Algae (singular alga) are a large and diverse group of photosynthetic, eukaryotic, plant-like organisms that use chlorophyll in capturing light energy, but lack characteristic plant structures such as leaves, roots, flowers, vascular tissue, and seeds. The designation algae includes diverse phyla, including diatoms (golden algae), green algae, euglenoids (flagellates), brown algae, and red algae, and range from single-celled organisms to giant seaweeds. The name alga (plural algae) comes from the Latin word for seaweed. The study of algae is called phycology or algology.
Algae range from single-celled organisms to multi-cellular organisms, some with fairly complex differentiated form and, if marine, called seaweeds. Some of the single-celled organisms may be as small as one micrometer. Multicellular algae may consist of a row of cells, appearing as a filament, or as a thin plate of cells, or even some larger ones may have bodies with a rudimentary division of labor. The multicellular giant kelp reaches 60 meters in length. Seaweeds themselves have many forms, including those that appear as if terrestrial plants with leaves and stems, looking like moss, mushrooms, leaf lettuce, or even palm trees.
The various types of algae play significant roles in ecology. Algae are the base of the aquatic food chain. Microscopic forms that live suspended in the water column-called phytoplankton-provide the food base for most marine food chains. The photosynthetic work done by algae is believed to produce more than three-quarters of the oxygen in the earth's atmosphere; far more than that produced by terrestrial plants.
In very high densities (so-called algal blooms), algae may discolor the water and outlast or poison other life forms.
General characteristics and ecology
Algae are usually found in damp places or bodies of water and thus are common in aquatic environments, but they are also found in terrestrial locales. Most unicellular and colonial algae are aquatic, and float near the surface of the water. The seaweeds grow mostly in shallow marine waters, but some, such as the red algae, can grow quite deep in the ocean. Terrestrial algae are usually rather inconspicuous and far more common in moist, tropical regions than dry ones, because algae lack vascular tissues and other adaptations to live on land. Algae can endure dryness and other conditions in symbiosis with a fungus as lichen.
All algae have photosynthetic machinery that is considered to derive from the cyanobacteria, and so produce oxygen as a by-product of photosynthesis, unlike the non-cyanobacterial photosynthetic bacteria. It is believed that more than three-quarters of the oxygen in the atmosphere comes from algae and cyanobacteria, rather than from plants. Although all algae utilize chlorophyll, at times other pigments mask the green color, resulting in organisms with red and brown colors.
In temperate zones, the photosynthesis of algae may be the sole source of oxygen in ice-covered lakes and ponds. If the ice remains thin and clear, such photosynthesis can help keep oxygen levels high enough to prevent fish kills by compensating for oxygen lost through respiration and decomposition. When sunlight is reduced through snow cover or thick ice, algal photosynthesis may be reduced to the point of threatening fish survival.
Some algae reproduce both sexually and asexually, such as the green algae (for example, Chlamydomonas, a unicellular green algae). The presence of sexual reproduction in some form is a nearly universal trait among living organisms, as seen even at this simple level.
Taxonomy of algae
The term algae is mainly used for convenience, rather than taxonomic purposes, as there appears little relationship between the various phyla. Although they have historically been regarded as simple plants, algae are generally classified in the kingdom Protista, rather than Plantae. Algae sometimes are defined as "photosynthetic protists"; however, some taxonomic schemes do not limit them to this kingdom.
Algae are distinguished from the other main protists, the protozoa, in that they are photoautotrophic (deriving energy from photosynthesis only), although this is not a hard and fast distinction as some groups contain members that are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon by such means as osmotrophy (by osmosis) or phagotrophy (enveloping by the cell membrane). Some scientists include as algae the prokaryotic (simple cell structure lacking a nucleus or organelles) cyanobacteria, which are aquatic, photosynthetic, and commonly known as "blue-green algae." However, in general, the designation of algae is limited to eukaryotic (cell structure with a differentiated nucleus and organelles), photosynthetic organisms.
