Algae are a vast group of photosynthetic organisms that live primarily in water. They range from microscopic single cells measured in micrometers to giant kelp that can grow tens of meters long. Unlike true plants, algae lack roots, stems, leaves, and internal plumbing to transport water and nutrients. Scientists have documented over 50,000 living species across multiple kingdoms of life, making “algae” less a single family and more a loose collection of organisms that share one key trait: they use sunlight to make energy, but they aren’t plants.
What Makes Algae Different From Plants
The easiest way to understand algae is to compare them to the plants you already know. Land plants have vascular systems, networks of tiny tubes called xylem and phloem that move water from roots up to leaves and shuttle sugars back down. Algae have none of this. They absorb water and nutrients directly through their cell surfaces, which is why they must live in or very near water. A seaweed clinging to a rock might look rooted, but that anchor point (called a holdfast) is just a grip. It doesn’t absorb anything the way a tree root does.
Algae also reproduce differently. Plants produce seeds or spores inside specialized structures with layers of protective sterile cells. Algal reproductive cells are simpler and lack that layered architecture. And while plant photosynthesis relies on a fairly standard set of pigments, algae carry a wider variety, which is why they come in so many colors.
The Major Groups
Since the 1830s, scientists have sorted algae largely by color, a reflection of the different pigments packed inside their chloroplasts.
- Green algae are the closest relatives of land plants. They contain the same primary chlorophyll pigments and thrive in freshwater ponds, lakes, and moist soil. Sea lettuce, the translucent green sheets you might spot in tide pools, is a familiar example.
- Red algae get their color from pigments called phycobiliproteins, which let them absorb light at depths where green and brown algae struggle. Many red algae are the source of agar and carrageenan, two substances you encounter in food more often than you realize.
- Brown algae contain chlorophyll c along with golden-brown pigments called xanthophylls. This group includes the giant kelps that form underwater forests off coastlines, as well as common rockweeds found along shorelines worldwide.
These three groups are not closely related to each other in evolutionary terms. Brown algae are actually more closely related to certain single-celled predatory organisms than they are to green algae. The word “algae” is a convenience label, not a branch on the tree of life.
Size: From Invisible to Enormous
Microalgae are single-celled organisms measured in micrometers, far too small to see without a microscope. A drop of pond water can contain thousands of them. These include diatoms, which build intricate glass-like shells from silica, and dinoflagellates, the spinning cells responsible for bioluminescent ocean waves.
Macroalgae, commonly called seaweeds, are the visible kind. Giant kelp can grow tens of meters tall, forming dense canopies beneath the ocean surface that shelter fish, invertebrates, and marine mammals. Between these extremes you’ll find filamentous algae, the slimy green strands that coat rocks in streams, and colonial forms where thousands of cells cluster into visible blobs.
Why Algae Matter to the Planet
Roughly half of all the oxygen produced on Earth comes from the ocean, and the organisms responsible are overwhelmingly algae and photosynthetic bacteria drifting in sunlit surface waters. Every other breath you take, in a sense, was made possible by these microscopic producers. They also form the base of nearly every aquatic food chain, converting sunlight into the organic matter that feeds everything from tiny shrimp to whales.
Coral reefs depend on algae in an especially intimate way. Tiny algae called Symbiodinium live inside coral tissue, converting sunlight and carbon dioxide into sugars and oxygen that fuel the coral’s growth and skeleton-building. In return, the coral provides the algae with nutrients and a safe, sunlit home. When ocean temperatures rise too high, this partnership breaks down. The coral expels its algae, turns white (a process called bleaching), and can die if the algae don’t return. Even corals that recover from bleaching grow more slowly and bleach more easily the next time.
Algae in Food and Industry
You’ve almost certainly eaten algae-derived ingredients this week without knowing it. Three compounds extracted from seaweed are workhorses of the food industry:
- Carrageenan, from red algae, thickens and stabilizes dairy products, deli meats, and plant-based milks. It also serves as a vegan alternative to gelatin in gummy candies and capsules.
- Agar, also from red algae, is the go-to gelling agent for everything from Asian jellies and marshmallows to the petri dishes used in microbiology labs worldwide.
- Alginate, from brown algae, shows up in bakery fillings, ice cream (where it controls melting), reformed foods like onion rings, and low-fat spreads.
Beyond food additives, edible seaweeds like nori, wakame, and kombu are dietary staples in East Asia. Nutritionally, they’re notable for extremely high iodine content. A small portion of kombu can deliver several thousand percent of the recommended daily iodine intake, which is beneficial for people who are deficient but potentially harmful for those with thyroid conditions. Nori is gentler: a 5-gram dried sheet provides roughly 80 micrograms of iodine, a much more moderate amount. Some seaweeds, particularly nori and sea lettuce, also contain significant vitamin B12, making them one of the few non-animal sources of that nutrient.
Microalgae have carved out their own market. Chlorella and spirulina dominate the global microalgae supplement industry, sold as powders, tablets, and smoothie ingredients. Algae are also being developed as feedstocks for biofuels, fertilizers, cosmetics, and even biodegradable packaging materials.
Harmful Algal Blooms
Not all algae growth is welcome. When nutrient pollution from fertilizer runoff, sewage, or agricultural waste floods a body of water, algae can multiply explosively into what’s called a harmful algal bloom, or HAB. In freshwater lakes and reservoirs, these blooms are typically caused by cyanobacteria (sometimes called blue-green algae, though they’re technically bacteria). Cyanobacteria don’t infect people the way disease-causing bacteria do. Instead, they produce toxins, the most common being microcystin, which can damage the liver, and saxitoxin, which affects the nervous system.
In saltwater, harmful blooms are usually driven by dinoflagellates or diatoms. These are the organisms behind “red tides,” which can turn coastal water rusty brown and produce toxins like brevetoxin, domoic acid, and ciguatoxins. Shellfish filter large volumes of water and can concentrate these toxins in their tissues, which is why health agencies close shellfish harvesting areas during bloom events. Domoic acid, produced by certain diatoms, causes amnesic shellfish poisoning and has also been linked to mass strandings of sea lions and seabirds along the Pacific coast.
Blooms are becoming more frequent and widespread as nutrient pollution increases and water temperatures rise, creating conditions that favor rapid algae growth.