What are algae?

Algae are pond scums, terrestrial algae, snow algae, seaweeds, and freshwater and marine phytoplankton. The plant body is relatively undifferentiated, and there are no true roots and leaves.

Algae are not a single taxonomic entity. Molecular phylogeny (gene sequencing) and other characters show they belong to four kingdoms: Kingdom Plantae (e.g. chlorophytes and rhodophytes - green and red algae), the Kingdom Chromista (e.g. phaeophytes - brown algae - dinoflagellates, and diatoms), the Kingdom Protozoa (e.g. Euglenozoa), and the Kingdom Bacteria (Blue-green algae).
 

Pronunciation: Algae ("al'jay" or "al'gay", both are currently in use) is the plural; alga ("al'ga") is the singular, and there is no such thing as "algaes"! 

Algae are very simple chlorophyll-containing organisms: some say that they are plants; others say that the are not, calling them protists or protoctists. According to the most recent phylogenetic studies, both are not quite correct. Some algae (most the greens and the reds) are related to land plants, and some flagellated algae are related to the protists, but there is no justification for including all algae in any generic term other than "algae" although it has little meaning taxonomically.

We use the term "algae" very loosely, simply because defining them is so very difficult. As conceived in the broadest sense, algae are oxygen-generating, photosynthetic organisms other than the embryophyte land plants, fungi and lichens. Quite simply, what we call "algae" is an artificial and highly heterogeneous aggregation of organisms belonging to many different evolutionary lineages, and therefore highly diverse from a genetic point of view. This genetic diversity is reflected in the enormous biodiversity exhibited by algae in terms of morphological, ultrastructural, ecological, biochemical, and physiological traits.

Marine macroalgae, or seaweeds, are plant-like organisms that generally live
attached to rock or other hard substrata in coastal areas. They belong to three
different groups, empirically distinguished since the mid-nineteenth century by the Irish botanist William Henry Harvey (1811–1866) on the basis of thallus color: red algae (phylum Rhodophyta), brown algae (phylum Ochrophyta, class Phaeophyceae), and green algae (phylum Chlorophyta, classes Bryopsidophyceae, Chlorophyceae, Dasycladophyceae, Prasinophyceae, and Ulvophyceae). Distinguishing these three groups, however, involves more substantial differences than indicated by this simple designation. In addition to the pigmentation, they differ considerably in many ultrastructural and biochemical features including photosynthetic pigments, storage compounds, composition of cell walls, presence/absence and type of flagella, ultrastructure of mitosis, connections between adjacent cells, and the fine structure of the chloroplasts. In general, we can say that they are simple organisms composed of one cell, or grouped together in colonies, or as organisms with many cells, sometimes collaborating together as simple tissues.

Spores. Most algae form some sort of spore, which is a cell - often motile - that serves to reproduce the organism without change genetically.  Some are colonial and motile in the adult phase like Volvox (right, photograph © Karl Bruun).

Sex? Algae also have sex, sometimes a very simple kind of sex where the algae themselves act as gametes, but also very complicated sex with egg and sperm-like cells and even sex pheromones. In all probability, an alga was the first organism to have something that we would recognise as sex, about 1.3 billion years ago (i.e. 1,300 million years ago). So sex was not something invented yesterday.

Internal transportation? Some of the larger kelps have translocation (transport of photosynthetic products) but most do not. They have no need for water-conducting tissues as they are, at least at some stage, surrounded by water. There are no seeds. Spores may be motile or non-motile, and this varies from phylum to phylum, e.g., the red and blue-green algae are non-flagellated and are essentially non-motile throughout their life-cycle.

Algae of one kind or another have been around for over 2 billion years. We are still discovering new algae, sometimes whole groups of them at a time. Some 600–900 new species of alhgaare discovered each year (data from AlgaeBase. 

Algae of other groups usually have two flagella (singular: flagellum). Reproduction may be isogamous, anisogamous, or oogamous. Female gametangia are not enclosed by a wall of sterile cells as in higher cryptogams. Mostly autotrophic (photosynthetic), pigments are very variable and are the basis of major classes since the 1840s ; all have chlorophyll a; some have b, others c; all have accessory pigments of some kind e.g. phycocyanin (blueish), phycoerythrin (reddish), carotenes (yellow-brown), xanthophylls (brown, notably fucoxanthin found in brown algae). These accessory pigments are of dietary importance.

Some algae are heterotrophic (get some energy from non-photosynthetic sources ). There is great variation in size - unicellular and 3-10 µm (µm: a micron is one-thousanth of a millimeter) to giant kelps up to 70 meters long and growing at up to 50 cm per day. Found in mostly aquatic situations (need water to reproduce and, generally, to photosynthesise).

Where are algae found? Algae are found just about everywhere on earth: in the sea, in our rivers and lakes, in the air, on soils and walls, in animals and plants (as symbionts - partners collaborating together); on plants and animals; in fact just about everywhere where there is light with which to photosynthesise. The Three-toed Sloth (right)  has a whole biosphere of algae growing on it's back in what is knowns as a "mutualistic relationship" (both partners benefit). It acts a camouflage but the sloth may also consume some of the algae.  The sloth’s hair is uniquely adapted to absorb water and has specialized cracks for the algae to thrive (Picture right ©  Vecteezy).        

We currently know of over 50,000 living species of algae: up-to-date figures and the numbers for each phylum and class are provided dynamically by AlgaeBase.

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