General characteristics of Cryptophyta
Division - Cryptophyta:-
General characteristics:-
1. Introduction:-
> This group is composed primarily of flagellates that occur in both marine and freshwater environments; palmelloid phases can also be formed, and some members are known to be zooxanthellae in host invertebrates or within certain marine ciliates.
> Cryptophyte algae are mixotrophic, capable of phototropy and phagotrophy.
> Phagocytotic ingestion of bacteria is thought primarily to provide a source of phosphorus and nitrogen in nutrient limiting conditions.
> These algae are also chemotactic, swimming in a straight line until they reach a patch of high-nutrient concentration.
> Cryptophytes are the dominant algae in the freshwater lakes of Antarctica.
> The lakes are fed by glacial melt streams that flow for 6–10 weeks during the brief austral (southern) summer.
> Cryptophytes dominate the lower stratified levels where they live heterotrophically during winter months, taking up about one bacterium per hour by phagocytosis.
> During the summer months, the cryptophytes are mixotrophic (combining heterotrophy and autotrophy by photosynthesis).
> A key to the survival of cryptophytes in this environment is maintaining the population in the vegetative state, rather than entering a resting state. The cryptophyte population can respond quickly when “good” conditions return in the short Antarctic summer.
2. Cell structure:-
> There are two apically or laterally attached flagella at the base of a depression.
> Each flagellum is approximately the same length as the body of the cell.
> Depending on the species, there are one or two rows of microtubular hairs attached to the flagellum.
> Small, organic scales are common on the flagellar surface and sometimes on the cell body.
> Corps de Maupas (CM):- It 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.
> The cells contain chlorophylls a and c and phycobiliproteins that occur inside the thylakoids of the chloroplast.
> The cell body is asymmetric with a clearly defined dorsiventrally construction. The asymmetric cell shape results in a peculiar swaying motion during swimming.
> Chloroplast:-
- Most cryptophytes have a single lobed chloroplast with a central pyrenoid. The chloroplast most likely evolved from a symbiosis between phagocytic organism and a red alga.
- The chloroplast is surrounded by two membranes of chloroplast endoplasmic reticulum and the two membranes of the chloroplast envelope.
- Between the outer pair of membranes and the inner pair of membranes of the chloroplast endoplasmic reticulum (periplastidial space) are starch grains and a nucleomorph, probably the remnant of the nucleus of the endosymbiont in the event that led to chloroplast E.R.
- The nucleomorph contains three minute paired-chromosomes with 531 genes that 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 by spherical globules is present inside the plastid.
- In the chloroplast, the thylakoids are grouped in pairs, and there are no connections between adjacent thylakoids. The Cryptophyta is the only group to have this arrangement of thylakoids.
- Chlorophylls a and c are present.
- The major carotenoid present is α-carotene, and the major xanthophyll, diatoxanthin.
- Phycobilins are present in the thylakoid lumen rather than in phycobilisomes on the stromal side of the thylakoids as occurs in the cyanobacteria and red algae.
- Each photosynthetic cryptophyte has only one species of phycobiliprotein – either a phycoerythrin or a phycocyanin – but never both.
- No allophycocyanin is present.
> Ejectisomes:-
- The Cryptophyceae have projectiles called ejectisomes, which are of different structure from the trichocysts of the Dinophyceae.
- There are usually large ejectisomes near the anterior depression and smaller ejectisomes around the cell periphery.
- Both sizes of ejectisomes have the same structure; they are made up of two unequal-sized bodies enclosed within a single membrane.
- The ejectisomes discharge when the organism is irritated.
- The discharge of the ejectisome results in a movement of the organism in the opposite direction. The discharge of the ejectisome could function as an escape mechanism, or it could be a direct defense mechanism causing damage to an offending organism.
> Cryptophytes will often undergo diel vertical migrations with an amplitude less than 5 meters.
> In small humic forest lakes, species of Cryptomonas are positively phototactic in the morning, moving into the phosphorus-depleted upper layer. Later in the day the cells move away from the uppermost water layer, avoiding high levels of irradiance, and move into the phosphorus-rich hypolimnion. A further advantage of this cycle is the reduction of grazing pressure by zooplankton (for which cryptophytes are a preferred food) which often migrate in the reverse direction.
3. Symbiotic associations:-
> Mesodinium rubrum is a marine planktonic holotrich ciliate of extremely wide geographical distribution that colors the water in which it is growing red.
> 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 is due to numerous reddish-brown chloroplasts, which belong to a single cryptophycean alga that lives symbiotically inside the ciliate.
> The association is probably similar to that of symbiotes in other classes, with the endosymbiont providing the host with photosynthate and the host providing the endosymbiont with a protected environment.
4. Reproduction:-
> 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 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.
> The gametes fuse producing a zygote that germinates into a star-shaped vegetative cell.
Classification:-
Class:- Cryptophyceae
Orders:-
i. Goniomonadales:- Eg.- Goniomonas
ii. Cryptomonadales:- Eg.- Cryptomonas
iii. Chroomonadales:- Eg.- Chroomonas