Algae: Ecosystem Roles, Biodiversity, and Environmental Impact

Algae underpin life on Earth in ways most people never see. From single-celled phytoplankton drifting in the open ocean to towering kelp forests anchored along rocky coasts, these organisms drive roughly half of the planet’s photosynthesis, feed marine food webs from their base, and cycle enormous quantities of carbon between the atmosphere and the deep sea. They also, under the wrong conditions, produce toxic blooms that poison wildlife and foul drinking water. The story of algae is ultimately a story about planetary plumbing: how nutrients, energy, and carbon move through ecosystems, and what happens when those flows are disrupted.

The Organisms That Made the Atmosphere Breathable

Algae have been shaping Earth’s chemistry for billions of years. Cyanobacteria, the photosynthetic microbes sometimes called blue-green algae, were responsible for the Great Oxidation Event roughly 2.45 to 2.32 billion years ago, when atmospheric oxygen levels rose dramatically and rewrote the rules for life on the planet.¹ Before that event, Earth’s atmosphere was largely devoid of free oxygen. Cyanobacteria cracked water molecules during photosynthesis and released oxygen as a byproduct, and over geological time, that oxygen accumulated enough to transform the atmosphere and enable the evolution of aerobic life.²

The evolutionary legacy did not stop there. The chloroplasts inside all modern algae and land plants trace back to a single ancient event in which a eukaryotic cell engulfed a cyanobacterium and kept it alive as an internal photosynthesis factory. This primary endosymbiosis gave rise to the green algae, red algae, and glaucophytes.³ Later rounds of secondary endosymbiosis, in which other eukaryotes engulfed green or red algae, spread photosynthesis into entirely new lineages, including the diatoms and dinoflagellates that dominate ocean phytoplankton today.⁴ In other words, the photosynthetic capacity scattered across the tree of life all traces back to a single bacterial partnership.

What Counts as an Alga, and How Many Kinds Exist

“Algae” is not a single evolutionary group. It is a grab-bag label covering an enormous diversity of photosynthetic organisms that range from single-celled diatoms smaller than a red blood cell to giant kelps tens of meters tall. The major phyla include green algae, red algae, brown algae, diatoms, dinoflagellates, euglenoids, and cyanobacteria, among others. Freshwater surveys illustrate just how mixed an algal community can be: in Hawaiian streams, for example, green algae made up about half of all identifications, with diatoms, red algae, cyanobacteria, xanthophytes, and euglenoids filling out the rest.⁵

A key distinction within this diversity is the split between microalgae and macroalgae. Microalgae are single-celled or form simple colonies; macroalgae are the seaweeds, with complex multicellular bodies. Genomic comparisons across all three major macroalgal phyla (green, red, and brown) have identified shared gene sets for cell adhesion, extracellular matrix formation, and cell differentiation that distinguish multicellular forms from their single-celled relatives.⁶ Multicellularity evolved independently in each of these lineages, yet they converged on similar genetic toolkits to build complex bodies.

Powering the Planet Through Primary Production

Marine phytoplankton are the workhorses of global photosynthesis. They fix carbon dioxide into organic matter using sunlight, generate oxygen, and form the energy base for virtually all marine food chains. Among phytoplankton groups, diatoms contribute the largest share of marine primary production at around 38%, followed by cyanobacteria at roughly 23% and chlorophytes (green algae) at about 22%.⁷ Together, these microscopic organisms rival all the forests, grasslands, and croplands on Earth in terms of total carbon fixation.

What makes phytoplankton so effective is speed. A diatom can double its population in a day or two under favorable conditions, while a tree takes decades to reach maturity. That rapid turnover means phytoplankton contribute a disproportionate share of the planet’s photosynthetic output relative to their standing biomass at any given moment. It also means they respond quickly to changes in nutrient supply, temperature, and light, which makes them both sensitive indicators of environmental change and potent drivers of it.

