Diatoms: Jewels of the Marine World
Diatoms are a major group of plants which float in open water, and they are one of the most successful types of microscopic algae known. The estimated over 100,000 known species are found in the oceans, in freshwater, in soils and even on damp surfaces. Most diatoms are unicellular, although some can form colonies in the shape of long filaments or ribbons. As eukaryotes or cells with a nucleus, they have highly complex cells, comparable to other eukaryotes such as mammals and even humans (Philippe, et al., 1994, Journal of Evolutionary Biology 7: 247).
A major feature of diatom cells is their unique geometrically designed cell houses made of silica (hydrated silicon dioxide). These frustule homes show an enormously wide diversity of forms, but usually consist of two unequal halves with a separation of some type between them (dia means two, toms, to cut or to separate into two). Basically they are like pill boxes.
The enormous variety of these glass home designs displays an architectural beauty rarely seen in the natural world. Thus we see in Thierstein and Young (editors. 2004. Coccolithophores: From Molecular Processes to Global Impact. Springer-Verlag): “Diatoms are unique among extant photoautotrophic taxa [photosynthetic organisms] in that they have an absolute requirement for orthosilicic acid, which they polymerize on a protein matrix to form strong shells called frustules. Silica is introduced into the oceans primarily by continental weathering, but the present day surface ocean is strongly undersaturated with respect to silica as a direct consequence of diatom growth. Diatoms are basically neritic [living in water at most 200 m deep], and blooms are largely confined to continental margins, and shallow seas, and such open ocean regions as the North Atlantic and Southern Ocean where silica can be supplied through upwelling. Hence, one possible clue to the rise of diatoms in the Cenozoic [recent times] may lie in an increased flux of silicic acid from the continents.” (p. 445). The variety of diatom shapes is enormous. Of the marine species, some look like the pattern of spokes on bicycle wheels, others like six pointed stars, yet others like pinwheels or triangles. In fresh water as well as centric designs (as above), there are many that look boat shaped or like long needles.
Diatoms provide an excellent means of studying evolution because they are the most common fossil types found, and their hard glass shells preserve them extremely well. As a result there now exists an “extensive fossil record of diatoms” (Sims et al 2006. Phycologia 45 no. 4 p. 361). Moreover the over 200 genera of living diatoms that have been identified, including approximately 100,000 extant species, allow one an ideal means for reflecting on their origins.
The commonality of well-preserved fossil diatom glass houses in the fossil record is a major reason why these algae are today a favored tool of modern evolutionary researchers for dating rocks and documenting evolution. If evidence for evolution exists, it would be found here, yet the first diatoms are clearly modern diatoms and no evidence for their evolution exists in the fossil record. The “first physical remains of diatoms are from the Jurassic [similar level to rocks with sauropod dinosaurs in them], and well-preserved, diverse floras are available from the Lower Cretaceous [immediately above the Jurassic rocks].” (Sims et al. 2006. p. 361).
The total lack of any evidence for diatom evolution is usually explained away by the claim that they have evolved far too rapidly to leave a fossil record. This, though, argues from absence of proof, not evidence. The fact is, the first diatoms are clearly closely similar, as far as we can tell, to modern diatoms, except that many early diatoms are extinct and these examples “bear little resemblance to modern taxa” (Sims. et al, 2006, p. 362). Consequently these are of little help in determining diatom phylogeny, or evolutionary relationships, if any.
Evolutionary science can only speculate about “possible diatom origins,” speculation that is not influenced by fossil or other evidence (Sims et al. 2006. p. 36). Lack of fossil evidence has not stopped speculation about diatom evolution however. The main theory is diatoms evolved during the Precambrian [before the appearance of many celled organisms] from a “naked photosynthetic cell [that] acquired a coating of siliceous scales” (Round and Crawford. 1981. Proceedings of the Royal Society of London B 221(1183) p. 237) If this were the case, evidence of the scales would exist. The process of wall formation is obviously complex, and why would the evolution of these organisms stop after this point?
Comparison of “molecular and paleontological data in diatoms suggests a major gap in the fossil record” according to Philippe et al. 1994. p. 247. One study has suggested that, in contrast to fossil data, “molecular clock calibrations indicate that the rRNA coding regions in the diatoms are evolving at approximately 1% per 18 to 26 Ma [million years], which is the fastest substitution rate reported in any pro- or eukaryotic group of organisms to date” (Kooistra et al. 1996.Phylogenetics and Evolution. 6 no. 3 p. 391). Potentially diatoms should show a fast rate of change, but a search for these changes comes up with zero results.
Another group, called Coccolithophores, share several important traits with diatoms. Coccolithophores are single-celled microscopic algae that are a major component of the upper layer of ocean microplankton. They are unique in both the animal and plant worlds for several reasons. One major reason is that their spherical cell is surrounded by many limestone (calcite) plates shaped like hubcaps called coccoliths. As a unit, coccolithophores look very much like microscopic fancy Christmas tree ornaments. When the coccolithophores die, reproduce, or make too many plates, they dump some or all of their plates into their watery world. It is estimated that dumping their plates adds over 1.5 million tons of calcite into the oceans annually. Like diatoms, their tiny plate-enclosed homes are assembled in such a way as to produce a wide variety of beautiful geometric designs. (See “Chalk Talk” in Dialogue Dec. 2002 at www.create.ab.ca )
The complexity of their plate homes and the lack of fossil evidence for their evolution provide the basis for the conclusion that calcification has only arisen once in the evolution of the Haptophyta [mostly marine, mostly single celled golden brown algae]” (Thierstein and Young. 2004. p. 261). Hypothetical evolutionary trees have been constructed, but DNA comparisons have “fundamentally altered our way of thinking about evolution and ecology of the group” (2004. p. 277). Of note is the conclusion that “there is no obvious long-term trend [in the fossil record] and the cause of the variations [existing in coccolithophores] is not known” (2004. p. 516). Nevertheless the fossil record is excellent, and both “the organic and inorganic remains of coccolithophores provide key geochemical records for study of past oceanographic, environmental, and biological conditions. Coccolithophores are useful for paleoceanographic reconstructions because they are widespread throughout the ocean and both organic and inorganic remains enjoy long term preservation in the marine sediment record. The inorganic record is especially durable and extends back to the evolution of coccolithophores in the Early Mesozoic [rock layers just below the dinosaurs].” (Thierstein and Young. 2004. p. 530). Thus these ample records from the past too, provide only problems and no comfort for evolutionary theories.
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