The algae are oxygen-producing, mainly aquatic organisms possessing enormous morphological, cytological, molecular and reproductive diversity. Modern studies have led to the recognition that algae represent a number of evolutionary lines or lineages, almost all of which are represented in the British freshwater flora. Most of these lineages probably arose independently as a result of endosymbiosis. There has been a tendency by protozoologists to adopt what has been termed a ‘protistan’ view and to place these lineages into the Protista, a kingdom that itself is unnatural (Corliss, 1994). The majority of the lineages are eukaryotic, in which the cells have a double membrane surrounding the nucleus and most other organelles such as chloroplasts. Another evolutionary line, the Cyanobacteria (Cyanophyta or blue-green algae) are often considered together with other algae, although these organisms are prokaryotic (membrane-bound organelles absent) and more closely related to the bacteria.

A few algae have photosynthetic structures known as cyanelles, which in many ways are intermediate between a chloroplast and a free-living bluegreen alga. In most such cases, the rest of the cell resembles quite closely other species of algae with normal chloroplasts. It is likely that the cyanelles have evolved relatively recently from free-living cyanobacteria. However, in the case of Glaucocystis nostochinearum, which has conspicuous cyanelles, there is still doubt about its relationships and it is here classified in its own phylum, the Glaucophyta. Some other organisms possess no photosynthetic structure, but are otherwise quite similar to those that have chloroplasts. These are often treated as protozoans, but clearly belong in the same phyla as the related photosynthetic organisms. Most examples are in the flagellated phyla (Euglenophyta, Cryptophyta, Dinophyta and Chrysophyta). The first three, but not the Chrysophyta, are nowincluded in this edition.

What constitutes an algal species has been the subject of much debate (see John and Maggs, 1997, for a review). Most species are recognized by discontinuities in sets of morphological characters observed with the light microscope, so that algal systematics is largely based on what has been termed ‘the morphospecies concept’. Culture studies have frequently shown that species concepts based solely on characters observed in field-collected material are often too narrow and the taxonomic validity of many of the characters used is open to question (see Trainor, 1998, and his comments in a review of the genus Scenedesmus sensu lato).

The green algae or chlorophytes are the most species-rich and morphologically diverse group in the Flora. Traditionally they have been classified into classes and orders according to the level of organization when in the vegetative state. As a consequence, motile forms are traditionally attributed to the order Volocales, coccoids to the Chlorococcales, filamentous forms to the Ulotrichales and Chaetophorales. A new classification was proposed by Mattox and Stewart (1984) based on the ultrastructural architecture of the basal body in flagellated cells and on cytokinesis during mitosis. Species with the basal bodies orientated in a clockwise (CW) or directly opposite (DO) were placed in class Chlorophyceae and those having a counterclockwise orientation in the classes Ulvophyceae and Pleurastrophyceae; the latter was renamed Trebouxiophyceae by Friedl (1995), who redefined it. Another class, the Charophyceae, was recognized by Mattox and Stewart (1984), who distinguished it from the other classes by the presence of a cell plate (phragmoplast) produced during cytokinesis and the subapical insertion of two similar flagella; the multilayered structure at the flagellar base originally used for separation is known now to be present in other classes.

Molecular data have shown that the green algae are separated in two major evolutionary lineages, the Chlorophyta and the Streptophyta. The Chlorophyceae, Trebouxiophyceae and Ulvophyceae belong to the Chlorophyta. The Streptophyta include the land plants and several groups of green algae; whereas in the past these were regarded as orders of the class Charophyceae, at present they are separated at class level: Mesostigmatophyceae, Chlorokybophyceae, Klebsormidiophyceae, Coleochaetophyceae, Zygnemophyceae and Charophyceae (Lewis and McCourt, 2004; Lemieux et al., 2007; Pröschold and Leliaert, 2007; Becker and Marin, 2009). In the Zygnemophyceae, the traditionally recognized order Zygnematales is often divided into the Zygnematales sensu stricto and the Desmidiales (see McCourt et al., 2000; Gontcharov et al., 2003). The classification of the green algae based upon ultrastructural details has received further support from the phylogenetic analysis of nuclear-encoded SSU and ITS rDNA sequences (Pröschold and Leliaert, 2007).

Key to Genera

The key below is divided into sections designed to facilitate the ready identification of the more than 250 chlorophyte genera recorded from the British Isles. The first step is to decide to which section your sample belongs and then to key it out under the appropriate section. Once having arrived at a genus, then compare it against the description and illustrations before using the key to species.

