Chlorarachniophyta Cryptophyta
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Chlorarachniophyta • These algae (Fig.1) 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 Chlorarachniophyta ingested green algal cells in the past, with the green algal cells evolving into endosymbionts within the amoeba host (Fig. 3). Cryptophyta Prymnesiophyta (Haptophyta) ١ .Fig. 1 Examples of algae in the Chlorarachniophyta ٢ • Chlorarachnion reptans is a marine amoeba that forms large plasmodia with the individual cells linked by a network of reticulopodia or filopodia. • Chlorarachnion means “green spider” for the web-like network of reticulopodia (pseudopodia) in which are embedded the green amoeboid cells. • The chloroplast (e.g., endosymbiont chloroplast) contains chlorophylls a • The cells are naked and contain a number of lobed chloroplasts, each with a central and b and xanthophylls. pyrenoid (Fig. 3) and has a nucleomorph. • The chloroplast is surrounded by four membranes, the innermost two • A vesicle containing the storage product carbohydrate, paramylon (β-1,3-glucan) membranes are those of the chloroplast envelope of the endosymbiont. caps the pyrenoid. The next membrane is the plasma membrane of the endosymbiont and the • The cells move over the reticulopodia and ingest other algal cells and bacteria as a outer membrane represents the food-vacuole membrane of the amoeba food source. host. •A nucleomorph or reduced nucleus is present between the second and third envelope of each chloroplast. • The origin of this organelle is different from the origin of the cryptophytes nucleomorph, because the chlorarachniophytes originated from a green algal endosymbiont. • 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 Plasmodial reticulum of Chlorarachnion, bright field Fig.3 Semidiagrammatic drawing of the cell host amoeba. structure of Chlorarachnion reptans. microscope image (a) and schematic drawing (b). (Bar: 4 µm.) Fig .3 Semidiagrammatic drawing of the cell structure .of Chlorarachnion reptans ٣ ٤ • Asexual reproduction is carried out by either normal mitotic cell division or zoospore formation. • Under nutrient deprivation, the star-shaped vegetative cells become resting cells by retracting their reticulopodia, rounding up and secreting a thin cell wall. • The resting cells apparently rely principally on photosynthate from the chloroplasts as a food source. • The resting cells germinate to star-shaped vegetative cells under favorable conditions. • Zoosporogenesis occurs by Cryptophyta a resting cell dividing twice to produce four ovoid zoospores, each with a single flagellum wrapped around the cell body. • The zoospores settle to produce the star-shaped vegetative cells. • Sexual reproduction characterized by heterogamy and occurs when a non-motile female gamete is approached by a motile, star-shaped, male gamete. gametes fuse producing a zygote The٥ • ٦ that germinates into a star-shaped vegetative cell. Fig. 4 Chlorarachnion reptans. ١ Cell structure CRYPTOPHYCEAE • There are two apically or laterally attached • This group is composed primarily of flagellates that occur in both marine and flagella at the base of a depression. freshwater environments; palmelloid phases can also be formed, and some • Each flagellum is approximately the same members are known to be zooxanthellae in host invertebrates or within length as the body of the cell. certain marine ciliates. • Depending on the species, there are one • The cells contain chlorophylls a and c2 and phycobiliproteins that occur inside or two rows of microtubular hairs attached the thylakoids of the chloroplast. to the flagellum. • The cell body is asymmetric with a clearly defined dorsiventrally construction. • Small, organic scales (Fig.9.2) are • The asymmetric cell shape results in a peculiar swaying motion during common on the flagellar surface and swimming. sometimes on the cell body. • Most cryptophytes have a single lobed chloroplast with a central pyrenoid. • The Corps de Maupas (CM) is a large vesicular structure in the anterior portion of the cell. Its main function is probably that of disposing of unwanted protoplasmic structures by digestion. Fig. 9.9 (a) Cryptomonas erosa. (b) Chilomonas paramecium. (c) Goniomonas truncata. (d) Rhodomonas lacustris. (e) Chroomonas nordstedtii. Fig. 9.1 Drawing of a cell of the Cryptophyceae as seen in the light and electron microscope. (CE) Chloroplast Fig. 9.2 Drawing of the most common type of flagellar envelope; (CER) chloroplast endoplasmic reticulum; scale found in freshwater cryptophytes. (CM) Corps de Maupas; (D) dorsal; (E) ejectisome; (L) lipid; (M) mitochondrion; (N) nucleus; (NM) ٧ ٨ nucleomorph; (P) pyrenoid; (PP) periplast