Other Algae

Several other groups of flagellated, motile algae - including the euglenoids (Division Euglenophyceae), cryptomonads (Division Cryptophyceae) and golden-brown algae or chrysophytes (Division Chrysophyeae) - are components of the freshwater phytoplankton. Euglenoids are common in fresh waters, especially in small ponds and farm dams where there is considerable organic pollution from animals, although members of this group also occur in brackish and marine waters. Cryptomonads also occur across a range of freshwater, brackish and marine environments, and are common components of most phytoplankton communities in lentic waters, although they are seldom present at high cell densities. In comparison, chrysophytes are a predominantly freshwater group of phytoplankton. Many species have a preference for cool, unpolluted soft waters that may be slightly acidic. They may be common in such locations, and form blooms sufficient to turn the water brown. They also tend to occur more in waters with low nutrient concentrations, rather than in phosphorus-enriched waters. Such situations include the dilute humic-acid stained coastal dune lakes of western Tasmania, and in wetlands in the coastal and tableland regions of New South Wales. They are less common in the warmer, harder waters of the Murray-Darling Basin, although they still occur as minor components of the phytoplankton communities of these ecosystems. One genus, Dinobryon, is however common in tropical and subtropical reservoirs. Populations may also have seasonally restricted growing seasons (Sandgren 1988b), so cells may not always be present within the phytoplankton community.

The distinctive features of euglenoids, cryptomonads and chrysophytes are provided in Boxes 5.8, 5.9 and 5.10, respectively.

Free swimming naked euglenoids typically have long cigar-shaped to oval-shaped or pear-shaped cells (such as Euglena, Figure 5.24, page 133), or a flattened leaf-shaped cell (such as Phacus, Figure 5.25, page 133) and move with a spiralling motion through the water. Their flexible cells allow them to change shape, especially under high light intensity under a microscope when they may withdraw their flagella and form into a spherical shape. When not swimming, the flexible pellicle also allows the cells to move across a surface by expanding parts of the cell while other parts contract. Armoured euglenoids - which have cells enclosed in a lorica - are typified by Trachelomonas (Figure 5.26, page 133).

Commonly occurring freshwater cryptomonads include Cryptomonas and Rhodomonas.

Common genera of chrysophytes that illustrate the diversity in morphology within this algal division include the unicellular Mallomonas and

BOX 5.8 DISTINCTIVE FEATURES OF EUGLENOIDS

Euglenoids are single-celled, motile algae. They usually have at least two flagella, but in many cases - especially in the freshwater species - only one is emergent, from a canal at the anterior end of the cell. Euglenoids often appear bright green under a microscope, due to the presence of both chlorophyll-a and b. Chlorophyll-b is something that euglenoids share in common with the Chlorophyceae, but not with any other division of algae. Other pigments include P carotenes and xan-thophylls, which can at times give blooms of euglenoids a brick-red appearance. Many other euglenoids are colourless - lacking any photosynthetic pigmentation -and they survive by purely heterotrophic means. Even pigmented euglenoids can exhibit both photosynthetic and heterotrophic nutrition and, if placed in the dark, can lose their photosynthetic pigmentation, or become 'bleached'.

Many euglenoids are naked - lacking a cell wall as such. They do, however, contain a structure known as a pellicle just inside the exterior cellular membrane, which is composed of overlapping proteinaceous strips that wind helically around the cell, and provide considerable flexibility to change shape. There is also a group of euglenoids where the naked cells are enclosed in a non-living outer layer surrounding the cell, known as a lorica. These are often ornamented with spines, and have a short neck or pore, through which the flagella emerge.

There are often numerous disc-shaped chloroplasts scattered throughout the cells of photosynthetic species, which may have paramylon - a carbohydrate storage product - associated with them. Eyespots are present in the anterior part of the cell, near the base of the flagella. The anterior of the cell also contains a contractile vacuole that assists with osmotic regulation within the cell. The nucleus is also sometimes visible under light microscopy in the centre of the cell. Reproduction is asexual - occurring by cell division. Sexual reproduction has yet to be demonstrated. Some euglenoids can form cysts to withstand periods of unfavourable environmental conditions. Some species also have phototaxic circadian rhythms, moving up and down the water column in response to light and at times, forming scums on the surface of the water. Common genera include Euglena Phacus, Lepocinclis, Trachelo-monas and Strombomonas.

Synura, which forms spherical to ovate colonies. Both genera have small siliceous scales and some species have spines or bristles. Another genus, Dinobryon, has cells enclosed in loricas and which form linear or branching colonies.

