What does Apicomplexa mean

what are apicomplexan parasites, ist other apicomplexa and their pathogenicity, how does apicomplexa reproduce
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Published Date:02-08-2017
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Apicomplexa Algae are organisms that have plastids, or organ- colorless plastid called an apicoplast (Fig. 8.1) isms that are derived from cells whose ancestors (Wilson, 1993; Wilson et al., 1994). Molecular possessed plastids. Until 1994, it was thought that studies have shown that the apicoplast and the apicomplexa did not have plastids (and con- dinoflagellate plastids originated from red algae sequently were not covered in phycology text- by a single endosymbiotic event that occurred rel- books). Then it was shown that a known organelle atively early in eukaryotic evolution (Fast et al., in many apicomplexa was actually a reduced 2001). Fig. 8.1 Drawing of the basic cytology of an apicomplexan cell.APICOMPLEXA 311 Fig. 8.2 General scheme by which an apicomplexan infects a blood cell. The apicomplexan has a characteristic laminin Toxoplasma. The second is that drugs effective polysaccharide on the surface of the plasma membrane that against prokaryotic organisms might be effec - binds with laminin receptors on the blood cell. This forms a tive against the apicoplast since all plastids origi- tight junction between the apicomplexan and blood cell. The nally evolved from endosymbiotic prokaryotic apicomplexan discharges its rhoptries. The blood cell cyano bacteria. Apicomplexans are absolutely phagocytoses the apicomplex into a parasitophorous vacuole. dependent on the apicoplast, which has led to spec- (Modified from Sam-Yellowe, 1996.) ulation that this curious organelle is a potential “Achilles heel” of parasites, such as Plasmodium. The discovery of the apicoplast generated con- The typical apicomplexan vegetative cell siderable interest since most apicomplexans are (merozoite) (Fig. 8.1) has an apicoplast sur- unicellular endoparasites that cause some of the rounded by four membranes. The inner two mem- most significant tropical diseases (Foth and branes are the inner and outer plastid membranes McFadden, 2003). Malaria in humans is produced while the outer membranes are derived from the by the apicomplexan Plasmodium. About 300 mil- vacuolar membrane and the plasma membrane of lion people are infected with malaria, leading to the endosymbiotic red alga. one million deaths annually (Ralph et al., 2004). The apical complex consists of a polar ring Apicomplexans cause other serious diseases in live- and a conoid formed of spirally coiled micro- stock and humans, such as cryptosporidiosis, tubules (Fig. 8.1). The apicomplexan has laminin babesiosis (Texas cattle fever), theileriosis (East polysaccharide on its surface while the host cell Coast fever), and toxoplasmosis. The realization has a laminin receptor (Fig. 8.2). The apicom- that these endoparasites were once algae raised plexan parasite attaches to the host cell with the hopes that the apicoplast might be a drug target conoid protruding to produce a stylet that forms for two reasons. The first is that the apicoplast is a tight junction between the apicomplexan para- essential for the survival of Plasmodium and site and host cell. The apicomplexan cell is taken312 CHLOROPLAST E.R.: EVOLUTION OF ONE MEMBRANE up into the host cell in the parasitophorous Katablepharis (Fig. 8.3) is a heterotrophic vacuole. The contents of the rhoptries and unicellular flag ellate that lacks a plastid. micronemes are emptied into the space between Katablepharis cells have ejectisomes and was clas- the apicomplexan plasma membrane and the par- sified with the Cryptophyceae. However, ultra- asitophorous vacuole membrane. structural studies (Lee and Kugrens, 1991; Lee et Apicomplexans have a layer of flattened mem- al., 1991) revealed the presence of an anterior branous sacs or alveoli (Fig. 8.1) beneath the conoid apparatus involved in phagocytosis of prey. plasma membrane that