Chromalveolata
The second supergroup is the Chromalveolata, including the alveolates ciliates, dinofla- gellates apicomplexa and the stramenopiles (diatoms, chrysophytes, and oomycetes). They have a unique cell surface where the cell plasma membrane is underlain by a layer of vesicles called alveoli, which can be empty or contain cellulose plates or scales generated from the Golgi. These vesicles fuse with the plasma membrane and provide the cell with an effective cell wall. The plates can be impregnated with silica or calcium carbonate. Other unique characters include the presence of micropores through the cell surface and the possession of extrusive organelles. Mitochondria have tubular christae. Nutrition in this group is very varied, and includes predation, parasitism, and photoautotrophy.
Alveolata includes three groups; the ciliates, dinoflagellates, and apicomplexans:
? Ciliates are a largely free-living group, and are characterized by their dense covering of cilia and complex nuclear arrangements. Many species are photosynthetic, having acquired chloroplasts from photosynthetic symbionts. Others are pathogenic, for example, Balantidium coli can infect humans when present in fecally contaminated food or water. It infects intestinal epithelial cells producing ulcers. Advanced cases show symptoms similar to amebic dysentery (i.e. vomiting, diarrhea, nausea) and the infection can lead to death.
? Dinoflagellates are predominantly unicellular, marine, free-living, motile organisms. Although unicellular, the dinoflagellates are structurally an extremely diverse group of unicellular organisms. They possess a unique nuclear structure, the dinokaryon, which lacks nucleosomes, is low in basic proteins, and is high in DNA con- tent. The chromosomes are very condensed and there is a prominent nucleolus. Most dinoflagellates have two flagella inserted into the cell at right angles to each other, around the midline of the cell. One is wrapped around the waist of the cell in a groove; the other extends from the posterior of the cell. There are also a number of extrusomes (organelles that can be extruded from the cell), including trichocysts, mucocysts, and nematocysts. Dinoflagellate chloroplasts contain chlorophylls a, c1, and c2 and the carotenoid fucoxanthin, and store chrysolaminarin, a b1–3-linked glucose. Some are capable of phagotrophy; others live within marine invertebrates and are termed zoo- xanthellae.
? Apicomplexa are a wholly parasitic group and have a body form much like an ameba, but they have an apical complex that aids in the attachment of the parasite to the host cell membrane and assists in cell invasion. The complex is formed of a tubulincontaining conoid and rhoptries, which can be extruded from the apicomplexan cell. The apicomplexa are all intracellular parasites, and the most significant in terms of human deaths is Plasmodium falciparum, the causative agent of malaria. There are other species of apicomplexans that are human pathogens but these seem to be distantly related and they likely diverged 5–8 million years ago at the point where humans and chimpanzees diverged from each other.
Stramenopiles include the photosynthetic diatoms and chrysophytes, and the heterotrophic or parasitic oomycetes, and opalines. Photosynthetic species have arisen after the symbiotic association between a nonphotosynthetic protistan and red or brown photosynthetic species some time ago in evolutionary history. The pigment content, the numbers of ER layers around the chloroplast, and the numbers of thylakoid lamellae con- firm this hypothesis. Those species that contain red pigments from the cryptophytes can occupy the very deepest layers of the photic zone. Most species in this group are unicellular, but some are colonial or filamentous. Life cycles are similar to those of the chlorophytes but the dominant vegetative stage is diploid:
? Diatoms contain chorophylls a and c and various accessory pigments, which give them a golden brown color. The chloroplasts have four membranes around them, demonstrating their origin as secondary symbionts from red algae. Diatoms differ from other members of the golden brown stramenopiles because their vegetative stage often lacks flagella, but they have a gliding motility on solid surfaces. They have a geometric shape supported by a silica-containing cell wall composed of a pair of ‘nested’ shells called frustules with a girdle band around them. The large half of the shell is termed the epitheca, the smaller is the hypotheca. Nuclei are always diploid.
? Chrysophytes contain chlorophylls a, c1, and c2 and have two flagella inserted into the cell at near right angles to each other. Some species are covered in radially or bilaterally symmetrical scales. They are also characterized by the formation of spores.
? Oomycetes and related groups. Members of the Oomycota are common aquatic species with a fungal-like thallus, predominantly saprophytes or parasites of animals and plants. They share many morphological characters of the chytrids, but dif- ferences include cellulose-containing cell walls, tubular mitochondrial cristae, and a life cycle where the dominant somatic phase is diploid rather than haploid or dikaryotic. They also possess biflagellate zoospores instead of the single flagellate zoospores found in the chytrids. These organisms are capable of causing devastating disease in both plants and fish.
Members of the Hyphochytrids also have cellulose-containing cell walls, and these organisms have only recently been separated from the chytrids. A haploid vegetative stage is dominant in the life cycle. Members of this phylum of organisms are destructive intracellular parasites of plants with an absorptive nutrition. They cause enlargement and multiplication of host cells, creating large and unsightly clubbed roots and often death of the plant. The best known disease is club root of crucifers. Like the Oomycota they share morphological and nutritional similarities with the chytrid fungi. They have some cellulose in their cell walls, and DNA sequence data also indicate a closer relationship with the dinoflagellate algae than that with the fungi.Often grouped together, both the cellular and acellular slime molds exist for most of their life cycle as soil-dwelling amebae, feeding on soil bacteria. In both groups they are haploid until the onset of reproduction.
? Opalines are a group of specialized endosymbiotic species living in the bowels of amphibians. In this microaerophyllic environment they obtain nutrients by pinocytosis. They are multinucleate, and have a pellicle-like cell covering with closely spaced flagella that beat together.