What exactly is an alga? A simple answer to this question has eluded botanists for centuries and even with the help of modern research the answer remains highly complex. Remember that the plant kingdom has been classified bytogether species which show common features. How then can one account for a single group containing, on the one hand, microscopic plants composed of a single cell and swimming freely in water, and on the other hand giant seaweeds over loom (328ft) in length, which may lay claim to being the longest-living things on earth? The is further complicated by some zoologists who claim the single-celled species, which swim by means of whiplike flagella, as animals!
Perhaps the simplest way to think of the group is not so much what they have in common, but rather what features, as a group, they do not have when compared with higher plants, as what does typify the group is their relative simplicity.
Algae have never evolved the complexand water plumbing system of higher plants and so have been unable to colonize dry land to any extent. They are plants of fresh or marine waters with seaweeds as the most obvious representatives in the intertidal and shallow water zone around every coastline. Here they are also able to survive without water-regulating features such as cuticle and stomata. Some of the most simple algae are able to colonize soil, rocks and tree trunks, but are only able to flourish so long as there is a film of surface moisture. When conditions become too dry, they quickly become dormant until suitable conditions return.
Although the photosynthctic pigment chlorophyll is found in all algalits characteristic green is often masked by others. These give some species a distinctive colour which was used in early attempts at classification. In particular, it was used to divide two of the major seaweed groups — the red and brown. The key to a modern classification of the algae has been the development of techniques for studying their biochemistry and cell structures. These studies have upheld the validity of dividing red and brown algae. The chemical responsible for the red colouring is the biloprotein, phycoerythrin, while the browns are coloured by a carotene, fucoxanthin. The red and brown algae (Rhodo-phyta and Phaeophyta) now form two of the ten divisions which are generally recognized. A third major division is the green algae (Chloro-phyta) where chlorophyll remains dominant.
One division, the blue-green (Cyanophyta), is composed of species which have cells quite unlike those of other algae. These cells, known as ‘procaryotic’ cells, are much more like those of bacteria and appear much simpler in organization. One of their unique features is that no true nucleus is present as in the normal ‘cucary-otic’ cells of other algae and all other plants. Because of their simplicity the procaryotic cells are thought of as being more primitive than eucary-otic ones. There is fossil evidence for this, as algae found in Precambrian rocks are thought to be of the blue-green type. These plants must have been alive over 600 million years ago when life on earth was still at a very simple stage. Speculating about the evolution of any group of living things is a fascinating exercise and the algae are no exception.
Form and reproduction
Chlamydotnonas—a unicellular alga
A unicellular organism is one which consists of a single cell, in which occur all the life processes —respiration, reproduction and, in the case of most algae,. The 325 or so species of Chlamydomonas occur widely in freshwater and are especially associated with water rich in ammonium compounds. They also occur on damp soil. The cell structure of Chlamydomonas has many features in common with cells found in higher plants, but the special features which allow Chlamydomonas to live as free- swimming individuals are the two, or sometimes four, flagella, a light-sensitive red pigment or ‘eye spot’ and a single cup-shaped chloroplast. Reproduction can be either sexual or asexual. Asexual reproduction is much the more frequent phenomenon, allowing Chlamydomonas to increase in numbers very rapidly. In this case the protoplast of an individual may divide several times within the parent cell wall to produce up to 32 daughter cells. Sexual reproduction occurs when two individuals act as sexual units or gametes. These then join to form a single unit known as a zygote. When in the zygote resting stage a thick wall protects the contents from drought; in this condition the zygote may be blown from place to place. When one lands in a suitable place development is completed and four new individuals are released and become mobile once more. This resistant zygote stage which results after sexual reproduction is a common phenomenon throughout the algae except for the blue-green, where sexual reproduction has never been observed. Although in most species of Chlamydomonas the two parents of the daughter cells are apparently identical, in other species distinct male and female parents are discernible.
Volvox—a colonial alga
Colonial algae usually rather resemble a mass of unicellular algae embedded in a gelatinous mass. In fact, the individual cells of Volvox look remarkably like Chlamydomonas. The disorganized appearance of the colony is misleading as its number of cells is determined when it is formed and does not increase during the life of the colony. A colony may contain as many as 20.000 cells each bearing two flagella. These cells are contained in a gelatinous sheath in the form of a sphere. The spheres which occur in freshwaterare just large enough to be seen by the naked eye and may be numerous enough to colour the water green. Movement of the many flagella is coordinated, so that the whole colony is able to move slowly through the water.