comprise the subpellicular The conoid apparatus is very similar to those of membrane complex, similar to that found in the the apicomplexans and it is likely that Katable - dinoflagellates. pharis should be classified as an apicomplexan. Fig. 8.3 Drawings of Katablepharis ovalis. Left: whole cell. Right: anterior part of cell. (From Lee and Kugrens, 1991; Lee et al., 1991.)APICOMPLEXA 313 Ralph, S. A., van Dooren, G. G., Waller, R. F., et al. REFERENCES (2004). Metabolic maps and functions of the Fast, N. M., Kissinger, J. C., Roos, D. S., and Keeling, P. J. Plasmodium falciparum apicoplast. Nat. Rev./Microbio. 2:203–16. (2001). Nuclear-encoded, plastid-targeted genes sug- gest a single common origin for apicomplexan and Sam-Yellowe, T. Y. (1996). Rhoptry organelles of the Apicomplexa: their role in host cell invasion dinoflagellate plastids. Mol. Biol. Evol. 18:418–26. Foth, B. J., and McFadden, G. I. (2003). The apicoplast: a and intracellular survival. Parasitol. Today 12:308–16. plastid in Plasmodium falciparum and other apicom- plexan parasites. Int. Rev. Cytol. 224:57–110. Wilson, R. J. (1993). Plastids better red than dead. Nature 366:638. Lee, R. E., and Kugrens, P. (1991). Katablepharis ovalis, a colorless flagellate with interesting cytological char- Wilson, R. J., Williamson, D. H., and Preiser, P. (1994). Malaria and other apicomplexans: the “plant” con- acteristics. J. Phycol. 27:505–15. Lee, R. E., Kugrens, P., and Mylnikov, A. P. (1991). nection. Infect. Agents Dis. 3:29–37. Feeding apparatus of the colorless flagellate Katablepharis (Cryptophyceae). J. Phycol. 27:725–33.Part V Evolution of two membranes of chloroplast endoplasmic reticulum and the Chlorarachniophyta Algae with two membranes of chloroplast endoplasmic reticulum (chloroplast E.R.) have the inner membrane of chloroplast E.R. sur- rounding the chloroplast envelope. The outer membrane of chloroplast E.R. is continuous with the outer membrane of the nuclear envelope and has ribosomes on the outer surface (Fig. V.1). The algae with two membranes of chloroplast E.R. evolved by a sec- ondary endosymbiosis (Fig. V.1) (Lee, 1977) when a phagocytic protozoan took up a eukaryotic photosynthetic alga into a food vesicle. Instead of being phagocytosed by the protozoan, the photosynthetic alga became established as an endosymbiont within the food vesicle of the protozoan. The endosymbiotic photosynthetic alga benefited from the acidic envi- ronment in the food vesicle that kept much of the inorganic carbon in the form of carbon dioxide, the form needed by ribulose bisphosphate/ carboxylase for carbon fixation (see Part IV for further explanation). The host benefited by receiving some of the photosynthate from the endosymbiotic alga. The food vesicle membrane eventually fused with the endoplasmic reticulum of the host protozoan, resulting in ribo- somes on the outer surface of this membrane, which became the outer membrane of the chloroplast E.R. Through evolution, ATP production and other functions of the endosymbiont’s mitochondrion were taken over by the mitochondria of the protozoan host, and the mitochondria of the endosymbiont were lost. The host nucleus also took over some of the genetic control of the endosymbiont, with a reduction in the size and function of the nucleus of the endosymbiont. The resulting cytology is characteristic of the extant algae in the Chlorarachniophyta and Cryptophyta, which have a nucleomorph representing the degraded endosymbiotic nucleus, as well as storage product produced in what remains of the endosymbiont cytoplasm. The type of chloroplast E.R. that exists in the Heterokontophyta and the Prymnesiophyta resulted from further reduction. The nucleomorph316 Fig. V.1 The sequence of events that led to the evolution of algae with two membranes of chloroplast endoplasmic reticulum. (Drawing by Brec Clay.) was completely lost and storage product formation was taken over by the host. The resulting cell had two membranes of chloroplast enve- lope surrounding the chloroplast. Outside