A small group of single-celled flagellates with two apical flagella differing in function and structure. One flagellum is directed forwards when cells are in motion, often longer than cell and easily visible; the second is thinner, rising close to the first but trailing, possibly acting as a rudder, initially in the apical groove. The large cells have no distinct wall and are often metabolic. Trichocysts or muciferous bodies are normally present in abundance just below the protoplast surface. Pigmented genera have numerous yellow-green, disclike chloroplasts containing chlorophylls a and c as well as several xanthophylls. The cells have oil as a storage product, an eyespot is usually absent, contractile vacuoles are numerous (sometimes in a prominent angular structure near apex), and the nucleus is large, sometimes with an endomembrane cap present.

Named the Chloromonadida by Klebs (1893) but changed to Rhaphidophyta by Bourrelly (1970), who believed the Chloromonadida could be confused with the genus Chloromonas (Chlorophyta, Volvocales). There are some similarities with the Cryptophyta, Pyrrophyta and Euglenophyta, but insufficient to warrant inclusion within any of them. Potter et al. (1997), on the basis of nucleotide sequencing, concluded that the Raphidophyceae are monophyletic, while 28S ribosomal sequencing had previously suggested affinities with the Chrysophyceae (Perasso et al., 1989). There are two families: Vacuolariaceae, containing photosynthetic genera, and the colourless Thaumatostigaceae, whose members have pseudopodia. Only the former family is considered here.

Nine genera are classified in the Raphidophyta and these occur in freshwater and marine habitats. Freshwater species are usually to be found in rather acid waters of some ponds and pools.

Some of the most important references on the phylum are Mignot (1967), Huber-Pestalozzi and Fott (1968), Bourrelly (1970), Spencer (1971),Heywood (1990) and Canter-Lund and Lund (1995).

The cells are extremely delicate and can be disrupted even by gentle pressure on a cover slip. Observation using ‘optical staining’, phase- or anoptral contrast is recommended.

The chrysophytes (‘Golden Algae’) constitute a group consisting mainly of the classes Chrysophyceae and Synurophyceae. They have in common their unequal flagella (a long hairy flagellum and a short smooth one) and pigment composition with the brown fucoxanthin giving the chloroplasts the characteristic golden-brown colour. They form specific resting stages, stomatocysts, which have a silicified wall with a pore closed by a non-silicified stopper. The two classes diverge in the position of the flagella, chlorophyll composition, and in the way the silica scales are formed. Another class, the Phaeothamniophyceae, includes most coccoid and filamentous forms previously assigned to the Chrysophyceae (i.e. Phaeothamnion, Stichogloea), but these genera are still retained in the Chrysophyceae here. The small class Dictyochophyceae is represented by the radially symmetric pedinellids.

The chrysophytes number about 1000 species. Most are unicellular or colonial flagellates, predominantly occurring in freshwater plankton. This treatment includes about 120 species found in the British Isles. Most of the silica-scaled forms (family Paraphysomonadaceae and the class Synurophyceae require electron microscopy for identification and are not included; only treated are those species which can be identified by means of the light microscope).

Some genera include representatives that are not only phototrophic, but phagotrophic. Sometimes remnants of engulfed organisms are visible.

DOUBTFUL TAXON

Microglena butcheri J.H.Belcher 1966

Pl. 72I (p. 284)

The position of the genus is problematical and on the basis of electron microscope investigations (Coute´ and Preisig, 1981) it might belong perhaps to the Raphidophyta rather than to the Chrysophyta. This species is characterized by its thick periplast with small lenticular bodies. It has a single yellowish green chloroplast with an eyespot and a funnel-shaped, non-contractile reservoir behind which the large nucleus is located.

The Eustigmatophytes are small, single-celled coccoidal algae, the term ‘eustigma’ referring to a conspicuous orange-red eyespot that is possessed only by flagellated forms. In such forms the flagella are inserted near the cell apex, and on the anteriorly directed flagellum are two rows of tripartite hairs (not visible with the light microscope). A second flagellum, if present, is usually very short, posteriorly directed and smooth. The eyespot has a unique structure and is at the anterior end of the cell where it is not associated with the chloroplast. It consists of a number of carotenoid-containing globules above which lies the basal expansion of the long flagellum. The one or more yellow-green chloroplasts contain chlorophyll a and violaxanthin is the principal accessory pigment. The pyrenoid characteristically projects on a short stalk from the inner side of the chloroplast and is surrounded by flat plates of an unknown food storage material; pyrenoids are not present in all eustigmatophycean algae and are never present in zoospores. Another unusual feature is the presence of a red-pigmented body whose role is unknown. Reproduction is by the formation of 2 or 4 autospores or zoospores.