BOX 5.9 DISTINCTIVE FEATURES OF CRYPTOMONADS

The cells of cryptomonads are flattened, giving them a bean- or heart-shaped appearance when viewed from the side. They are mainly single-celled, free-living and highly motile flagellates - having two flagella, one of which may be slightly shorter than the other. These typically emerge from a ventrally located depression or gullet, which, if present, opens towards the anterior end of the cell. The gullet is often lined with small organelles known as ejectosomes, which are discharged when the cell experiences some disturbance, unreeling long threads. These ejectosomes also occur on other parts of the cell.

The cells of cryptomonads are naked - lacking a cell wall. The cell itself most usually contains either one or two chloroplasts. In most cells, a single chloroplast is present, which contains two lobes joined in the middle by a pyrenoid. Cryptomonads possess both chlorophyll-a and c2, plus several other distinctive accessory pigments including carotenes, xanthophylls, phycocyanin and phycoerythrin. Cryptomonads can therefore display a variation in colouration, including red, blue, yellow, brown and green. Some are colourless (as they lack a chloroplast), and are heterotrophic. Starch is the main storage product. Asexual reproduction occurs with the cell dividing longitudinally, but no sexual reproduction has been recorded.

BOX 5.10 DISTINCTIVE FEATURES OF CHRYSOPHYTES

Planktonic chrysophytes are motile, and swim with the aid of two flagella -although in many species the second of these may be reduced to only a short stub. An eyespot may be present in the cell near the base of the flagella. Some chrysophytes may also undergo diurnal migrations up and down the water column of water bodies, indicating they may be responsive to light availability within the water body. In general, planktonic chrysophyte cells are ovate to tear drop in shape. The outside of the cell varies considerably, with some genera being naked - with nothing covering the cell membrane - while other genera have coverings of ornate siliceous scales and spines and, in yet others, the cells are contained within a funnel- or urn-shaped lorica secreted by the cell itself. There may be one or a few chloroplasts present within the cell. Chrysophyte pigmentation includes chlorophyll-a and both c1 and C2, and also fucoxanthin, which gives the typical golden-brown colour. Pyrenoids occur within the chloroplasts, and the cells contain a storage product know as chrysolaminarin. In addition to being photosynthetic, many chrysophytes have been shown to also be heterotrophic - actively ingesting bacteria, and even other algae. The chrysophyte nucleus is located in the anterior section of the cell. Asexual reproduction takes place through the binary fission of cells. Sexual reproduction has been reported for only a few species, with two vegetative cells fusing to form a zygote. Chrysophyte vegetative cells can also form resting cysts, which have ornamented siliceous external walls.

Figure 3.2 (a) Dark-field light microscopy images of a red-tide-forming dinoflagellate Noctiluca scintillans (100-1000 |jm diameter), and (b) the prey - the chain-forming diatom Thalassiosira (2-86 |jm diameter); (c) aerial photo of a Noctiluca bloom off Manly in Sydney, 1990s Beachwatch; (d) sea surface temperature image depicting EAC in summer, 2003 NOAA/CSIRO Marine Research, (a-c courtesy of NSW Department of Environment and Climate Change.)

Figure 3.2 (a) Dark-field light microscopy images of a red-tide-forming dinoflagellate Noctiluca scintillans (100-1000 |jm diameter), and (b) the prey - the chain-forming diatom Thalassiosira (2-86 |jm diameter); (c) aerial photo of a Noctiluca bloom off Manly in Sydney, 1990s Beachwatch; (d) sea surface temperature image depicting EAC in summer, 2003 NOAA/CSIRO Marine Research, (a-c courtesy of NSW Department of Environment and Climate Change.)

Figure 5.1 Colony of Microcystis aeruginosa. Note almost spherical cells - often in doublets - within a gelatinous matrix. Scale bar 50 pm.
Figure 5.3 Filament of Anabaena circinalis. Note the specialised cells - known as heterocytes - within the filament. These are sites of nitrogen fixation. Scale bar 50 pm.
Figure 5.5 Filaments of Cylindrospermopsis raciborskii. The specialised cells within the filaments are akinetes (resting spores). Tiny conical heterocytes occur at the ends of some filaments. Scale bar 50 pm.

Figure 5.2 Colony of Microcystis flos-aquae. Similar to M. aeruginosa, but cells are generally more dispersed within the gelatinous matrix, which has a more compact shape. Scale bar 50 pm.

Figure 5.4 Filament of Anabaena spiroides, also with heterocytes. Compare the tight spirals with the open spirals of A. circinalis. Scale bar 50 pm.

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