of this was the inner membrane of chloroplast E.R. that was the remains of the plasma mem- brane of the endosymbiont. Outside of this was the outer membrane of317 chloroplast E.R. which was the remains of the food vesicle membrane of the host. Although the above evolutionary scheme is discussed in one sequence, it is probable that two membranes of chloroplast E.R. evolved at least three times, with one line leading to the Chlorarachniophyta, a second to the Cryptophyta, and the third (or more) leading to the Heterokontophyta and Prymnesiophyta. The algae with two membranes of chloroplast E.R. are: Chlorarachniophyta: chloroplast derived from a green alga, chlorophyll a and b present, nucleomorph between inner and outer membrane of chloroplast E.R. Cryptophyta: Chlorophyll a and c, phycobiliproteins, nucleomorph between inner and outer membranes of chloroplast E.R., starch in grains between inner membrane of chloroplast E.R. and chloroplast envelope, periplast inside plasma membrane, tripartite hairs on flagella. Heterokontophyta: tripartite hairs on anterior tinsel flagellum, posterior whiplash flagellum, chlorophyll a and c, fucoxanthin, storage product usually chrysolaminarin in vesicles in cytoplasm. Prymnesiophyta (haptophytes): two whiplash flagella, haptonema present, chlorophyll a and c, fucoxanthin, scales common outside cell, storage product usually chrysolaminarin in vesicles in cytoplasm. Chlorarachniophyta These algae (Fig. V.2) represent an intermediate stage in the evolution of two membranes of chloroplast endoplasmic reticulum. This group has a small number of green amoebae that have ingested green algal cells in the past, with the green algal cells evolving into endosymbionts within the amoeba host (Fig. V.3) (Hibberd and Norris, 1984). A nucleo- morph or reduced nucleus occurs in the green algal symbiont. The reduced nature of the nucleomorph implies that some of the functions originally coded by the DNA of the endosymbiont nucleus have been taken over by the nucleus of the host amoeba. The chloroplast (e.g., endosymbiont chloroplast) contains chlorophyll a and b and is surrounded by four membranes. The innermost two membranes are those of the chloroplast envelope of the endosymbiont. The next mem- brane is the plasma membrane of the endosymbiont and the outer membrane represents the food-vacuole membrane of the amoeba host. Thus, the algae in the Chlorarachniophyta represent an intermediate stage in the evolution of the chloroplasts of some of the algae in the Heterokontophyta. Chlorarachnion reptans is a marine amoeba that forms large plasmodia with the individual cells linked by a network of reticulopodia (Geitler, 1930; Hibberd and Norris, 1984). The cells are naked and contain a number of lobed chloroplasts, each with a central pyrenoid (Fig. V.3). Four318 Fig. V.2 Examples of algae in the Chlorarachniophyta. ((a) adapted from Calderon-Saenz and Schnettner, 1989; (b) adapted from Ishida et al., 1996; (c) adapted from Hibberd and Norris, 1984; (d) adapted from Moestrup and Sengco, 2001.) Fig. V.3 Semidiagrammatic drawing of the cell structure of Chlorarachnion reptans. (Adapted from Hibberd and Norris, 1984.) membranes surround the chloroplast, which has a pyrenoid and nucleo- morph. A vesicle containing the storage product caps the pyrenoid. Chlorarachnion means “green spider” for the web-like network of reticu- lopodia (pseudopodia) in which are embedded the green amoeboid cells.319 Fig. V.4 Chlorarachnion reptans. (Adapted from Hibberd and Norris, 1984; Grell, 1990.) The cells move over the reticulopodia and ingest other algal cells and bac- teria as a food source. Under nutrient deprivation, the star-shaped vegetative cells become resting cells by retracting their reticulopodia, rounding up and secret- ing a thin cell wall (Grell, 1990). The resting cells apparently rely princi- pally on photosynthate from the chloroplasts as a food source. The resting cells germinate to star-shaped vegetative cells under favorable conditions. Zoosporogenesis occurs by a