The phylum was created to include a number of algae previously classified in the Xanthophyta (Hibberd, 1980; Santos, 1996). It currently consists of a single order (Eustigmatales) divided into 4 families containing about 7 genera and 15 species; most are freshwater or soil-dwelling, although there are some marine representatives. One unusual genus (Corvomyenia) is a symbiont within a freshwater sponge. Other eustigmatophytes reported from the British Isles are only known from brackish waters (e.g. Nannochloris ocelata Droop).

The eustigmatophytes are commonly mistaken for green algae when examined with the light microscope. Features useful for distinguishing flagellated forms are the separation of the prominent eyespot from the chloroplast and, in the case of coccoidal cells, the presence of a stalked pyrenoid, although this is not always very evident. The carbohydrate storage material, which does not stain with Lugol’s solution or iodine, is another useful distinguishing feature. Otherwise the identification of some genera and subgeneric taxa requires examination by transmission electron microscopy.

Most species of Haptophyta (also known as Prymnesiophyta) are single-celled flagellates, but some may have amoeboid, coccoid, palmelloid or filamentous stages. The flagellate cells have usually two flagella of equal or subequal length (unequal in the order Pavlovales), both of which are smooth. In most members of the Pavlovales the forward one is covered with small knob-like scales and fine non-tubular hairs that are invisible in the light microscope; tubular flagellar hairs as in the Chrysophyta or other heterokont flagellates do not exist in the Haptophyta. The two flagella may have a similar action (homodynamic) or move differently (heterodynamic). Between them there is a characteristic additional appendage, the haptonema. Depending on the species, this organelle is long, thread-like, exceeding the cell in length, or shorter, bulbous, or vestigial. It consists of 6 or 7 microtubules enclosed in two or three sheathing membranes (not visible with light microscopy). Haptonemata are capable of bending and flickering and, if long, can coil rapidly and uncoil slowly in response to shock. Haptonemata do not play a role in locomotion, but may be used in prey capture or anchoring the cell. The 1–2(–4) chloroplasts are usually golden or yellow-brown, because accessory photosynthetic pigments mask the green of the chlorophyll (chlorophylls a, c1/c2 and sometimes c3). The surface of the cell is covered with minute scales or granules of organic composition (not visible in light microscope) and, in addition, there may be calcified scales (coccoliths) visible in the light microscope; species with such scales are called coccolithophorids.

The Haptophyta comprises at present some 80 genera and 300 species; a large number of additional genera and species of fossil coccolithophorids have been described. Especially common in marine waters are species of Chrysochromulina, Phaeocystis and coccolithophorids (e.g. Emiliania huxleyi). Only about a dozen haptophyte species are known from freshwater or terrestrial habitats. Five have been recorded from the British Isles.

Introduction

The blue-green algae (Cyanobacteria) are unicellular or filamentous organisms that sometimes form structures recognizable with the naked eye, but usually require a microscope for identification. They differ from all other groups in this Flora in that they are prokaryotes: their cell contents are not differentiated into membrane-bound structures such as the nucleus, chloroplasts and mitochondria. The popular name for the group, blue-green algae, comes from the colour of the cells seen under the microscope. The photosynthetic pigments in the membranes inside the cells contain chlorophyll a, which gives a green colour, but almost all species can form the blue pigment, phycocyanin, under some conditions and some also forma red pigment, phycoerythrin. The cells therefore often appear blue-green, but sometimes shades of purple, when all three pigments are present. Visually obvious growths are also often blue-green, but sometimes they are brownish, purple or orange. This is because many species have a sheath around individual cells or the whole filament and this sheath is often golden or dark brown, or sometimes a shade of red.

Many blue-green algae are easy for someone without specialist knowledge to recognize under the microscope to the genus or even the species. However, the group has gained a reputation for being difficult to identify reliably. This is partly because some species are morphologically highly variable and in some cases groups of species which appear distinct in one place all tend to merge with each other elsewhere. However, there are also problems due to the different approaches which have been used to name the organisms. The most important difference is between the botanical and bacteriological approaches. The bluegreens are anomalous in that they have been treated by some authors under the conventions of the International Code of Botanical Nomenclature, while others under the International Code of Bacteriological Nomenclature.

