The golden-brown algae include both the chrysophytes and the synurophytes. The motile chrysophytes are sometimes referred to as the chrysomonads. Both the chrysophytes and synurophytes are most abundant and diverse in freshwaters of neutral or slightly acidic pH with low conductivity, alkalinity, and nutrient levels and colder temperatures, but may also inhabit a variety of environments.
Chrysophytes and synurophytes are heterokonts in either the vegetative or reproductive stage. Heterokont cells have two unequal flagella. One is long, covered in two rows of tripartite hairs, and is located on the cell anterior, while the other is short, smooth, and directed laterally or posteriorly, perpendicular to the longer flagellum. The flagella may be covered in scales. The longer flagellum moves with a flat, S-shaped motion to propel unicellular organisms forward; the short flagellum beats helically to allow colonial forms to swim with a rotational motion.
Chrysophytes are both photosynthetic and heterotrophic. They may be phagotrophic – engulfing particulate matter such as bacteria or small algae, or osmotrophic – absorbing organic molecules. The long flagellum is used as a feeding apparatus in some species by directing the water current and food particles towards the cell. The unicellular genus Ochromonas is often considered the “model chrysophyte” with cellular morphology and function typical of the group.
The synurophytes, commonly called the scaled chrysophytes, are motile flagellates covered by siliceous scales. Each cell has one or two golden-colored chloroplasts and two parallel flagella that emerge from an anterior pore or from in between scales. This group lacks the pigment chlorophyll c2 seen in the chrysophytes, and relies on photosynthesis as their sole energy source since they have lost their mixotrophic abilities. Most species have different types of scales that are found in specific locations on the cell. The scales vary in form, but generally consist of a base plate with an upturned rim and often have domes, ribs, pores, or spines. The scale design is unique to a particular species and is used in identification. The synurophytes currently include only four genera. Mallomonas, Synura, and Chrysodidymus are found in North America, while Tessellaria is found only in Australia.
Like the siliceous remains of diatom cells, synurophyte scales persist in sediments and are valuable tools for paleolimnologists and ecologists interested in the changes in ecological conditions over time. The synurophytes are excellent bioindicators as they inhabit particular environmental niches and are especially sensitive to pH and the presence of pollutants. Many species are useful in long-term monitoring of water quality and environmental conditions, and species distributions along dissolved salt, trophic, and temperatures gradients are of great use to ecologists.
All synurophytes undergo a siliceous resting stage. Both sexual and asexual reproduction can produce a cyst, often in response to changes in environmental conditions or population density. Like the synurophyte scales, the hollow, rounded cysts are formed within silica deposition vesicles.
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Phaeophyceae: Brown Algae
The brown colour of these algae results from the dominance of the xanthophyll pigment fucoxanthin, which masks the other pigments, Chlorophyll a and c (there is no Chlorophyll b), beta-carotene and other xanthophylls. Food reserves are typically complex polysaccharides, sugars and higher alcohols. The principal carbohydrate reserve is laminaran, and true starch is absent (compare with the green algae). The walls are made of cellulose and alginic acid, a long-chained heteropolysaccharide.
There are no known unicellular or colonial representatives; the simplest plant form is a branched, filamentous thallus. The kelps are the largest (up to 70 m long) and perhaps the most complex brown algae, and they are the only algae known to have internal tissue differentiation into conducting tissue; there is, however, no true xylem tissue as found in the ‘higher’ plants.
Most brown algae have an alternation of haploid and diploid generations. The haploid thalli form isogamous, anisogamous or oogamous gametes and the diploid thalli form zoospores, generally by meiosis. The haploid (gametangial) and diploid (sporangial) thalli may be similar (isomorphic) or different (heteromorphic) in appearance, or the gametangial generation may be extremely reduced (Fucales). The brown Giant Kelp Macrocystis pyrifera (above) is harvested off the coasts of California for feeding abalone. It used to be used for alginate extraction, but this now mostly comes from Atlantic Ascophyllum nodosum and Laminaria hyperborea. Alginates, derivatives of alginic acids, are used commercially for toothpastes, soaps, ice cream, tinned meats, fabric printing, and a host of other applications. It forms a stable viscous gel in water, and its primary function in the above applications is as a binder, stabilizer, emulsifier, or moulding agent. Saccharina japonica, formerly Laminaria, and other species of the genus are grown on ropes in China, Korea and Japan for food and alginate production. Undaria pinnatifida is also cultivated in Japan, Korea and China for production of Wakame, a valuable food kelp. Small amounts are also grown in Atlantic France for the European market.
About 16,000 tonnes of Ascophyllum nodosum (Feamainn bhuí in Irish, referring to the yellow colour in summer) are harvested each year in Ireland, dried and milled in factories at Arramara Teo., Cill Chiaráin (Kilkerrin), Co. Galway; and some 3,000 t of the resulting seaweed meal is exported and processed in Scotland for the production of alginic acid. Laminaria hyperborea stipes (sea rods) are harvested in Norway and used to be collected in drift in Scotland and Ireland. The rods are used for the manufacture of high-grade alginates. Other brown algae are used for the extraction of agricultural sprays (‘liquid seaweed extracts‘). These extracts are used at low concentrations on crops and their hormone-like activities are thought to be due to betaines, cytokinenins, etc.