resting cell dividing twice to produce four zoospores, each with a single flagellum wrapped around the cell body (Fig. V.1(c) and V.4). The zoospores settle to produce the star- shaped vegetative cells. Sexual reproduction occurs when a non-motile female gamete is approached by a motile, star-shaped, male gamete. The gametes fuse producing a zygote that germinates into a star-shaped veg- etative cell (Grell, 1990). REFERENCES Calderon-Saenz, E., and Schnetter, R. (1989). Morphology, biology, and systemat- ics of Cryptochlora perforans (Chloroarachniophyta), a phagotrophic marine alga. Pl. Syst. Evol. 163:165–76.320 Geitler, L. (1930). Ein grunes Filarplaasmodium und andere neue Protisten. Arch. Protistenkd. 69:615–36. Grell, K. G. (1990). Some light microscope observations on Chlorarachnion reptans Geitler. Arch. Protistenkd. 138:271–90. Hibberd, D. J., and Norris, R. E. (1984). Cytology and ultrastructure of Chlorarachnion reptans (Chloroarachniophyta division nova, Chloroarachniophyceae classis nova). J. Phycol. 20:310–30. Ishida, K., Nakayama, T., and Hara, Y. (1996). Taxonomic studies on the Chlorarchniophyta. II. Generic delimitation of the chlorarachniophytes and description of Gymnochlora syellata gen. et sp. nov. and Lotharella gen. nov. Phycol. Res. 44:37–45. Lee, R. E. (1977). Evolution of algal flagellates with chloroplast endoplasmic retic- ulum from the ciliates. South African J. Sci. 73:179–82. Moestrup, Ø., and Sengco, M. (2001). Ultrastructural studies on Bigelowiella natans, gen. et sp. nov., a chlorarachniophyte flagellate. J. Phycol. 37:624–6.Chapter 9 Cryptophyta and Lee, 1987). The number and shape of these CRYPTOPHYCEAE plates are used to characterize genera taking into consideration that the haploid and diploid phases This group is composed primarily of flagellates of a single genus can have different plates (Hoef- that occur in both marine and freshwater envir - Emden and Melkonian, 2003). New periplast plates onments. The cells contain chlorophylls a and c are added in an area adjacent to the vestibulum 2 and phycobiliproteins that occur inside the thy- (Brett and Wetherbee, 1996). Sulfated fucose-rich lakoids of the chloroplast. The cell body is asym- polysaccharides can be excreted outside of the cell metric with a clearly defined dorsi-ventral/ (Giroldo and Vieira, 2002). right-left sides (Figs. 9.1, 9.9, 9.10). The asymmetric The chloroplast most likely evolved from a cell shape results in a peculiar swaying motion symbiosis between an organism similar to the during swimming. Most cryptophytes have a phagocytic cryptomonad Goniomonas and a red single lobed chloroplast with a central pyrenoid. alga (Kugrens and Lee, 1991; Liaud et al., 1997; McFadden et al., 1994). The chloroplast is sur- rounded by two membranes of chloroplast endo- Cell structure plasmic reticulum and the two membranes of the chloroplast envelope (Fig. 9.1). Between the outer There are two apically or laterally attached membrane and the inner membrane of the chloro- flag ella at the base of a depression. Each flagel- plast endoplasmic reticulum are starch grains and lum is approximately the same length as the body a nucleomorph (Figs. 9.1, 9.4). The nucleomorph of the cell (Figs. 9.1, 9.8, 9.9, 9.10). Depending on contains three minute paired-chromosomes with the species, there are one or two rows of micro- 531 genes (humans have at least 31000 genes) that tubular hairs attached to the flagellum. In encode 30 proteins targeted into the chloroplast Cryptomonas sp., the hairs on one flagellum are 2.5 (Douglas et al., 2001; Cavalier-Smith, 2002). The m long and in two rows whereas the hairs on the nucleomorph is probably the remnant of the other flagellum are only 1 m long and arranged nucleus of the endosymbiont in the event that led in a single row (Heath et al., 1970; Kugrens et al., to chloroplast E.R. The nucleomorph is sur- 1987). Small, 150-nm-diameter organic scales (Fig. rounded by an envelope that has pores similar to 9.2) are common on the flagellar surface and those in a