Algae are a significant and crucial component of our global ecosystem, however they are usually a focus of attention due to some of their undesirable impacts. These are discussed in the Freshwater Algae and Marine Algae pages.
This overview of algae will focus on their ecological significance with some interesting examples.
Algae is an informal name given to what is a large and diverse group of organisms which are not necessarily closely related.
Traditionally, the term algae has referred to all photosynthetic organisms that are not plants – that is, they lack the range of cell and tissue types found in plants e.g., the transport tissues (xylem and phloem) and stoma (pores through which gaseous exchange occurs).
The algae are a polyphyletic group which means that they are all grouped together (formally or informally) but do not share an immediate common ancestor. This is particularly the case when the term algae includes, as it usually does, the cyanobacteria (blue-green algae).
As a group, algae range from microscopic, unicellular forms which may be less than 5 µm in size to large, multicellular forms such as the giant kelps which can be more than 50 m in length. This is a size difference of 7 orders of magnitude.
Algae can be unicellular (existing as individual cells), colonial (several to many cells living in a colony), filamentous (several to many cells forming a chain or filament) or multicellular (usually macroscopic with several to many layers of cells).
Some examples of microscopic, unicellular algae
Some examples of microscopic colonial and filamentous algae
Some examples of macroscopic algae (seaweeds)
Algae are eukaryotic organisms. In very simple terms, this means that they have a “complex” cell structure. The cells of eukaryotic organisms have a nucleus enclosed within membranes as well as other membrane-bound organelles. Eukaryotes may also be multicellular and include organisms consisting of many cell types forming different kinds of tissue.
According to the “Six Kingdom” system of classification of life (there are other systems which have fewer or more Kingdoms), the algae belong to the Kingdom “Protista”. Just as the informal group “algae” is diverse, the Kingdom Protista is also a very diverse formal classification. All Protista are eukaryotic, and most are unicellular, but there are some relatively simple multicellular forms. They include both autotrophic and heterotrophic forms. This group is extremely diverse and includes organisms previously considered to be plants (algae), animals (amoebas, foraminiferans, heliozoans & radiolarians, flagellates and ciliates) and fungi (slime molds).
The Six Kingdoms of Life
Algae is a term that also generally encompasses the Cyanobacteria (blue-green algae). The Cyanobacteria belong to the Kingdom “Eubacteria” (also referred to as “Bacteria”). Not only is this a different Kingdom to the rest of the algae but it is also part of an entirely different Domain. The three Domains of life (the classification level above Kingdom) are “Eubacteria”, “Archaea” and “Eukaryota”. The Eukaryota is the Domain that contains all of the eukaryotic organisms (defined above) including the protista (which includes the other “algae”), fungi, plants and animals.
Organisms grouped within the Eubacteria and Archaea are all prokaryotic (hence they are sometimes grouped as a single Domain the “Prokaryota”). Prokaryotic organisms are all unicellular and lack a membrane-bound nucleus and other membrane-bound organelles. In very simplistic terms, they are simple, “primitive” cells.
The Bacteria and the Archaea evolved from an ancient common ancestor which was the first form of life to appear on Earth, about 4 billion years ago. Archaea and Bacteria are generally morphologically quite similar (similar size and shape), but the archaea possess genes and several metabolic pathways that are more closely related to those of the eukaryotes. The Archaea utilise a greater variety of energy sources than eukaryotes; these range from organic compounds, such as sugars, to ammonia, metal ions, hydrogen gas and sunlight (although not via photosynthesis).
Bacteria display a wide diversity of shapes and sizes and are typically 0.5–5.0 micrometres (µm) in length. Bacteria exhibit a wide variety of metabolic types, and these differences are used to split them into three informal groups (informal because these groupings do not correspond with genetics-based classification). These three groups are based primarily on the source of energy used by the cells and the source of carbon used for growth as follows:
|Nutritional type||Source of energy||Source of carbon||Examples|
|Phototrophs||Sunlight||Organic compounds (photoheterotrophs) or carbon fixation (photoautotrophs)||Cyanobacteria|
Green sulfur bacteria
|Lithotrophs||Inorganic compounds||Organic compounds (lithoheterotrophs) or carbon fixation (lithoautotrophs)||Thermodesulfobacteria|
|Organotrophs||Organic compounds||Organic compounds (chemoheterotrophs) or carbon fixation (chemoautotrophs)||Bacillus|
The Cyanobacteria are a Phylum (formal classification) within the Kingdom Bacteria. Cyanobacteria obtain their energy through photosynthesis and are the only photosynthetic prokaryotes able to produce oxygen. This is significant because the Cyanobacteria are thought to have converted the early oxygen-poor atmosphere of the Earth into a relatively oxygen-rich one (referred to as the Great Oxygenation Event) which dramatically changed the environment of the entire planet and led to the near-extinction of anaerobic organisms and a proliferation of aerobic organisms. In addition to this, an oxygenated atmosphere facilitated the formation of the ozone layer shielding the planet from much of the sun’s damaging ultra-violet radiation which allowed complex life forms to evolve and to move out of aquatic environments into terrestrial environments.
During this time, many cyanobacteria formed structures called stromatolites (and other similar structures). Stromatolites were present within the photic zone of most aquatic environments and are accretionary structures formed in the shallow water of this zone by the trapping, binding, and cementation of sedimentary grains by cyanobacteria. The earliest stromatolite fossils are thought to be around 3.7 billion years with their global abundance reaching a maximum around 1.25 million years ago after which they declined in both abundance and diversity.