How Algae Move Carbon Into the Deep Ocean

Photosynthesis at the ocean surface is only the first step. What matters for the climate is how much of that fixed carbon gets exported to depth, where it stays out of the atmosphere for decades to centuries. This transport system is collectively called the biological carbon pump, and it works through several pathways. Sinking particles, mostly dead phytoplankton cells and zooplankton fecal pellets, account for about 70% of total global carbon export from surface waters.⁸ Of that sinking material, zooplankton fecal pellets make up about 85%, with phytoplankton aggregates contributing the remaining 15%.

Two other pathways round out the picture. Zooplankton that migrate vertically each day, feeding at the surface at night and retreating to depth during the day, actively shuttle carbon downward, contributing about 10% of total export. Physical mixing and subduction of particle-laden water accounts for the remaining 20%. These pathways differ in how long the carbon stays sequestered: sinking particles and migrant zooplankton lock carbon away for an average of 140 to 150 years, while the mixing pump keeps it sequestered for only about 50 years.⁸ In coastal upwelling zones like the California Current, simultaneous measurements of all three pathways found that sinking particles exported the most carbon across the 100-meter depth line, at about 9 millimoles of carbon per square meter per day.⁹

Feeding the Food Web With Essential Fats

Beyond carbon and oxygen, algae provide something no other organisms can make in large quantities: omega-3 long-chain fatty acids, specifically EPA and DHA. Microalgae are the main source of these essential fats, which are critical for the healthy development of most marine and terrestrial animals, including humans.¹⁰ Fish do not actually manufacture omega-3s themselves; they accumulate them by eating algae or eating organisms that ate algae. The entire supply chain traces back to phytoplankton membranes.

This matters because warming water threatens the supply. Phytoplankton adjust the fatty acid composition of their cell membranes in response to temperature: colder water favors more unsaturated fats like EPA and DHA, while warmer water shifts the balance toward less beneficial saturated fats. Modeling work suggests that climate warming could significantly reduce the global production of these essential fatty acids by phytoplankton, with cascading effects through aquatic and terrestrial food webs.¹¹ In the Arctic, where ice algae and pelagic algae form the nutritional base for everything from copepods to seals, depletions have already been documented.

Kelp Forests as Coastal Ecosystem Engineers

If phytoplankton are the open ocean’s invisible foundation, kelp forests are the visible equivalent along temperate and polar coastlines. These underwater forests of large brown algae line about a quarter of the world’s coastlines and function as foundation species, creating physically complex habitats that support exceptionally productive ecosystems.¹² Kelp forests provide elevated secondary production, nutrient cycling, energy capture, coastal defense against wave action, and serve as biodiversity hotspots.¹³

A single kelp forest can harbor hundreds of species of invertebrates, fish, and marine mammals. Sea urchins graze the kelp, sea otters eat the urchins, and that trophic cascade keeps the whole system in balance. When kelp declines, whether from warming waters, urchin overgrazing, or pollution, the loss cascades outward: fish nursery habitat disappears, coastal erosion accelerates, and the carbon sequestration service drops.¹⁴

Algae in Extreme Environments

Algae do not confine themselves to temperate seas and sunny lakes. In polar regions, certain species thrive in biological soil crusts: thin living layers on the surface of desert-like tundra soils. Genera like Chloromonas, often associated with polar biological soil crusts, include snow algae species adapted to freezing conditions. Some of these organisms synthesize photoprotective pigments that shield the rest of the crust community from ultraviolet radiation, acting as a living sunscreen layer. Meanwhile, filamentous algae and cyanobacteria produce mucilage that glues soil particles together, physically forming the crust matrix and stabilizing soils against erosion.¹⁵ These crusts are often the dominant source of biological activity in polar and arid landscapes, making algae essential players even in the harshest terrestrial environments.