nuclear envelope. The nucleomorph sometimes on the cell body (Lee and Kugrens, exhibits a rudimentary type of division utilizing 1986). microtubules (Morrall and Greenwood, 1982). The outer portion of the cell, or periplast The nucleomorph divides in preprophase of the (Gantt, 1971), is composed of the plasma mem- main nucleus following basal body replication, brane and a plate, or series of plates, directly under but before division of the chloroplast and the the plasma membrane (Figs. 9.1, 9.10) (Kugrens chloroplast endoplasmic reticulum (McKerracher322 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES Fig. 9.1 Drawing of a cell of the Cryptophyceae as seen in the light and electron microscope. (CE) Chloroplast envelope; (CER) chloroplast endoplasmic reticulum; (CM) Corps de Maupas; (D) dorsal; (E) ejectisome; (L) lipid; (M) mitochondrion; (N) nucleus; (NM) nucleomorph; (P) pyrenoid; (PP) periplast plate; (S) starch; (V) ventral. and Gibbs, 1982). The only cryptophyte that is known to lack a nucleomorph is Goniomonas (Figs. 9.8, 9.9(c)), a colorless cryptophyte that lacks a plas- tid. A second colorless cryptophyte, Chilomonas (Fig. 9.9(b)), is a reduced form of a photosynthetic cryptophyte and contains a leucoplast and a nucle- omorph (McKerracher and Gibbs, 1982). In the chloroplast, the thylakoids are grouped in pairs (Fig. 9.3), and there are no connections between adjacent thylakoids. The Cryptophyta is the only group to have this arrangement of thy- lakoids. Chlorophylls a and c are present. The 2 Fig. 9.2 Drawing of the most common type of flagellar major carotenoid present is -carotene, and the scale found in freshwater cryptophytes. (From Lee and major xanthophyll, diatoxanthin. There are three Kugrens, 1986.) spectral types of phycoerythrin and three spectralCRYPTOPHYTA 323 Fig. 9.3 Transmission electron micrograph of part of a chloroplast of Chroomonas mesostigmatica. The thylakoids are the amount of pigments under different light- grouped in pairs. The dense contents of the thylakoids intensity conditions. Cells of Cryptomonas grown represent the phycobilisomes. Also present are lipid droplets 2 under low light-intensity conditions (10 E m (l) and a large starch grain (s). 50 000. (From Dodge, 1969.) 1 s ) contain twice as much of chlorophylls a and c , and six times as much phycoerythrin per cell, 2 as those grown under high light-intensity condi- 2 1 types of phycocyanin, all of which are different tions (260 E m s ) (Thinh, 1983). Under from the phycobiliproteins found in the cyanobac- low light-intensity conditions there is a higher teria and red algae (Hill and Rowan, 1989). The concen tration of phycoerythrin and the thy- phycobiliproteins are in the intrathylakoid space lakoids are thicker. (inside the thylakoids (Fig. 9.3) (Gantt et al., 1971; The reserve product (similar in appearance to Spear-Bernstein and Miller, 1984), and are not on starch grains) is appressed to the pyrenoid area the stromal side of the thylakoids in phycobili- outside of the chloroplast envelope but inside the somes as occurs in the cyanobacteria and red chloroplast E.R. The cryptophytes are the only algae. Each photosynthetic cryptophyte has only algae that form their storage product in this area. one species of phycobiliprotein – either a phyco- The starch is an -1,4-glucan composed of about erythrin or a phycocyanin – but never both. 30% amylose and amylopectin. Cryptophycean No allophycocyanin is present (Gantt, 1979). starch is similar to potato starch and starch found Allophycocyanin acts as a bridge in the transfer of in the green algae and dinoflagellates (Antia et al., light energy from phycoerythrin and phycocyanin 1979). to chlorophyll a of the reaction center in red Some of the Cryptophyceae have eyespots. The algae and cyanobacteria. The presence of allophy- eyespots that have been reported consist of lipid cocyanin may not be necessary in the crypto- granules inside the chloroplast envelope. In phytes because the greater absorption range of Chroomonas mesostigmatica, the