Diatom

“ But only one group of these organisms, the diatoms, will receive our closer attention. They surpass the bacteria in size; still, a good microscope is needed to observe them. Whereas the bacteria present only few, and at that, simple forms, but colors of varied hue, the reverse is true of the diatom family. Their color, when they possess any, is a more or less dark brown or a green, but the delicacy and beauty of their forms is striking. ”

“ Paleoceanographic studies frequently examine the role changing abundances of diatoms have had on the evolution of ocean environments and the carbon cycle. While studies may sometimes make use of quantitative estimates of past diatom export productivity from measurements of sedimentary opal abundance, over longer time periods and global scales, sedimentary opal abundance estimates are not available, and recourse is often made to diatom diversity (diversity here means species richness) as a proxy for diatom ecologic significance and export productivity (e.g. [5] , [6] ) . ”

“ In the case of partly siliceous species, washing in pure water repeatedly is all that can be done. The larger and heavier diatoms may be separated from the sand by elutriation or by whirling in a casserole, by rocking in a shallow dish the shape of a watch crystal, or by pouring slowly over a strip of plate-glass at least two feet in length inclined at an angle of thirty degrees. The sand will cling to the glass, while the greater portion of the diatoms will run off. ”

“ Various devices, including hooks, spiral bundles, horns and processes exuding threads of plasma, exist for holding together the frustules. When marine forms are found in quiet waters some of these devices, being no longer of any value, cease to grow, although free swimming diatoms are rare. They either occur in long chains or are stipitate or sessile. If it is further assumed that the fresh-water diatoms are found in greater abundance in later periods, the action of running streams makes necessary the provision of some means by which the species may continue to colonize. ”

“ Sometimes the frustules, which in all cases are single, are crowded very closely upon each other and almost wholly hide the substance on which they repose. Fig. 30 is another diatom of a flag-like shape, named Achnanthes, having a long slender filament attached to one end of the lower frustule, representing the flag-staff. There are many wonderful species of such diatoms, some running almost end to end like a bundle of sticks, and therefore called Bacillária; others spreading out like a number of fans, such as the genus Licmophora ”

“ Certain species will grow wherever there are moisture, light and heat, but the greater number require the presence, in small amounts, of substances produced by the decay of animal and vegetable life. An abundance of diatoms in fresh water is usually an indication of its potability, while their entire absence in shallow water may be due to an excess of bacteria. ”

“ In one of the earliest classifications of diatoms, the individual cell received less consideration than the nature of the filament or thallus in which many species occur in the first stages of their growth. Those, however, which exist in colonies at first are, sooner or later, broken up into separate frustules, either before or at the time of their maturity or previous to conjugation, while very many species are never seen except in a free state. ”

“ There is considerable a-priori evidence from diatom biology and biogeography to expect diatom diversity to respond to climate, and thus to expect the long-term as well as short term correlations to be at least in part causal, not chance. ”

“ Grasslands
Rabosky and Sorhannus [16] suggested that the relatively low post-Eocene diversity of diatoms in their reconstruction argued against both a grassland expansion-driven increase of silica to the oceans, with consequent diversification of marine diatoms; and against co-evolutionary, inverse relationships between Cenozoic diatom and coccolithophore diversification. Our results do not support their arguments, although there are other reasons to question a grassland-marine diatom hypothesis, as proposed by Falkowski et al. [6] . ”

“ To illustrate this with the parallel case of silicious deposits, there are great beds made up of silicious shields of diatoms. These during their lifetime extracted from the waters the dissolved silica, which, but for their intervention, might have accumulated till it was at length deposited in the form of schist or of crystalline quartz. In either case the function of the coral, the rhizopod, or the diatom is limited to assimilating the carbonate of lime or the silica from its solution, and the organised form thus given to these substances is purely accidental. ”

“ When seen upon the upper surface, it looks at first sight like a mere row of cells with a band running along them; but by careful arrangement of the light its true form may easily be made out.
Fig. 28 represents a very common fresh-water diatom, named Melosíra várians. The plants of this genus look like a cylindrical rod composed of a variable number of segments, mostly cylindrical, but sometimes disc-shaped or rounded. An end view of one of the frustules is seen at the left hand, still coloured with its dots of “endochrome,” and showing the cylindrical shape. ”

“ The late Professor Smith, in his “Synopsis of Diatoms,” refers to their movements in the following terms: “I am constrained to believe that the movements observed in the Diatomaceæ are due to forces operating within the frustule, and are probably connected with the endosmotic and exosmotic action of the cells. The fluids which are concerned in these actions must enter, and be emitted through the minute foramina at the extremities of the silicious valves.” Schultze’s researches, which were made at a later date, carried this debatable question somewhat further. ”

“ They reproduce on an average about once in five days, so that from a single diatom the offspring possible under favorable conditions would amount to over 16,000,000 in four months or over 60 tons in a year. Of such an order is the potential increase of animals or plants, no matter how small, if the rate of reproduction is high. ”

“ In colour, Diatoms are usually brown or brownish, although they contain chlorophyll, the green colouring [84] matter of higher plants. In shape they may be rod-shaped, crescent-shaped, circular, wedge-shaped, oblong or oval. Some float about freely in the water, some are attached to supports by means of stalks. Some lead a solitary life and others dwell together in colonies. One feature they have in common, a curious flinty cell wall, and this is their most interesting point to the microscopist. ”

