Algae - Occurrence and Distribution

ALGAE - OCCURRENCE AND DISTRIBUTION Benthos.—Most algae grow permanently submerged, and are either attached (benthos) or free-floating (plankton). In fresh waters the algae of the benthos grow on stones, twigs, and larger aquatics, while the benthic seaweeds are nearly all lithophytes (i.e., fixed to rocks) . Few algae (Chara, Caulerpa) can obtain a foothold in loose sand or mud, and a rock on a sandy beach often stands covered with vegetation like an oasis in a desert. Mem bers of the benthos may become detached from their substratum and float freely, like the tangles of filamentous algae found in ponds, or the seaweed Sargassum, innumerable plants of which drift into the North Atlantic from the Gulf of Mexico and the Caribbean sea and thus give rise to the Sargasso sea. Unattached species of Fucus are not uncommon in salt marshes, but most seaweeds are doomed when torn away from their substratum. In rivers too all algal growth, other than the plankton, is attached, either encrusting rocks and pebbles (Myxophyceae, the red Hil denbrandtia) or forming long tresses trailing out with the current (Cladophora, Ulothrix, various red algae). All benthic forms bear numerous epiphytes and their dense tangles usually harbour a wealth of smaller algae and animal life.

Zonation.—Thebenthos usually exhibits more or less pro nounced zoning. In lakes and pools this is due to the gradual diminution in light-intensity with increasing depth of water. The epiphytes on the submerged Phanerogams often show such zoning clearly, while in deeper water the growth consists in the main of Charales, Nitella usually thriving at greater depths than Chara. Further out, where the light is reduced to a minimum, the bottom bears only diatoms and sometimes cladophoraceous forms. Zon ation is much more marked on rocky shores, where the littoral zone (i.e., the region between tide-levels) is of considerable extent. The highest levels are occupied by forms like Pelvetia, Bangia, Prasiola, etc., which are only under water for a few hours or indeed may grow at such a height, as to be merely wetted by spray and submerged only at spring tides. This spray-zone has its equivalent at the margins of large lakes, where the stones bear a growth of Spirogyra adnata, Myxophyceae, etc. Below the spray-zone, on northern shores, there follows a broad girdle of Fucus, various species of which occupy different levels; in the lower stretches they are mingled with Ascophyllum, while near low tide level Himanthalia, and Corallinaceae are often conspic uous features. In the sublittoral zone Laminariales are the domi nant forms in all the colder seas. In the Mediterranean the littoral zone is specially characterized by Corallina mediterranea. Between tide levels are also found many green (Ulva, Cladophora) and red algae, but the latter, both as species and individuals, are most abundant in deeper water, where they are never exposed.

The various pigments (brown, red, blue, etc.) that accompany the chlorophyll probably serve in the first place to screen the latter against the intense illumination to which seaweeds are exposed when uncovered by the tide. The Rhodophyceae of the littoral zone usually possess a dark red, almost black colour, and the brighter tints are met with only in the permanently sub merged forms. Red seaweeds are, in fact, so sensitive to strong light that in aquaria they have to be grown behind dark glass.

The accessory pigments in the chromatophores, however, also effect an alteration in the region of maximum absorption of light. While in green plants this is in the red part of the spectrum, in the case of red algae it is in the green. So that at considerable depths, where the red and yellow rays have been absorbed, while the green and blue ones are relatively undiminished, red algae can still carry on photosynthesis and will be more successful than green or brown forms. It has been thought that their charac teristic colour may have been acquired as a direct response to the quality of the light, but this is a debated point. It is well known, however, that some Oscillatorias will take on a colour complementary to that of the light to which they are exposed. Various Myxophyceae that grow in deep water, moreover, assume a red colour. It appears, however, from recent research that other factors than the colour of the light (e.g., lack of nitrogen) may bring about a change of pigmentation.

Phytoplankton.—Themembers of the phytoplankton are largely unicellular or colonial and are often highly adapted to a floating life. Many take the shape of flat plates (e.g., Pediastrum, Merismopedia, fig. 4, J), others bear numerous bristles (Micrac tinium, fig. 2, B) which heighten their buoyancy, while some (e.g., the many Myxophyceae responsible for "water-blooms") possess structures in their cells (pseudovacuoles) which render them lighter than water. A number of plankton algae (e.g., Volvocales), moreover, are freely motile and can thus maintain themselves in the surface layers where alone they obtain adequate illumination.

Marine phytoplankton consists largely of Diatoms, Peridinieae, and a few Heterokontae (Halosphaera, Meringosphaera), although a red filamentous member of Myxophyceae (Trichodesmium) is sometimes abundant in the warmer seas, hence the name of the Red sea. Freshwater phytoplankton is more diverse, including apart from Diatoms and Peridinieae, many Isokontae and blue green algae. Since the phytoplankton constitutes the diet of many smaller animals, its abundance is often intimately related to the productivity in fish of oceanic or lacustrine waters. It varies con siderably, both in quantity and quality, in different seasons of the year, and there is usually a more or less marked annual succession of forms (Diatoms, Peridinieae, etc.), many of which appear in swarms of rather limited duration.