red eyespot is in cryp tophycean phycobiliproteins in conjunction the center of the cell (Fig. 9.4) and is an extension with chlorophyll c overlaps the chlorophyll a of the chloroplast beyond the pyrenoid (Dodge, absorption spectrum. There is a variation in 1969). In Cryptomonas rostella, the eyespot is324 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES Fig. 9.4 Transmission electron micrograph of a cell of Chroomonas mesostigmatica showing the eyespot (E) present smaller body is joined to the first and sits at an in the chloroplast. (Ey) Ejectisome; (F) flagellum; (G) Golgi; angle within the V-shaped portion of the larger (N) nucleus; (Nu) nucleomorph; (S) starch. (Micrograph body. The two bodies actually constitute one long provided by Paul Kugrens.) tape with two spirals. The bodies are always arranged so that the smaller body is near the sur face. The ejectisomes discharge when the beneath the chloroplast membrane near the organism is irritated (Fig. 9.5), the discharged depression. Some of the Cryptophyceae exhibit ejectisome being a long tubular structure with a positive phototaxis, with maximum sensitivity of short portion at an angle to the long portion. The the colorless Chilomonas being in the blue at discharged small ejectisome from the cell 366 nm (Halldal, 1958). periphery is 4 m long, whereas that of a larger The Cryptophyceae have projectiles called ejectisome from under the anterior depression is ejectisomes, which are of different structure about 20 m long. The discharge of the ejecti- from the trichocysts of the Dinophyceae and some results in a movement of the organism in which are probably closely related to the R-bodies the opposite direction. The discharge of the of the kappa particles of the ciliates (Hovasse ejectisome could function as an escape mecha- et al., 1967; Kugrens et al., 1994). Within a cell nism, or it could be a direct defense mechanism there are usually large ejectisomes near the ante- causing damage to an offending organism. rior depression and smaller ejectisomes around Ejectisomes originate in vesicles in the area of the cell periphery (Figs. 9.1, 9.4, 9.8). Both sizes of Golgi bodies. ejectisomes have the same structure; they are The Corps de Maupas is a large vesicular struc- made up of two unequal-sized bodies enclosed ture in the anterior portion of the cell (Fig. 9.1). Its within a single membrane (Fig. 9.5). Each of these main function is probably that of disposing of bodies is a long tape curled up on a very tight unwanted protoplasmic structures by digestion spiral. The tape is tapered, with the greatest (Lucas, 1970a,b). width being on the outside of the ejectisome. TheCRYPTOPHYTA 325 Fig. 9.5 (a) General organization of an ejectisome showing the two subparts. (b) A model of an ejectisome being fired outside of the cell. (c) A drawing of a discharged ejectisome. (After Hovasse et al., 1967.) (a) (b) (c) the day the cells move away from the uppermost water layer, avoiding high levels of irradiance, Ecology and move into the phosphorus-rich hypolimnion In comparison with other algal groups, the (Knapp et al., 2003). A further advantage of this Cryptophyta appear to be especially light sensitive, cycle is the reduction of grazing pressure by zoo- often forming the deepest living populations in plankton (for which cryptophytes are a preferred clear oligotrophic lakes (Nauwerk, 1968). In higher food) (Loret et al., 2000) which often migrate in the mountain and north temperate lakes, cryptomon- reverse direction. ads and other flagellates are present in the water Cryptophyte algae are mixotrophic, capable of column throughout the winter. Because of the low phototropy and phagotrophy. Phagocytotic inges- light intensity under snow and ice cover, these tion of bacteria is thought primarily to provide a algae concentrate in surface waters to receive suffi- source of phosphorus and nitrogen in nutrient- cient light from net photosynthesis (Wright, 1964; limiting conditions (Urabe et al., 2000). These Pechlauer, 1971). Survival