Cyanobacteria – Earth’s Ancient Oxygenators

When we think of oxygen producers, we often picture great forests, lush rainforests, or vast underwater kelp beds. But the organisms we truly should thank for much of the air we breathe are invisible to the naked eye: microscopic cyanobacteria. Over 2.5 billion years ago, they were the first lifeforms to master oxygenic photosynthesis. By splitting water molecules and releasing oxygen, they initiated the Great Oxidation Event – one of the most dramatic shifts in Earth’s history. This transformation not only altered the atmosphere but also paved the way for all oxygen-dependent life, including ourselves.

Nature’s own fertilizers

Beyond producing oxygen, many cyanobacteria can “fix” nitrogen – turning nitrogen gas from the atmosphere into forms that plants and other organisms can use. This ability makes them nature’s own fertilizer factories and central players in both terrestrial and aquatic ecosystems.

The species Anabaena, for example, thrives in freshwater lakes and in the brackish waters of the Baltic Sea. Within specialized cells called heterocysts, Anabaena shields the delicate nitrogen-fixation process from oxygen. It can also form resting cells that endure winter buried in sediments, only to awaken with the return of light and warmth. In this way, Anabaena and its relatives sustain ecosystems season after season, enriching soils, crops, and aquatic food webs.

At the base of life – and of research

As primary producers, cyanobacteria form the foundation of the food chain. Tiny planktonic cyanobacteria feed microscopic grazers, which in turn nourish larger organisms all the way up to fish, birds, and mammals. Even the smallest solitary species, such as Chroococcus, contribute enormously to Earth’s primary production and oxygen generation despite their minute size.

But cyanobacteria are more than the base of ecosystems – they are also a focus of modern biotechnology. The genus Nostoc, for instance, is often described as a living ancestor of today’s plants, surviving only on light, air, nutrients, and water. In laboratories, Nostoc and other cyanobacteria are studied for their ability to produce bioactive compounds: antibacterial, antifungal, antioxidant, and even anticancer molecules. Research projects explore how these organisms could serve as sources of new medicines, functional foods, and sustainable biofuels.

Pigments, sunscreens, and innovation

Life under the sun comes with challenges. To survive intense radiation, cyanobacteria produce a variety of colorful pigments that both capture light for photosynthesis and protect against harmful UV rays. These same protective molecules are now inspiring human innovation. A Finnish company, for example, is developing natural sunscreens based on cyanobacterial pigments. Beyond skincare, scientists are testing how such pigments might be used in food, cosmetics, and even environmental technologies – from natural colorants to UV-protective coatings.

Blooms and toxicity

Under favorable conditions – particularly when nutrient levels in the water are high – cyanobacteria can multiply explosively in what are called blooms. These massive growths are visible as green, blue, or reddish surface scums on lakes and seas. While visually striking, blooms can cause serious ecological problems. Some species release toxins that are harmful to humans, pets, and wildlife. When blooms die and sink to the bottom, the decomposition process consumes oxygen, sometimes leaving deep waters depleted of life-supporting conditions.

In sensitive seas like the Baltic, this creates a vicious cycle: oxygen-depleted sediments release stored nutrients, which fuel yet more blooms. Even as external nutrient pollution has decreased in recent decades, internal nutrient loading means the effects of cyanobacterial blooms can persist for years or even decades.

Why cyanobacteria produce toxins at all remains an open scientific mystery. Hypotheses range from chemical defense against grazers, to protection against oxidative stress, to by-products of metabolism. The reality is likely that different toxins serve different purposes under different ecological conditions.

Research in motion

At the Swedish University of Agricultural Sciences (SLU), researcher and docent Malin Olofsson investigates how cyanobacteria regulate toxin production, and how nitrogen fixation varies both between species and between individuals within a species. Using advanced methods that can measure activity in single cells, her work sheds light on questions that once seemed impossible to answer: Which species contribute most to nitrogen fixation? Under what conditions do cyanobacteria produce and release toxins? How does environmental change – such as rising temperatures or altered light regimes – affect their behavior?

Her findings show that cyanobacteria are not static or predictable, but dynamic partners and challengers in our ecosystems. Sometimes they act as allies, enriching food webs and fertilizing soils. At other times, they pose threats to water quality, human health, and ecological balance.

From ancient oxygen to future solutions

Cyanobacteria remind us that some of Earth’s smallest lifeforms have had the greatest impact on our planet. They transformed the atmosphere, continue to fuel ecosystems, and today inspire innovations ranging from medicine to renewable energy. Understanding when they are friends and when they are foes is essential as we navigate both environmental challenges and biotechnological opportunities.

Text developed in collaboration with Malin Olofsson, researcher and docent, SLU Swedish University of Agricultural Sciences