Egregia menziesii Chemistry
- Pigments, Storage Products, Cell Wall Chemistry, and Cell Wall Structure
Like all browns, Egregia menziesii contains chlorophylls a and c. Also like other browns, the major accessory pigment is fucoxanthin, which gives the plants their yellowish-brown color (absorbs in the range of 500-540 nm), followed by b-carotene and violaxanthin. The browns have also been shown to contain seven minor carotenoids, as well as traces of acetyleric pigments, diadinoxanthin, and diatoxanthin. and violaxanthin. The browns have also been shown to contain seven minor carotenoids, as well as traces of acetyleric pigments, diadinoxanthin, and diatoxanthin.
The major storage product of the browns is laminarin, a soluble b-1,3-linked glucan, located in membrane bound vesicles. Other storage products include mannitol and, in a smaller amount, sucrose, glycerol, and other miscellaneous molecules.
CELL WALL Egreagia menziesii's cell wall, again like other browns, is formed by a meshwork of fibrils in an amorphous matrix, much like rods and cement in a concrete wall. The major fibrillar material of E. menziesii is cellulose, along with others structural polysaccarides.
The major matrix polysaccharide is alginic acid, composed of b(1-4)-D-mannuronic acid anda(1-4)-L-guluronic acid. The proportion of these two sugars in the cell wall effects the strength. For example, stiff, guluronate-rich alginates are typical of the holdfast, while mannuronate-rich alginates, being more elastic, are more typical of the cell walls in the blades(1).
This paradigm of structural adaptation to mechanical stress on cell walls does not always hold, however. For example, blades of E. menziesii growing in high energy environments (more wave stress) were shown to have blades made of a weaker alginates, and should have been more flexible than blades of plants growing in calmer water. However, contrary to this, the blades were stiffer and stronger(1).
Other matrix polysaccharides include fucoidan and xylan.
ON THE CELL WALL: CHLOROPLASTS AND PHYSODES
The chloroplasts of Egregia, like all plants, lay close to the cell wall to gain maximum light harvesting efficiency. As with other browns, the thylakoids of Egregia lay in stacks of three to five. The chloroplasts are tighlty associated with endoplasmic reticulum of the outside surfaces. Also on the surface of the chloroplasts lay many small vacuoles known as pyrenoids, where storage products seem to be formed.
Unique to the brown algae are small bodies laying just inside the cell wall which are assumed to be some kind of vacuole, called physodes. The physodes may acts as storage spaces for secondary storage products (not laminarin). They also contain many phenolic and polyphenolic compounds, which are thought to limit growth of epiphytes (which increase drag and shade the plant), and possibly act to deter herbivores(1).
Egregia is one of only a few algae that have been found to have plasmodesmata connecting adjacent cells. Groups of plasmodesmata forming "pit-fields," of Egregia are similar to the plasmodesmata of many angiosperms(2).
This picture may help
you visualize these structures:
References for this page:(1). Lobban, Christopher S., and Paul J. Harrison. Seaweed Ecology and Physiology. Cambridge University Press, New York. 1997.
(2). Dodge, John D. The Fine Structure of Algal Cells. Academic Press, London. 1973.
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