fossil record of the Prymnesiophyceae is known from the Carboniferous (approximately 300 000 000 years ago) (Faber and Preisig, 1994; Jordan and Chamberlain, 1997).

The cells are commonly covered with scales. In many cases, the scales are calcified, thereby producing coccoliths. The chloroplasts lack girdle lamellae and most contain chlorophylls a and c1/c2, β-carotene, diadinoxanthin, and diatoxanthin (Zapata et al., 2004). The storage product is chrysolaminarin (leucosin) in vesicles in the posterior end of the cell (Janse et al., 1996). The anterior end of the cell has a large Golgi apparatus and sometimes a contractile vacuole.

The Prymnesiophyceae are primarily marine organisms, although there are some freshwater representatives. They make up a major part of the marine nannoplankton and constitute about 45% of the total phytoplankton cells in the middle latitudes of the South Atlantic.

EUGLENOPHYCEAE

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. 6.11, 6.14(c)), and Klebsiella occurring exclusively in marine or brackhish 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, 6.3, 6.7, 6.14(c)) 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 (Ngô and Bouck, 1998; Bastin and Gul, 1999; Talke and Preisfeld, 2002). The paraflagellar rod is composed of two major proteins forming an elongated alpha-helical stalk that parallels the axoneme.

BIODIVERSITY AT THE PERIPHERY

Biodiversity has now become an integral element in environmental monitoring. Much attention has been given in recent years to the species-rich areas of the Earth, as there is an obvious concern that regions that contain so much of the world's evolutionary heritage do not become biologically impoverished. There is, however, a strong case for giving attention to marginal areas in the preservation of biodiversity even if the numbers of species that they contain cannot compare with the biological hotspots of the world.

Marginal areas, as has already been pointed out, are areas where climatic change is liable to cause disturbance either in location or the nature of the vegetation that survives in these potentially labile localities. Historically, they are areas that will have experienced climatic change in the past and therefore the species that live in these areas may be pre-adapted to climatic change and should therefore be considered particularly relevant in the study of species responses to fluctuating environments. It has been argued (Safriel et al., 1994) that peripheral populations have to be genetically more variable than those from core areas, since the variable conditions induce fluctuating selection, which maintains high genetic diversity. Alternatively, due to marginal ecological conditions at the periphery, populations there are small and isolated: the within-population diversity is low, but the between-population genetic diversity is high due to genetic drift.

CHALLENGES OF THE MARITIME ENVIRONMENT

Coastal regions present both opportunities and challenges to plants. On one hand, reduction of temperature extremes, combined in many instances with freedom from frost and drought, extends the potential distribution for species that are intolerant of climatic extremes. On the other hand, the constant threat of habitat destruction, coupled with the physical stresses of wind exposure, salt drenching, burial and flooding, renders coastal regions marginal areas for many species (Fig. 7.1).

Climatic warming and its consequences for sealevel rise are already having an increasing impact on coastal habitats. Coastal vegetation has always had to adjust to changes in sea level. Since the peak of the last ice age about 18,000 years ago the sea has risen more than 120 metres. The greater part of this change took place over 6000 years ago. Over the past 3000 years the sea level has been largely constant, rising by about 0.1 to 0.2 mm per year. Over the last 50 years, however, the average rate of sea-level change obtained from tidal gauges has risen to + 1.8 ± 0.3 mm yr−1. The rate now seems to be rising more rapidly. Since 1992 satellite altimetry measurements have shown an average rise of +3.1 ± 0.4 mm yr−1 (Nerem et al., 2006). It is now probable that this recent acceleration (Fig. 7.2) represents the first signs of the effect of global warming on sea level (Houghton et al., 2001).