Sometimes the prokaryotic cyanobacteria, given their aquatic and photosynthetic characteristic, have been included among the algae, and have been referred to as the cyanophytes or blue-green algae. Recent treatises on algae often exclude them, and consider as algae only eukaryotic organisms. Cyanobacteria are some of the oldest organisms to appear in the fossil record, dating back about 3.8 billion years (Precambrian). Ancient cyanobacteria likely produced much of the oxygen in the Earth's atmosphere.
Cyanobacteria can be unicellular, colonial, or filamentous. They have a prokaryotic cell structure typical of bacteria and conduct photosynthesis directly within the cytoplasm, rather than in specialized organelles. Some filamentous blue-green algae have specialized cells, termed heterocysts, in which nitrogen fixation occurs.
As commonly defined, algae are eukaryotes and conduct photosynthesis within membrane-bound structures (organelles) called chloroplasts. Chloroplasts contain DNA and are similar in structure to cyanobacteria, with the speculation that they represent reduced cyanobacterial endosymbionts. The exact nature of the chloroplasts is different among the different lines of algae, possibly reflecting different endosymbiotic events.
There are three groups that have primary chloroplasts:
- Green algae (together with higher plants)
- Red algae
In these groups, two membranes surround the chloroplast. The chloroplasts of red algae have a more or less typical cyanobacterial pigmentation, while the green algae and higher plants have chloroplasts with chlorophyll a and b, the latter found in some cyanobacteria but not most. There is support for the view that these three groups originated from a common pigmented ancestor; i.e., chloroplasts developed in a single endosymbiotic event.
Red and green algae have an "alternation of generations" life cycle. This is the same life cycle as the mosses, suggesting that green algae were ancestral to mosses. Green aquatic, the most diverse algae with over seven thousand identified species, are generally aquatic, and the majority are freshwater organisms. They range from unicellular organisms to marine species of large, multicellular seaweeds. Most of the seaweeds of the warm oceans are red algae. They absorb the deep penetrating blue light, allowing them to exist deeper than other algae.
Two other groups have green chloroplasts containing chlorophyll b:
- euglenids and
Three and four membranes surround these, respectively, and it is speculated that they were retained from an ingested green alga. Those of the chlorarchniophytes contain a small nucleomorph, which is the remnant of the alga's nucleus.
The remaining algae all have chloroplasts containing chlorophylls a and c. The latter chlorophyll type is not known from any prokaryotes or primary chloroplasts, but genetic similarities with the red algae suggest a relationship there. These groups include:
- Heterokonts (e.g., golden algae, diatoms, brown algae)
- Haptophytes (e.g., coccolithophores)
In the first three of these groups (put together in the supergroup Chromista, along with various colorless forms), the chloroplast has four membranes, retaining a nucleomorph in cryptomonads, and it is speculated that they share a common pigmented ancestor. The typical dinoflagellate chloroplast has three membranes, but there is considerable diversity among chloroplasts in the group. The Apicomplexa, a group of closely related parasites, also have plastids, though not actual chloroplasts, which share similarities with that of the dinoflagellates. The brown algae include the major seaweeds found on the shores in the temperate zones and the large, offshore beds of kelps.
Note many of these groups contain some members that are not photosynthetic, but are considered to have once been photosynthetic. Some retain plastids, but not chloroplasts, while others are considered to have lost them entirely.
Forms of algae
Most of the simpler algae are unicellular flagellates or amoeboids, but colonial and non-motile forms have developed independently among several of the groups. Some of the more common organizational levels, more than one of which may occur in the life cycle of a species, are:
- Colonial - small, regular groups of motile cells
- Capsoid - individual non-motile cells embedded in mucilage (thick, gluey, sugary substance)
- Coccoid - individual non-motile cells with cell walls
- Palmelloid - non-motile cells embedded in mucilage
- Filamentous - a string of non-motile cells connected together, sometimes branching
- Parenchymatous - cells forming a thallus with partial differentiation of tissues
In three lines, even higher levels of organization have been reached, leading to organisms with full tissue differentiation. These are the brown algae-some of which may reach 60 meters in length (kelps)-the red algae, and the green algae. The most complex forms are found among the green algae, in a lineage that is considered to have eventually led to the higher land plants. The point where these non-algal plants begin and algae stop is usually taken to be the presence of reproductive organs with protective cell layers, a characteristic not found in the other algal groups.