Periodicity.—Suchperiodicity is also observable in the ben thos and may be very striking in ponds, where four or more distinct phases can sometimes be distinguished during the year and certain forms (Spirogyra, Tribonema, etc.) are commonly only present during a limited period. The flora is usually poorest in the height of summer. Seaweeds likewise exhibit a periodicity, some being annuals and absent in the unfavourable season (winter or summer, as the case may be) , whilst others though lasting for two or more years periodically shed parts of the thallus (cf. Laminaria and Desmarestia above) . Reproduction is, moreover, commonly confined to definite periods; thus, many species of Spirogyra and Oedogonium are found fruiting mainly in spring, Coleochaete in the summer, the Laminariales produce their spor angia especially during the winter months, etc. Sexual reproduc tion in freshwater algae usually occurs during periods of bright sunshine.

Conditions of Existence.

Theeffect of light on the distribu tion of algae has already been considered. Second in importance is temperature, which probably in large part conditions the broad differences in the composition of the algal flora over the surface of the earth. In the colder seas other seaweeds are overshadowed by the large brown forms (Laminariales, Fucales), which are far less abundant in tropical and subtropical waters where red algae and Siphonales come to the front. Such regional differences are not so marked in freshwaters, although certain forms (Clado phora), common in temperate zones, are rare in the tropics, where moreover Myxophyceae play a bigger role than in colder latitudes. Some of these differences are no doubt determined by the higher oxygen-content of colder waters.

In extreme temperatures the algal flora acquires a very dis tinctive stamp. Thus, in hot springs the vegetation consists almost entirely of Myxophyceae and Diatoms, some of which are able to thrive at very high temperatures (80°C and more). Many algae, on the other hand, can withstand low temperatures and may be melted out from ice in a perfectly healthy condition. The frigid Antarctic lakes contain huge sheets of the blue-green Phor midium bearing a host of epiphytes. A peculiar flora consisting mainly of green algae occupies the surface of the perpetual snow fields of the Alps, Andes, etc., and of the polar regions. Over wide areas the snow may exhibit a red colour due in the main to the resting-cells of Chlamydomonas nivalis, with which other forms are associated. Yellow snow, with a different flora, occurs in the Antarctic and is also known from the Alps.

Few algae can stand appreciable variations in the concentra tion of the water. Freshwater forms are not very tolerant of an excess of salts and seaweeds cannot thrive in dilute water, so that in estuaries and on salt-marshes only a limited number of algae are to be found.

Endophytes and Parasites.

Themajority of epiphytic algae are attached merely to the surface of their substratum, but espe cially among marine forms some of the threads not uncommonly penetrate the latter, either invading only the external membranes of their host or burrowing deeper, so that they become active parasites. In most cases such forms afford little outward indica tion of their parasitism, but in the red alga Harveyella (Chore colax) mirabilis the cushionlike thalli are devoid of all colour. It is rare for algae to be parasitic on higher plants, but Cephaleuros, one of the Chaetophorales, attacks the leaves of various tropical Phanerogams (including tea and coffee), causing more or less serious diseases.

A number of algae (endophytes) live almost entirely within the interior of other plants, without as a general rule being more than space-parasites. Such are the species of Chlorochytrium which occur in the fronds of the duckweed, etc., the Nostoc found in the liverwort Anthoceros, and the species of Anabaena that inhabit Azolla and the roots of various higher plants. In other cases the endophyte merely lies within the thick surface mem branes of its host (many marine algae). A more intimate relation (symbiosis) exists in the case of the "green cells" (Chlorella spp.) found within the bodies of various lower animals, and the asso ciation of Trebouxia, Trentepohlia, and certain Myxophyceae with fungi to form lichens. Here the alga receives both protec tion and food from its partner (animal or fungus), which in turn profits by absorbing some of the products of photosynthesis of the alga.

Terrestrial

Algae.—Numerous algae can exist more or less successfully out of water. The amphibious seaweeds of the littoral zone form a case in point. Somewhat similarly situated are the forms (Botrydium, V aucheria) that grow on the damp mud round the edges of pools and ditches and which form resting spores as soon as the mud dries. There are, however, many algae living in and on soil, which are more highly adapted to withstand pro longed drying. The subterranean community comprises green (Chlorococcum) and blue-green forms, as well as diatoms, culti vated soils in general having a richer flora than natural ones. There can be no doubt that these algae fulfil an important role in the soil. A capacity to fix atmospheric nitrogen has been attributed to them, but no decisive proof is yet forthcoming. The gelatinous sheaths of many soil-algae, especially Myxophyceae, may well serve as a home and as a source of carbohydrate-supply for nitrogen-fixing bacteria, and herein may lie their chief importance.