There are about 1800 species of brown algae, and most are marine. In general, brown algae are larger and more species are found in colder waters. Virtually all the biomass worldwide comes from a relatively small number of species in the orders Laminariales and Fucales. The total wholesale value of dried brown algae worldwide collected in the wild or cultivated is less than $100 million dollars.
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Characteristics: Green colour from chlorophyll a and b in the same proportions as the ‘higher’ plants; beta-carotene (a yellow pigment); and various characteristic xanthophylls (yellowish or brownish pigments). Food reserves are starch, some fats or oils like higher plants. Green algae are thought to have the progenitors of the higher green plants but there is currently some debate on this point.
Green algae may be unicellular (one cell), multicellular (many cells), colonial (living as a loose aggregation of cells) or coenocytic (composed of one large cell without cross-walls; the cell may be uninucleate or multinucleate). They have membrane-bound chloroplasts and nuclei. Most green are aquatic and are found commonly in freshwater (mainly charophytes) and marine habitats (mostly chlorophytes); some are terrestrial, growing on soil, trees, or rocks (mostly trebouxiophytes). Some are symbiotic with fungi giving lichens. Others are symbiotic with animals, e.g. the freshwater coelentrate Hydra has a symbiotic species of Chlorella as does Paramecium bursaria, a protozoan. A number of freshwater green algae (charophytes, desmids and Spirogyra) are now included in the Charophyta (charophytes), a phylum of predominantly freshwater and terrestrial algae, which are more closely related to the higher plants than the marine green algae belonging to the Chlorophyta (known as chlorophytes). Other green algae from mostly terrestrial habitats are included in the Trebouxiophyceae, a class of green algae with some very unusual features.
Asexual reproduction may be by fission (splitting), budding, fragmentation or by zoospores (motile spores). Sexual reproduction is very common and may be isogamous (gametes both motile and same size); anisogamous (both motile and different sizes – female bigger) or oogamous (female non-motile and egg-like; male motile). Many green algae have an alternation of haploid and diploid phases. The haploid phases form gametangia (sexual reproductive organs) and the diploid phases form zoospores by reduction division (meiosis). Some do not have an alternation of generations, meiosis occurring in the zygote. There are about 8,000 species of green algae, about 1,000 of which are marine chlorophytes and the remainder freshwater charophytes. Unfortunately, just because algae are green no longer means that they are related: two major aggregation of green algae, the Chlorophyta and the Charophyta have turned out not be remotely related to each other.
Commercial uses: Organic beta-carotene is produced in Australia from the hypersaline (growing in high salinity water often known as brine) green alga Dunaliella salina grown in huge ponds. Carotene has been shown to be very effective in preventing some cancers, including lung cancer. Caulerpa, a marine tropical to warm-temperate genus, is very popular in aquaria. Unfortunately, this has led to the introduction of a number of Caulerpa species around the world, the best-known example being the invasive species Caulerpa taxifolia.
Chlorella, a genus of freshwater and terrestrial unicellular green alga with about 100 species, is grown like yeast in bioreactors, where it has a very rapid life history. It may be taken in the form of tablets or capsules, or added to foods such as pasta or cookies. Taken in any form, it is said improve the nutritional quality of a daily diet. According to the Taiwan Chlorella Manufacturing Company the increase in processed and refined foods in the diet of modern man make Chlorella an important food supplement for anyone interested in better health.
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Characteristics: The red colour of these algae results from the pigments phycoerythrin and phycocyanin; this masks the other pigments, Chlorophyll a (no Chlorophyll b), beta-carotene and a number of unique xanthophylls. The main reserves are typically floridean starch, and floridoside; true starch like that of higher plants and green algae is absent. The walls are made of cellulose and agars and carrageenans, both long-chained polysaccharide in widespread commercial use. There are some unicellular representatives of diverse origin; more complex thalli are built up of filaments.
A very important group of red algae is the coralline algae, which secrete calcium carbonate onto the surface of their cells. Some of these corallines are articulated (right, Corallina, with flexible erect branches; others are crustose (below). These corallines have been used in bone-replacement therapies. Coralline algae were used in ancient times as vermifuges, thus the binomial Corallina officinalis.
However, Nori, popularised by the Japanese is the single most valubable marine crop grown by aquaculture with a value in excess of US$1 billion. More information on aquaculture.
The red algae Kappaphycusand Betaphycus are now the most important sources of carrageenan, a commonly used ingredient in food, particuarly yoghurts, chocolate milk and repared puddings. Gracilaria, Gelidium, Pterocladia and other red algae are used in the manufacture of the all-important agar, used widely as a growth medium for microorganisms and for biotechnological applications.
There are about 6000 described species of red algae, the vast majority of which are marine. These are found in the intertidal and in the subtidal to depths of up to 40, or occasionally, 250 m. The main biomass of red algae worldwide is provided by the Corallinaceae and Gigartinaceae.?