Asexual reproduction occurs by the formation of a number of daughter colonies within the sphere, which are then expelled. Sexual reproduction is accomplished by the formation of free-swimming male gametes known as antherozoids. These fuse with egg cells which form individually within the sheath. As in
Chlatnydomonas, a resting zygote is the result of the union.
Spirogyra—a filamentous alga
This well-known alga is widely distributed in freshwater ponds where its unbranched strands may form a dense mat sometimes referred to as blanket weed. Each filament consists of a row of cylindrical cells placed end to end. Within the protoplast of each cell is situated one or more of the distinctive spiral chloroplasts which give these algae their name. Each strand grows longer by cell divisions. These are in one plane only so that the filament remains a single row of cells. Accidental breakage of these filaments appears to enable new filaments to grow and provides a means of asexual reproduction.
Sexual reproduction occurs when two filaments come to lie side by side in a gelatinous mass. Cells from each of the two filaments produce outgrowths of the cell wall which link with the cell in front. At this stage the two filaments look remarkably like a minute ladder. These connections are known as conjugation tubes. No flagellate gametes are produced, as the contents of one cell merely flow into the other where the zygote is formed. Ulva—a parenchymatous or leafy alga These green seaweeds all occur in the marine mid-tidal zone where they are known as sea lettuce. Unlike the algae already described, Ulva contains cells of different kinds, specialized to form a ‘and holdfast. This flat ‘ or thallus is produced by division of cells in two or three planes, while the holdfast is composed of colourless rhizoids which arise from certain cells at the base of the thallus. On emerging from the thallus they become matted together to form the holdfast which anchors the plant to the rocks. Although the holdfast acts as a true in functioning as an anchor, it does not function as a water- or nutrient-absorbing organ. A new holdfast can be produced from a fragment of thallus, but this form of asexual reproduction does not appear to be common.
Sexual reproduction, which is the norm in Ulva, provides us with an example of alternation of generations which is not only a feature of the algae, especially seaweeds, but also in modified form of all higher plants. In Ulva both of these generations produce free-swimming single-celled ‘swarmers’. These generations are the haploid gamctophytc producing true gametes, two of which fuse to form a zygote, and the diploid sporophyte producing zoospores by meiosis. These do not fuse with one another but develop directly into a new haploid plant after a short resting phase. Swarmers of both types may be liberated in such vast numbers as to colour the water green. Along the Californian coast the release of swarmers occurs bi-weekly giving regular ‘green tides’. The stoneworts (Charophyceae) — specialized parenchymatous algae Members of this specialized class of algae may easily be confused with higher plants as their thalli, usually about 20 centimetres in height, consist of distinct nodes and internodes. They occur in fresh and brackish water, although they thrive best in lime-rich water. Here they may completely encrust themselves with lime or ‘marl’. Not only is the thallus complex in structure, but the organs producing the sexual cells are also highly specialized. Although mobile male gametes are produced, the egg cell remains within the parent plant until after fertilization.
Red and brown algae (Rhodophyta and Phaeophyta)
All the preceding examples are members of the green algae (Chlorophyta), which have been used to illustrate some of the range of structures and methods of reproduction found in the algae. Such a range of structures can be found in other divisions, but it is in the red and brown algae that the most specialized parenchymatous forms have evolved. It is these which form the majority of true seaweeds. Although the red algae have evolved the most complex reproductive systems, it is the brown algae that have developed the most elaborate plant forms. The group includes such well-known seaweeds as the giant kelps Nereocystis, the various rock-weeds or wracks of the genus Fucus and the floating seaweeds Sargassum. Both divisions exhibit forms of alternation of generations.
Ecology and economic uses
Seaweeds in general are very specific in their environmental requirements. The brown algae, for instance, are typically seaweeds of cool waters including the Arctic and Antarctic, while the red are more abundant in warm waters. Each species is also usually associated with a particular tidal band or depth of water. The brown alga, Fucus, prefers the higher tidal belts, while Nereocystis is characteristic of deeper water. Both species are anchored by means of a holdfast. As some species of giant kelp may be over 100m (328ft) in length, the holdfast is often a substantial structure. Sargassum filipendula, on the other hand, is free-floating and collects in vast floating mats such as in the notorious Sargasso Sea in the midatlantic.