The numerous subaerial algae are mainly green and blue-green, the latter playing a dominant part in the warmer humid regions of the world, where they constitute important primary colonizers of rock-surfaces and may often cover the latter for many. hun dreds of square yards with sheets of distinctive tint, giving a characteristic coloration to the landscape. In temperate regions green terrestrial algae are more conspicuous. Every one will be familiar with the green covering on tree-trunks, etc., due to Pleuro coccus and other unicellular forms, which possess such a capacity to resist desiccation that they will survive months of extreme drying in a desiccator over concentrated sulphuric acid without harm. Other examples of green subaerial algae are Zygogonium ericetorum, one of the Conjugatae whose purple or greenyl wefts cover extensive tracts of peaty soil, Hormidium found on clayey soils, and Trentepohlia whose orange tufts are particularly com mon in hilly districts and in the damp tropics. All these can sur vive long periods of drought without any appreciable change, resuming growth as soon as wet weather sets in.

Geographical Distribution.

Whilethe broad differences in the character of the algal flora in diverse situations and in differ ent regions of the earth are without difficulty related to special factors, the analysis of the conditions that determine local vari ations is still in its infancy. Such variations are patent to any one who makes a study of the algae of a limited region. Thus the algal flora of a moorland pool differs from that of one situated in meadow land, that of a lake differs according as the bottom is rocky or covered with silt, calcareous waters are poor in algal growth, etc. Similarly the vegetation of the sea-shore varies with the nature of the rocks, the extent of wave-action, the presence of currents, and the amount of detritus borne by the waves. Other differences appear to depend rather on geographical than on ec ological factors. Thus, the desmid flora has a distinctive character in different parts of the world, and desmids appear to be entirely absent from the Antarctic continent, though abundant in the Arctic. Similar observations may be made with reference to the geographical distribution of seaweeds.

Uses.—Relativelyfew algae are of economic importance. Sea weeds are in many places employed as a convenient form of manure. A species of Porphyra (P. laciniata) and the red sea weed Rhodymenia palmata are used as food in some places, the latter being known as dulse. Agar-agar, a substitute for gelatine, is derived from a species of Gracilaria, while another red sea weed, carrageen (Chondrus crispus), has been used as an invalid food. Iodine was at one time mainly obtained from the ash of seaweeds.

Fossil Algae.

Thedense incrustation of carbonate of lime found in various Siphonales and in the Corallinaceae has already been mentioned, and very similar forms are luiown as fossils in the Trias as well as in the Palaeozoic from the Silurian onwards. Such algae have evidently contributed to no small extent to the building of limestones, just as the Lithothamnions, etc. (Nullipores) are important agents in the formation of coral-reefs at the present day. Algae, which are not thus encrusted, are not so readily pre served as fossils, and of such we have little more than a fairly reliable record of their presence in earliest times; some of the forms in question are of large dimensions (e.g., the Devonian Nematophycus) and may have been similar to Laminariales. For fossil diatoms, see BACILLARIALES.

BIBLIOGRAPHY.-The following three works contain an up-to-date Bibliography.-The following three works contain an up-to-date account of the various classes of algae and afford a complete bibliog raphy up to the time of their publication: F. Oltmanns, Morphologie und Biologie der Algen (Jena, 1922) • G. S. West and F. E. Fritsch, British Freshwater Algae (1927) • G. 'S. West, Algae, Cambridge Bo tanical Handbooks, Vol. I. (191'6). A more popular account is G. Murray, An Introduction to the Study of Seaweeds (1895). Of general systematic works the most important are: J. B. de Toni, Sylloge Al garum, vol. i. Chlorophyceae, vol. ii. Bacillariaceae, vol. iii. Fucoideae, vol. iv. Florideae, vol. v. Myxophyceae (by A. Forti) (Padua, 1889– '9°7) ; Engler and Prantl, Natiirliche Pfianzenfamilien, Chlorophyceae (vol. 3, 1927, by H. Printz) ; Phaeophyceae (I. Teil, Abt. 2, by F. R. Kjellman and supplement by N. Svedelius, 1897 and 19°9) ; Rhodo phyceae (I. Teil, Abt. 2, by Fr. Schmitz and N. Svedelius, 1897 and 1911) ; Schizophyceae (i. Teil, Abt. by O. Kirchner, 'goo) , A. Pascher, Siisswasserfiora Deutschlands, Oesterreichs, und der Schweiz (Jena, 1912-27)—the last copiously illustrated and semi-popular. For seaweeds, see also W. H. Harvey, Phycologia Britannica (1846-55) ; Nereis Boreali-Americana (1851-58): Phycologia Australica (1858– 63) ; E. Wuitner, "Les Algues marines des cotes de France," Encyclo pedie pratique du naturaliste, VII. (1921) ; and L. Newton, Handbook of British Seaweeds (pubd. by British Museum, 1928). Many other less comprehensive systematic works, as well as the many monographs dealing with separate groups, will be found cited in one or other of the above. The biological aspects are more particularly treated in H. B. Ward and G. C. Whipple, Freshwater Biology (1918). (F. E. F.)