Stromatolites are still found today but are largely restricted to hypersaline lakes and marine lagoons where extreme conditions due to high saline levels prevent animal grazing. It was the arrival of complex organisms which subsequently grazed on the stromatolites which is thought to be the main reason for their decline.
Stromatolites in Shark Bay, Western Australia (left) and Lake Thetis, Western Australia (right).
Cyanobacteria (sometimes referred to as blue-green algae) are still a very ecologically important group of organisms with respect to nitrogen-fixation, algal blooms, potential toxicity and other harmful effects. They are considered to be the most successful group of microorganisms on earth and play a vital role in the nitrogen, carbon and oxygen cycles of the planet.
Similarly to the algae, Cyanobacteria can be unicellular (existing as individual cells), colonial (several to many cells living in a colony) or filamentous (several to many cells forming a chain or filament). However, there are no multicellular forms.
Some examples of unicellular, colonial and filamentous Cyanobacteria
Algae, including the Cyanobacteria, are found in almost all aquatic and terrestrial habitats including oceans, rivers, lakes etc. but also in damp soil, on damp rocks and in ice. Algae are also found in environments considered to be “extreme” such as extreme cold (inside ice), extreme heat (thermal vents and hot springs), extreme salinity (salt lakes) and extreme acidity.
Algae in extreme environments. Antarctic ice (top left), hot springs in Yellowstone National Park, USA (top right), Salt lake, Port Gregory, Western Australia (bottom left), acidic hot spring in Rotorua, New Zealand.
Algae exist in the environment as free floating plankton (phytoplankton), associated with bottom sediments (benthic algae), attached to various substrates, e.g. wood, rocks (periphytic algae), attached to other algae or plants (epiphytic algae) or attached to other protists or animals (epizoic algae). Macroscopic algae (seaweeds) are usually attached to the bottom sediments or another substrate.
Significantly, algae can also have symbiotic relationships with other organisms. A symbiotic relationship is one where two organisms have a close, long term biological interaction (often with one living inside the other) where both organisms benefit from the relationship. Some symbioses are obligatory, which means that one or both of the symbionts entirely depend on each other for survival, whilst others are facultative (optional) where both organisms can generally live independently.
Common aquatic (predominantly marine) algal symbioses include those between algae and corals, jellyfish, sea anemones and bivalve molluscs such as clams. The bright colours of these organisms are largely attributable to the endosymbiotic algae within their tissues. Coral bleaching occurs when a change in environmental conditions (especially an increase in water temperature) causes the algae living in the coral to begin to produce substances that are harmful to the coral (instead of the beneficial substances it usually produces) and the coral, although dependant on the algae for its survival, ejects the algal cells out of its tissues. This causes the coral to lose its colour (become bleached) and die.
Algal symbioses in coral polyps (close up of polyps, top left), jellyfish (top right), anemones (bottom left) and a giant clam (bottom right).
Other examples of algal symbioses are lichens which are a symbiosis between fungus and algae (green algae or cyanobacteria), the symbiotic relationship between the cyanobacterium Nostoc with Cycads and several types of Bryophyte (non-vascular land plants) such as mosses, hornworts and liverworts and the symbiosis between the cyanobacterium Anabaena azollae and the water fern Azolla sp. This latter association is of considerable economic significance as the fast growing Azolla has been used for over 1,000 years as a companion plant in rice paddies. The nitrogen-fixing ability of the Azolla symbiont Anabaena azollae means that the growing rice has access to a constant supply of the crucial nutrient.
Some more peculiar algal symbioses include those between algae and vertebrates. The first of these, a symbiotic association between algae (Oophila amblystomatis, a green algae) and the eggs and embryos of a salamander (Ambystoma maculatum) is the only known endosymbiotic relationship between an algae and a vertebrate (a relationship where the algae lives stably inside the cells of the vertebrate). The second is an interesting symbiotic relationship between algae (Trichophilus welckeri, a green algae) and several species of three-toed sloth.
The fur of these sloths often has a greenish colouration, a result of the algae living on the long, coarse hairs. For some time it was thought that this was a very simple relationship by which the algae gain a supply of water and nutrients from the fur and skin of the sloth as well as (possibly) greater exposure to sunlight as the sloth lives in the tree canopy and the sloth gains camouflage. However, this is a much more complicated relationship involving a third party, several species of moth collectively called sloth moths. This intertwined relationship has also explained why these sloths expend so much energy climbing down to the forest floor once a week to defecate rather than simply do this from up in the canopy.
The adult moths live on and within the fur of the sloth where they get nutrients from the secretions of the sloths’ skin and the algae present on the fur, as well as protection from predators (particularly birds). When the sloth makes its weekly climb down to the forest floor, female moths temporarily leave their home in the sloths fur to lay their eggs in the fresh sloth dung. When the new moths emerge from the dung, they fly up into the canopy in search of a sloth host. The adult moths live their entire life in the fur of the sloth when they die the moths decompose in the fur releasing nutrients vital to the growth of the algae. In addition to obtaining camouflage from the algae in its fur, it is thought that the sloth also supplements its vegetarian diet by feeding off the algae.
Both the algae and the moths are found exclusively on three-toed sloths.