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Heterokont

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Heterokont
Diatoms.jpg
Scientific Classification
Classes

Colored groups

Colorless groups

Heterokonts, also known as stramenopiles, are a main group of eukaryotes that includes around 10,500 species. The majority of heterokonts are algae, such as the diatoms (a key element of plankton) or giant kelp. Some other more common members of Heterokontophyta include Pythium, and the oomycetes, including Phytophthora. [1]

Anatomy

Heterokonts are tubular protists that have tripartite tube-like hairs. A majority of heterokonts are unicellular flagellates, and nearly all others create flagellate cells sometime in their life cycle, an example being zoospores or gametes.

The name heterokont refers to the distinguishing appearance of the cells that normally have two uneven flagella. The flagella are positioned sideways, and are generally maintained by four microtubule roots that are in a unique pattern. One flagellum is smooth and frequently shorter, or is occasionally condensed to a basal body; while the other is the tinsel (anterior) flagellum is enclosed with cross bristles called mastigonemes. Glycoproteins in the cell's endoplasmic reticulum produce mastigonemes before being transferred to the cell’s surface. A backwards current is created when the tinsel flagellum moves, which brings in food or pulls the cell throughout the water. Possibly one of the defining characteristics of the group, the mastigonemes have a strange tripartite makeup, thus consisting of a few protists that don’t manufacture cells with the classic heterokont figure. [2]

Four membranes border chloroplasts in heterokont algae. First, it is surrounded by two membranes from the chloroplast envelope or cover, and then is enclosed by two membranes of chloroplast endoplasmic reticulum; the last one is constant with the endoplasmic reticulum that is continuous with its nuclear envelope. This construct of membranes suggests that the heterokont chloroplasts were resultant from symbiotic eukaryotes, seemingly red alga. The chloroplasts typically include chlorophyll a and c, and also commonly the accessory pigment fucoxanthin, which often gives them a brownish-green or a golden-brown color. Colorless heterokonts are usually basal heterokonts and propose that they may have divided before the emergence of chloroplasts inside the group. Nonetheless, chloroplasts that contain fucoxanthin are also found along with the haptophytes. There is evidence that these two groups (haptophytes and heterokonts) share a general derivation, and perhaps even the cryptomonads as well. If that were the case the ancestral heterokont would be an alga, and all the colorless groups would arise by chloroplast loss. [3]

Reproduction

The reproduction of heterokonts differs between classes. Some heterokonts, such as Saprolegniales reproduce with asexual reproduction using zoospores, and also reproduce with sexual reproduction. Others, for example, oomycetes, have oogamous sexual reproduction to reproduce. Heterokonts reproduce relatively quickly and quite often. [4]

Ecology

Heterokont algae can be found in just about all environments wherever life exists, however, the occurrence differs extensively between the classes. Pinguiophyceae, Chrysomophyceae, Bolidophyceae, Schizocladophyceae, and Pelagophyceae are simply identified from marine environments. Phaeophyceae are more or less entirely marine organisms, although there are five freshwater genera that are known. Synurophyceae are most likely limited to freshwater, though a couple of uncertain marine occurrences were reported. Some heterokonts such as Dictyochophyceae occur in both marine and freshwater habitats. Then there are heterokonts for example, Eustigmatophyceae, which occur in freshwater, marine, and terrestrial habitats. Lastly, diatoms can be found in all ordinary habitats sustaining life. As regards to haptophytes, they are generally marine organisms, but some freshwater species seem to be well known. [5]

Gallery

Related References