at these extremely low algae are also chemotactic, swimming in a straight light levels depends not only on a highly efficient line until they reach a nutrient patch, at which photosynthetic system, but also on slow rates of time the cells stop and tumble in the volume of cell respiration at low water temperatures and high-nutrient concentration (Lee et al., 1999). reduced winter zooplankton grazing. In spring, Cryptophytes are the dominant algae in the with the disappearance of snow and resulting freshwater lakes of Antarctica. The best studied sudden increase in light in Arctic and mountain lakes are those in the McMurdo Dry Valleys lakes, cryptomonads suffer considerable light (Fig. 9.6) (Roberts and Laybourne-Parry, 1999; stress, such that the biomass maximum moves to McKnight et al., 2000). The McMurdo Dry Valleys deeper waters (Kalff and Welch, 1974). are the largest ice-free areas in Antarctica (about 2 Cryptophytes will often undergo diel vertical 4000 km ) and constitute a polar desert with tem- migrations with an amplitude less than 5 meters. peratures ranging from 45 C to 5 C. These val- In small humic forest lakes, species of Cryptomonas leys remain ice free because the Trans-Antarctic are positively phototactic in the morning, moving Mountains not only block the flow of the ice sheet, into the phosphorus-depleted upper layer. Later in but also block the flow of moisture, the valleys326 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES distribution that colors the water in which it is growing red. It has been recorded from neritic locations such as bays and fjords; away from the coast it is usually associated with regions of upwelling and in such conditions the blooms have been recorded as extending over areas as large as 100 square miles. The color of the ciliate (Fig. 9.7) is due to numerous reddish-brown chloro- plasts, which belong to a single cryptophycean alga that lives symbiotically inside the ciliate (Gustafson et al., 2000). The cryptophyte is sur- rounded by a single membrane, and has a nucleus and the normal cytology and pigments of the Cryptophyceae. The endosymbiotic cryptophyte is 14 able to fix C in the light, evolve oxygen in photo- 32 synthesis, and assimilate P, indicating that it is Fig. 9.6 The location of the McMurdo Dry Valleys in a functioning autotroph. The association is prob- Antarctica. ably similar to that of symbiotes in other classes, with the endosymbiont providing the host receive only about 10 cm of snow a year. The lakes with photosynthate and the host providing the are fed by glacial melt streams that flow for endosymbiont with a protected environment. 6–10 weeks during the brief austral (southern) Blooms of Mesodinium rubrum are a regular feature summer. The lakes are perennially covered by of upwelling ecosystems. The organism has three debris-containing ice caps up to 5 m thick that characteristics that enable it to compete effec- reduce light penetration. In addition, the sun tively with other autotrophic plankton (Smith does not arise above the horizon for a number of and Barber, 1979). (1) It is motile, swimming at 1 months during the austral winter. These lakes are rates of 2.0 to 7.2 m h , an order of magnitude highly stratified because of a lack of forces that greater than the maximum swimming speeds could generate turnover of the water column (e.g., attained by dinoflagellates. (2) It has strong pho- wind, water temperature changes). Cryptophytes totropisms, being positively phototactic in an dominate the lower stratified levels where they increasing light regime in the morning and nega- live heterotrophically during winter months, tively phototactic in decreasing light and in nutri- taking up about one bacterium per hour by ent-depleted waters. (3) It has extremely high 3 1 phagocytosis (Roberts and Laybourn-Parry, 1999). photosynthetic rates (1000 to 2000 mg C m h ), During the summer months, the cryptophytes equaling the highest ever observed for oceanic are mixotrophic (combining heterotrophy and plankton. Conventional dinoflagellate or diatom 3 1 autotrophy by photosynthesis). A key to the sur- blooms typically have only 60 to 70 mg C m h . vival of cryptophytes in this environment is main- taining the population in the vegetative state, Classification rather than entering a resting state. The crypto- phyte population can respond quickly when There are three recognizable groups within the “good” conditions return in the short Antarctic Cryptophyceae (Marin et al., 1998; Deane et al., summer. 