What is algae?
The term algae is used for everything from microscopic bacteria and plankton to pond slime and seaweeds, but unlike animals and many other living things, algal groups are an assortment without a single common ancestor. That said, one feature is shared by most algae in every group: photosynthesis – the ability to make food from carbon dioxide and sunlight, releasing oxygen, just like plants.
Are algae plants?
It’s the other way around! Plants are actually a form of algae. Chloroplasts – the structures where photosynthesis begins inside plant cells – evolved from symbiotic cyanobacteria that were incorporated into what became green algae (chlorophytes). As descendants of this group, you could say that plants are actually green algae.
Other algal groups originated via similar ‘endosymbiosis’ events (when one organism lives within another) independently, which is how the tree of life ended up with separate branches collectively known as algae. Those groups are distinguished from plants based on distinct features.
What's the difference between plants and algae?
One difference between algae and plants is anatomy. An algae’s body is generally much simpler than a plant's. Green algae have up to five types of cell, red and brown have up to 14. Plants have up to 44 cell types, organised into complex tissues and organs, such as leaves or roots, and they often transport liquid nutrients via a network of phloem and xylem vessels.
Another difference between plants and algae is reproduction. Although some algae reproduce through sex, combining sperm and egg, a plant’s life cycle is more sophisticated and usually includes a period when a parent nurtures an embryo (plants are embryophytes).
What kinds of algae are there?
Algae are grouped by ancestry. Some groups have a popular name that’s based on the coloured pigments they use to capture energy from light during photosynthesis. Red algae (rhodophytes) use the pigment phycoerythrin, not just chlorophyll, for instance. But each species’ colour reflects the combination and concentration of pigments it carries, which is why a ‘red’ alga might look green or even black.
Algae are also categorised by size. Microalgae are single-celled phytoplankton, up to 1mm (if cyanobacteria are ‘blue-green algae’, as some biologists say, the smallest are under one-thousandth of a millimetre wide). Macroalgae or seaweeds have multicellular bodies whose cells may be fused together to form a large, flat, plant-like body or ‘thallus’. The record-holder is the giant kelp (Macrocystis pyrifera), a brown alga that can grow up to 60m long.
What’s an algal bloom and what causes it?
An Algae bloom is a sudden accumulation of algae. Blooms change water colour and are normally short-lived, lasting days or weeks.
They can be caused by factors such as a rise in light or nutrients or a drop in water turbulence, and factors such as grazing by microscopic animals (zooplankton) will cause them to disappear. To protect themselves, 3-5 per cent of species release toxins, which explains why algae in ponds and lakes can be poisonous to fish, livestock and pets.
A bloom can cover tens of thousands of square kilometres – an area so vast that it’s visible from space, allowing satellites to track periodic cycles of bloom-and-bust.
Where are algae found?
While plants are largely terrestrial, the majority of algae are aquatic. Some species grow in soil (even desert crusts!) but most live in freshwater or marine environments at depths up to 300m. Although many algae are free-living, seaweeds can remain attached to a substrate, such as the seafloor.
Some microalgae exist in mutually beneficial, symbiotic relationships with other life-forms: algae and fungi combine as lichen, for example, while Symbiodinium species (known as zooxanthellae) reside in animals, most famously reef-building corals.
Why are algae important?
Brown algae form kelp forests that line a quarter of coasts to create habitats and sustain ecosystems. Their tiny relatives, diatoms, play an even bigger role. Despite contributing only 0.2 per cent to the biomass of Earth’s photosynthetic species, diatoms and other phytoplankton produce almost half of the organic carbon that’s vital to life.