plate; (S) starch; (V) ventral. • The chloroplast most likely evolved from a symbiosis between phagocytic • The outer portion of the cell, or periplast, is composed of the plasma organism and a red alga. membrane and a proteinaceous plate, or series of plates, directly under the • The chloroplast is surrounded by two membranes of chloroplast endoplasmic plasma membrane (Figs. 9.1, 9.10). reticulum and the two membranes of the chloroplast envelope. • The number and shape of these plates are used to characterize genera • Between the outer pair of membranes and the inner pair of membranes of the taking into consideration that the haploid and diploid phases of a single chloroplast endoplasmic reticulum (periplastidial space) are starch grains and a genus can have different plates. nucleomorph, probably the remnant of the nucleus of the endosymbiont in the • Sulfated fucose-rich polysaccharides can be excreted outside of the cell. event that led to chloroplast E.R. • The primary method of reproduction is simply by longitudinal cell division, • The nucleomorph contains three minute paired-chromosomes with 531 genes that but sexual reproduction has recently been documented. encode 30 proteins targeted into the chloroplast. • The nucleomorph is surrounded by an envelope that has pores similar to those in a nuclear envelope. • The cryptophytes are the only algae that form their storage product in the periplastidial space. • The starch is an α-1,4-glucan composed of about 30% amylose and amylopectin. • Cryptophycean starch is similar to potato starch and starch found in the green algae and dinoflagellates. • Sometimes an eyespot formed Chroomonas oblonga has multiple periplast plates (P) by spherical globules is under the plasma membrane present inside the plastid. ٩ ١٠ • The Cryptophyceae have projectiles called ejectisomes, which are of different structure from the trichocysts of the Dinophyceae. • In the chloroplast, the thylakoids are grouped in pairs (Fig. 9.3), and there are no connections between adjacent thylakoids. The • There are usually large ejectisomes near the anterior depression and Cryptophyta is the only group to have this arrangement of thylakoids. smaller ejectisomes around the cell periphery. • Chlorophylls a and c are present. • Both sizes of ejectisomes have the same structure; they are made up of 2 two unequal-sized bodies enclosed within a single membrane (Fig. 9.5). • The major carotenoid present is α-carotene, and the major xanthophyll, diatoxanthin. • The ejectisomes discharge when the organism is irritated. • Phycobilins are present in the thylakoid lumen rather than in • The discharge of the ejectisome results in a movement of the organism phycobilisomes on the stromal side of the thylakoids as occurs in the in the opposite direction. The discharge of the ejectisome could function cyanobacteria and red algae. as an escape mechanism, or it could be a direct defense mechanism causing damage to an offending organism. • Each photosynthetic cryptophyte has only one species of phycobiliprotein – either a phycoerythrin or a phycocyanin – but never both. • No allophycocyanin is present. Fig. 9.3 Transmission electron micrograph of part of a chloroplast of Chroomonas mesostigmatica. The thylakoids are grouped in pairs. The dense contents of the thylakoids represent the phycobilisomes. Also present are lipid droplets (l) and a large starch grain (s). × 50 000. Fig. 9.5 (a) General organization of an ejectisome showing the two subparts. (b) A model of an ejectisome ١١ ١٢ being fired outside of the cell. (c) A drawing of a discharged ejectisome. ٢ Ecology • In comparison with other algal groups, the Cryptophyta appear to be especially light sensitive, often forming the deepest living • Cryptophytes will often undergo diel vertical migrations populations in clear oligotrophic lakes. with an amplitude less than 5 meters. • Because of the low light intensity under snow and ice cover, these • In small humic forest lakes, species of Cryptomonas are algae concentrate in surface waters to receive sufficient light for positively phototactic in the morning, moving into the photosynthesis. • Survival at these extremely low light levels depends not only on a phosphorus-depleted upper layer. Later in the day the highly efficient photosynthetic system, but also on cells move away from the uppermost water layer, – slow rates of cell respiration at low water temperatures and avoiding high levels of irradiance, and move into the – reduced winter zooplankton grazing. phosphorus-rich hypolimnion. A further advantage of this • In spring, with the disappearance of snow and resulting sudden increase in light in Arctic and mountain lakes, cryptomonads suffer