2002): Order 1 Goniomonadales: colorless cells with no Symbiotic associations plastids. Order 2 Cryptomonadales: cells usually reddish Mesodinium rubrum is a marine planktonic in color with chloroplasts containing holotrich ciliate of extremely wide geographical the phycobiliprotein Cr-phycoerythrin.CRYPTOPHYTA 327 Fig. 9.7 Mesodinium rubrum with its cryptomonad symbiont. (a) Light micrograph of the ciliate showing the chloroplast (C) and pyrenoid (P) of the cryptomonad endosymbiont. (b) Transmssion electron micrograph. The dotted lines indicate the boundary between the cytoplasm of the ciliate and the cryptomonad symbiont; the difference in density of the two cells is particularly clear. The symbiont nucleus, one of the macronuclei, and the micronucleus of the ciliate are out of the plane of the section. (CM) Ciliate mitochondrion; (ER) endoplasmic reticulum; (Mac) macronucleus; (P) pyrenoid; (SM) symbiont mitochondrion; (V) vacuole. The large arrowhead indicates a possible region of Golgi activity. 4500. (From Hibberd, 1977.) Order 3 Chroomonadales: the remainder of the Cryptomonadales cryptophyte algae, often blue-green in Cryptomonas (Figs. 9.9(a), 9.10, 9.11) and Chilomonas color due to chloroplasts containing the (Fig. 9.9(b)) are the only two genera in the order. phycobiliprotein Cr-phycocyanin. Cryptomonas spp. are reddish in color due to the presence of the phycobiliprotein Cr-phycoerythrin Goniomonadales in a bilobed chloroplast joined in the center by Goniomonas (Figs. 9.8, 9.9(c)), a colorless alga with apyrenoid. Chilomonas is a reduced form of freshwater and marine species, is the sole alga Cryptomonas (Hoef-Emden and Melkonian, 2003) in the order. Goniomonas is colorless and does containing a leucoplast without photosynthetic not contain a plastid. Food organisms are taken pigments. up by an anterior tubular invagination, the Cryptomonas has an asymmetric shape, which infundibulum, and digested in food vacuoles in can be attributed, in part, to a subapical depres- the cytoplasm. Storage granules occur inside sion called the vestibulum which may extend an extension of the outer membrane of the internally to form a gullet or progress along the nuclear envelope. Large ejectisomes occur ventral surface into a furrow (Fig. 9.10(b)) (Kugrens under the anterior plasma membrane and and Lee, 1991). Large ejectosomes occur in rows small ejectisomes occur between the periplast under the furrow. Sexual reproduction occurs in plates. Cryptomonas (Fig. 9.11) (Kugrens and Lee, 1988).328 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES Fig. 9.8 Reconstruction of a cell of Goniomonas truncata. (f) Flagellar roots; (s) storage granules; (e) ejectisome; (dv) digestive vacuole; (i) infundibulum. (After Mignot, 1965.) Fig. 9.9 (a) Cryptomonas erosa. (b) Chilomonas paramecium. (c) Goniomonas truncata. (d) Rhodomonas lacustris. (e) Chroomonas nordstedtii.CRYPTOPHYTA 329 Fig. 9.10 Scanning electron micrographs of Chroomonas oblonga (a) and Cryptomonas sp. (b). Chroomonas oblonga has multiple periplast plates (P) under the plasma membrane, no furrow is present, and the flagella (F) arise from an anterior vestibular depression. Cryptomonas sp. has a smooth surface that is produced by a single periplast plate under the plasma membrane. The furrow (Fu) is an extension of the anterior vestibulum. A vestibular ligule (VL) overlaps the vestibulum. (From Kugrens et al., 1986.) Fig. 9.11 The life cycle of Cryptomonas sp. (Adapted from Kugrens and Lee, 1988.)

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