“ However, I wander from the microscope. These shortbread things are fussing about hither and thither across the field, and now and then an amœba comes crawling into view. These are invertebrate jelly-like things of no particular shape, and they keep on thrusting out a part here, and withdrawing a part there, and changing and advancing just as though they were popular democratic premiers. Then diatoms keep gliding athwart the circle. These diatoms are, to me at least, the most perplexing things in the universe. ”

“ In Diatoma and Fragilaria, we find a punctum or pore at one end of a valve, but not in line with the pseudoraphe; in Synedra, a minute pore is usually found in the position of the terminal nodule and, in some species, indications of a central nodule are observed; the median line is wider but there is no raphe. In the fresh-water Synedræ, many of which are among the longest of diatoms, living in running streams, the terminal nodules are much more indistinct, while the marine forms have distinct terminal nodules, are not, as a rule, found in bands, and assume a more naviculoid outline. ”

“ The relationship to diversity can either be a direct influence of diversity on the relative abundance of diatoms in the plankton (the effect of more diverse evolutionary adaptations on relative nutrient capture) , or indirect via the response of both diatom abundance and diatom diversity to changing amounts of nutrient input into the oceans due to changing Cenozoic climate. ”

“ It often happens that gatherings are made consisting almost entirely of sand. Attempts at cleaning in the usual way will cause the loss of nearly all of the diatoms. In this case, after the material has been treated with acid until nothing remains but sand and a few diatoms, the mechanical finger must be used. ”

“ It was on the third day of the diatoms that Hapley became aware of a novel addition to the local fauna. He was working late at the microscope, and the only light in the room was the brilliant little lamp with the special form of green shade. Like all experienced microscopists, he kept both eyes open. It is the only way to avoid excessive fatigue. One eye was over the instrument, and bright and distinct before that was the circular field of the microscope, across which a brown diatom was slowly moving. ”

“ Bearing in mind that these organisms, as figured, have been obtained by destroying the organic matter with nitric acid, we find Bacillaria figured as it exists only in the living condition—the frustules being joined together in the peculiar way which has given to this form the specific name paradoxa. For this diatom to have passed through a bath of nitric acid, and come out in the condition figured, would have been almost as great a miracle as the passing of Shadrach, Meshach, and Abednego, unscathed through the fiery furnace of Nebuchadnezzar. ”

“ It is customary, especially among writers who are familiar with other classes of plants, to decry any classification of diatoms according to the markings of their siliceous envelopes. As, however, one of the chief distinctions of the class is the possession of a more or less siliceous and indestructible frustule, and as the cell and its contents are never seen except within the valves, their variety forms the only available method of identification. ”

“ The diatoms of the Polar Regions, however, are not all marine forms. I have examined hundreds of land forms in the Arctic Regions, especially during my wintering in 1896 and 1897 in Franz Josef Land. Doubtless, also, there are species of diatoms that belong to Antarctic lands. "On several occasions, notably on December 18th and 20th, 1892, I saw bergs which were fringed with pale brown streaks, like a vein apparently sandwiched in their main mass," and I believe that this coloration was due to diatoms or some other forms of algæ. ”

“ Diatoms increase in numbers, like many of the Foraminifera, by simply dividing into two. When the living cell thus parts, each half takes as its share one side of the box, and each then makes another side, or “frustule,” to complete itself. ”

“ Perhaps the most remarkable of all diatoms. Many species possess the power of motion, which, however, is evident only in the free frustule. In N. paxillifer, the movement of the frustules occurs without the loss of continuity or adherence to each other, so that, while at one time the adnate frustules form a narrow filament, like that of Fragilaria, at another {120}time they move laterally to their extreme length and form a thread of frustules adherent at their ends, later resuming their original position. ”

“ Diatoms do not live in absolutely pure or non-aërated water. The individual plates or granules of the endochrome are called chromatophores. Their number and significance will be referred to in the description of genera. ”

“ GROWING.
a, b, Flint skeleton inside the jelly-cell. a, c and d, b, Two flint skeletons formed by new valves, c and d, forming within the first skeleton.
Another species of Diatoma (Fig. 8) called Diatoma vulgare, is a very simple and common form, and will help to explain how these plants grow. The two flinty valves a, b inside the cell are not quite the same size; the older one a is larger than the younger one b and fits over it like the cover of a pill-box. ”

“ The method which is the most frequently made use of is the following:—Place the “gathering” containing the Diatomaceæ in a small glass or porcelain vessel, add strong nitric acid, and, by the aid of Bunsen’s burner or spirit-lamp, boil for some minutes. From time to time a drop of the mixture may be put upon a slide, and examined under the microscope to see if all foreign matter be got rid of. When the valves are clean, the vessel containing them must be filled with water, and the whole left for an hour or two, so that all the diatoms may settle perfectly. ”

“ The city of Berlin is built upon a bed of clay of from six to one hundred feet thick, two thirds of the mass of which consist of diatoms. There is a puzzling feature in the life of these diatoms. They contain a coloring matter, diatomin, which is similar to chlorophyl in its properties and in having the power of abstracting carbonic acid from the air and water. It is hard to understand how this power can be exercised where the light does not penetrate. Yet a great mass of the diatom bed under Berlin is living and active, and streets and houses have been disturbed by its growth. ”

“ Diatoms, which likewise secrete a siliceous skeleton, often of great beauty. The skeletons of Diatoms are found abundantly at the present day in lake-deposits, guano, the silt of estuaries, and in the mud which covers many parts of the sea-bottom; they have been detected in strata of great age; and in spite of their microscopic dimensions, they have not uncommonly accumulated to form deposits of great thickness, and of considerable superficial extent. ”