Although of such lowly status in the plant kingdom algae may significantly affect the environments in which they live. The lowly unicellular and colonial algae form the plant components of plankton and as such form the vital first stage in the food chain of marine animals. This vast mass of simple plant and animal life found floating near the surface occurs not only in the sea but also in freshwater lakes. A number of species of whale live entirely by filtering plankton from the oceans. Some of the larger brown and red algae are more directly of benefit as a human resource. Some of the chemicals extracted from seaweeds include colloidal gels like pectin, carragheen and agar, as well as iodine from various red algae. One of the most important gelling agents extracted from seaweed is algin, which has a wide range of industrial uses: its salts are used to ‘dress’ textiles, thicken dyes, join tweeds in weaving, glazing paper, as a suspending agent in the preparation of medicines, paints, cosmetics, insecticides, etc, for waterproofing fabric, and as a stabilizer in ice-cream. It is harvested commercially from species of Laminaria along a number of European coastlines. It is off the Pacific coast of the USA, however, that the industry has become most mechanized. Machines, rather like giant combine harvesters, cut and scoop 300tonnes of the giant kelp Macrocystis per day.
Because of its economic value there have been a number of proposals to introduce the productive American giant kelp Macrocystis pyrifera into European waters. This has caused alarm among marine biologists who consider that this productive seaweed could cause profound changes in the cco-system of the shallow waters around the coast, replacing the native Laminaria and affecting fish and shellfish grounds. A lesson has already been learnt from the accidental introduction of the Japanese seaweed Sargassum muticum to the USA and elsewhere. This has caused considerable problems and its recent appearance on the south coast of the United Kingdom has been regarded with alarm, and attempts are being made to eradicate it.
Algae can be less than beneficial in other ways. Those which form the plankton have an enormous capacity for growth and where conditions are right, ‘blooms’ occur which may colour water the colour of the dominant algae. Organisms present in such vast numbers may cause many problems. Their waste products may cause chronic poisoning of fish and other marine life. Today many freshwater lakes are undergoing eutrophication —enrichment by sewerage effluent or nitrogen fertilizers which have been washed off adjacent farm or forestry land. In such nutrient-rich conditions algae flourish. Although during the day the oxygen produced bymasks that used for respiration, at night oxygen in the water is quickly used up by the algae, often causing extensive fish deaths through suffocation.
The relative simplicity of algal structures makes them an obvious group in which to look for the ancestral forms of higher plants. It is easy to imagine the range from simple to relatively complex structures found in a number of divisions as stages in an evolutionary sequence leading through to higher plants. There is certainly evidence for this in the case of Chloro-phyta which like higher plants contain chlorophyll as the dominant photosynthetic pigment. As the first fossil vascular plants occur in rocks of Silurian age over 405 million years ago, this split must have occurred even earlier and it is thus most unlikely that forms showing direct ancestry would have survived. Similarly, the algal divisions themselves may have had a common ancestor but each division has evolved in its own way. Evolution in the algae has produced a number of complex forms, some of which show parallels with the evolution of higher plants. These include cell differentiation to perform specific functions within the plant, including translocation. Sexual reproduction has been refined to a point where the female cell is retained in the parent plant until after fertilization, and alternation of generations, fundamental to the evolution of higher plants, is present.
Cradle of the deep the sea otter and kelp
The Californian sea otter lives permanently in the great beds of kelp off the coast—it even sleeps there, wrapping the fronds round its body for anchorage, sure of reasonable comfort because the kelp damps down wave motion to a considerable degree. Many invertebrates also live among the seaweeds, notably small molluscs and
Crustacea which the otter eats. The sea otter does in fact maintain a natural balance, because these small animalson the kelp, and where the otter has been destroyed by man the kelp gradually disappears as a result.
Between 1910 and 1930 kelp was harvested off the western coast of U.S.A. as a rich natural source of potash, but this finally proved uneconomic, as did wartime production of acetone. The major value of kelp today, in California as elsewhere, is in producing alginates, which have many industrial applications.