Algae and symbioses
Algae frequently form part of a symbiosis with other organisms. In a symbiotic relationship, the alga photosynthesises and supplies photosynthates to its host. The host organism is then capable of deriving some or all of its energy requirements from the alga. Examples include:
- lichens - a fungus is the host, usually with a green alga or a cyanobacterium as the symbiont. Both fungi and algae found in lichens are capable of living independently.
- corals - several algae form symbioses (zooxanthellae) with corals. Notable among these is the dinoflagellate Symbiodinium, found in many hard corals. The loss of Symbiodinium, or other zooxanthellae, from the host leads to coral bleaching.
Uses of algae
Algae are helpful in reducing pollutants. They assist in capturing the runoff fertilizers that enter lakes and streams from nearby farms. Algae are used in many wastewater treatment facilities, reducing the need for harmful chemicals, and are used in some power plants to reduce carbon dioxide emissions. The carbon dioxide is pumped into a pond, or some kind of tank, on which the algae feed. The natural pigments produced by algae can be used as an alternative to chemical dyes and coloring agents.
Algae is commercially cultivated as a nutritional supplement. Among algal species cultivated for their nutritional value include chlorella (a green algae) and dunaliella (Dunaliella salina), which is high in beta-carotene and is used in vitamin C supplements.
One of the most popular microalgal species is spirulina (Arthrospira platensis), which is a cyanobacteria, and has been hailed by some as a superfood. Algae is used in the Chinese "vegetable" known as fat choy (which is actually a cyanobacterium).
Many common products, such as hand lotion, lipstick, paint, and ice cream, contain derivatives from algae.
Algae can be used to produce biodiesel fuel, and by estimates can potentially produce superior amounts of oil compared to land-based crops. Because algae grown to produce biodiesel do not need to meet the requirements of a food crop, it is much cheaper to produce. Also, it does not need fresh water or fertilizer (both of which are quite expensive). Currently, most research into efficient algal-oil production is being done in the private sector, but if predictions from small-scale production experiments bear out, then using algae to produce biodiesel may be the most viable method by which to produce enough automotive fuel to replace current world gasoline usage. The per unit area yield of oil from algae is at least 15 times greater than the next best crop, palm oil. The difficulties in efficient biodiesel production from algae lie not in the extraction of the oil, which can be done using methods common to the food-industry such as hexane extraction, but in finding an algal strain with a high lipid content and fast growth rate that is not to difficult to harvest, and a cost-effective cultivation system that is best suited to that strain. Research into algae for the mass-production of oil is mainly focused on microalgae (which is generally referred to as organisms capable of photosynthesis that are less than two millimeters in diameter), as opposed to macroalgae (i.e., seaweed). This preference towards microalgae is due largely to its less complex structure, fast growth rate, and high oil content (for some species).
Algae can be grown in tanks, raceway-type ponds, and lakes. However, due to the fact that these systems are "open" to the elements, sometimes called "open-pond" systems, they are much more vulnerable to being invaded by other algal species and bacteria. Only a relatively small number of species have been successfully cultivated for a given purpose in an outdoor system (for example, as a food source, for oil production, or for pigments). In open systems, one does not have control over water temperature and little control over lighting conditions. In temperate climates, the growing season is limited to the warmer months. Some of the benefits of this type of system are that it is one of the cheaper methods: at the most basic, all that is needed is to dig a trench or pond. It also has one of the largest production capacities compared to other systems.
A variation on the basic "open-pond" system is to close it off, by covering the pond or pool with a greenhouse. While this usually results in a smaller system for economic reasons, it resolve a number of the challenges associated with an open system. It allows the preferred species to stay dominant, and it extends the growing season (only slightly if unheated, but if heated, it can produce year round.)
Algae also can be grown in polyethylene sleeves, and in a photobioreactor. A photobioreactor is basically a bioreactor that incorporates some type of light source. Because these for the most part are closed systems when used to cultivate algae, everything that the algae need to grow (carbon dioxide, nutrient-rich water, and light) must be introduced into the system.
Algae can be harvested using microscreens, by centrifugation, or by flocculation.
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