The present account includes marine as well as freshwater and terrestrial species. Some species occur in both types of environment: many freshwater species extend into brackish environments and a few marine species also extend into brackish environments. Morphologically similar forms occurring in non-marine and marine environments have sometimes been given different names and in other cases the same name. At least with the current state of knowledge for the British Isles, there are relatively few strictly marine blue-green algae, so it seems best to provide an account of the whole Flora.

The study of freshwater dinoflagellates in the British Isles has been sporadic and partial. The majority of work describing species and their ecology took place between 1920 and 1960. Workers of note were the Wests (e.g. W. and G.S. West, 1906) and Harris (1940). Since that time there have been some detailed studies on individual genera, most notably Ceratium in the English Lake District (Chapman et al., 1981, 1985; Heaney et al., 1983, 1988), but little further descriptive work. As far as possible, all historical accounts of records for the British Isles have been brought together in the production of the present account, which aims to describe all the species recorded. However, it is likely that many more species remain to be discovered; indeed, in the course of writing this account a number of hitherto unrecorded species were found. So the keys provided here should be used with caution. Whilst they cover the species most likely to be found, studies on new water bodies may well lead to the discovery of species not described here. If an organism does not conform to any of the descriptions here, the most useful accounts for trying to identify it are those of Huber-Pestalozzi (1950) and Popovský and Pfeister (1990).

Dinoflagellates are characterized by their nucleus, which is distinctive by virtue of the permanently condensed chromosomes. Under the light microscope the nucleus frequently appears rather large in proportion to the cell and with obvious sausage-shaped chromosomes. Where the attribution of a cell is in doubt it is always worth checking this point, as a number of unusual cell forms exist within the class. However, the most common form for dinoflagellates is a motile unicell, often with a defined groove (cingulum) encircling the cell body, and with two dissimilar flagella. One transverse flagellum is placed within the cingulum and the other, longitudinal, follows a second groove (sulcus) at right angles to the cingulum. These flagella produce a characteristic whirling swimming motion from which the name dinoflagellate is derived. The cell wall consists of vesicles, which in various genera contain differing amounts of cellulosic material. These vary from strongly armoured species, which have relatively thick plates, to those with little reinforcement, which are referred to as unarmoured. Under the light microscope these differences may be very evident, the armoured forms having strongly ornamented, angular outlines which are retained even after lengthy examination.

The primitive green algae may be classified into three classes, the Pedinophyceae, the Prasinophyceae and the Charophyceae. The class name Prasinophyceae (Christensen, 1962) is based on the Greek word prason (πράσου), meaning a leek. It refers to the yellowish green colour of the cell in many prasinophyceans, reflecting the presence of pigments which are slightly different from those of most other green algae. Prasinophycean green algae are mainly flagellates. The cell surface and the flagella are commonly covered with minute organic scales, but these are usually too small to be visible under the light microscope (exception: the largest scales in Mesostigma). Therefore, for critical species determination, scale structure often needs to be ascertained by electron microscopy. In some genera all traces of scales have been lost and recently an increasing number of minute flagella-lacking species of themarine picoplankton have been proved by gene sequencing to be prasinophyceans. Prasinophyceans are generally accepted to be the oldest group of green algae fromwhose ancestors all other groups of green algae have arisen (in cladistic terms they constitute a paraphyletic group of green algae). They are thought to be very ancient, a theory also based on the presence of prasinophycean-like cells in fossil material dating perhaps as far back as the Precambrian.

Ultrastructurally, the flagellated prasinophyceans usually have parallel and notably long flagellar bases, which are inserted in a groove. The genera are morphologically very different, probably reflecting the ancient origin of the group; thus the number of flagella varies from zero or one in a few genera, to 2 and 4 in several genera, to 8 and even 16 flagella in one species, the only 16-flagellated algal flagellate known. The total number of extant species is probably around 100.

The ultrastructural features separating the prasinophytes from the chlorophytes, eustigmatophytes and raphidophytes are not distinguished using the light microscope. For this reason the prasinophyte genera are keyed out with other flagellates in Section IV of the keys to chlorophytes (principally Volvocales) and the phyla mentioned above (see p. 367).

CLASS PEDINOPHYCEAE

The Pedinophyceae, so named by Moestrup (1991), comprises 3 genera of small, naked (scale- or wall-less) flagellates, with around 10–15 species. The cells show a number of unusual traits, e.g. during mitosis in Pedinomonas, where the nuclear envelope remains intact throughout, surrounding the entirely internal spindle apparatus.