When Algae Turn Dangerous

The same nutrient-hungry growth that makes algae so productive can become destructive when nutrients pour in faster than ecosystems can handle. In freshwater systems, excess phosphorus from agricultural runoff and wastewater promotes cyanobacterial blooms, a problem worsening worldwide.¹⁶ Species like Microcystis aeruginosa can produce microcystins, toxins that damage the liver and are a growing concern for drinking water supplies. Research in Lake Erie showed that bloom growth and microcystin concentrations responded more frequently to additions of dissolved nitrogen than phosphorus alone, and that combined nitrogen-plus-phosphorus additions yielded the highest toxin concentrations in more than half of experiments.¹⁷ The nutrient story behind toxic blooms, in other words, is more complicated than just “too much phosphorus.”

In the ocean, the most notorious example is Florida’s red tide, caused by the dinoflagellate Karenia brevis. These blooms produce brevetoxins, potent neurotoxins that cause respiratory illness in humans and mass die-offs of fish, sea turtles, and marine mammals.¹⁸ Unlike freshwater cyanobacterial blooms, Karenia brevis blooms are a naturally occurring phenomenon in the Gulf of Mexico, though nutrient pollution from coastal development likely intensifies them.¹⁹ Brevetoxins can even persist in coastal sediments after blooms subside, meaning the environmental damage outlasts the visible event.

Climate Change and Shifting Algal Communities

Rising ocean temperatures and acidification are rearranging algal communities in ways that could reshape marine ecosystems. Experimental work in the coastal waters of the Southern East China Sea found that under combined high-CO₂ and high-temperature conditions, the proportion of diatoms in the community fell while dinoflagellates increased.²⁰ Under high-CO₂ conditions alone, diatom proportions dropped by about 23% relative to the control, with dinoflagellates filling the gap.

These shifts matter because diatoms and dinoflagellates play very different roles. Diatoms are heavy, glassy-shelled cells that sink quickly and are the most efficient phytoplankton group at exporting carbon to the deep ocean. Dinoflagellates tend to stay in the upper water column longer and are more often associated with harmful blooms. A community that swings from diatom-dominated to dinoflagellate-dominated could weaken the biological carbon pump and increase bloom risk simultaneously. Combined with the projected declines in omega-3 fatty acid production described earlier, these compositional shifts represent a threat to both climate regulation and food web nutrition.

Farming Seaweed to Clean Coastal Water

One of the more practical applications of algal biology is using seaweed cultivation to pull excess nutrients out of polluted coastal waters. The idea is straightforward: if nutrient runoff drives harmful blooms, why not grow seaweed that absorbs those same nutrients, then harvest it and remove the nutrients from the system entirely? Field trials have confirmed this works. Cultivation of the red seaweed Porphyra yezoensis in eutrophic Chinese coastal waters reduced ammonium concentrations by 50 to 94%, nitrate by 21 to 38%, and phosphate by 42 to 67% compared to control areas without seaweed farms.²¹ Annually, 300 hectares of Porphyra cultivation removed roughly 14,700 kilograms of nitrogen and 2,370 kilograms of phosphorus through harvested biomass alone.

In temperate North American waters, Gracilaria aquaculture has shown promise as a nutrient bioextraction tool in urbanized estuaries around New York City and Long Island Sound.²² Kelp farming is being tested in the Gulf of Maine as well, where harvesting sugar kelp (Saccharina latissima) after six to seven months could remove an estimated 19 to 176 kilograms of nitrogen per hectare depending on farm configuration.²³ That wide range highlights a real limitation: site-specific factors like current speed, water temperature, and ambient nutrient levels vary so much that pilot studies are needed even within a single bay.

Algae as an Industrial Feedstock

The same fast-growing, lipid-rich properties that make algae ecologically powerful also make them attractive for industrial use. Microalgae can simultaneously capture CO₂ and accumulate oils suitable for conversion to biodiesel, and researchers have explored a range of bioenergy products from algal biomass, including biodiesel, bio-oil, bioethanol, biogas, and biohydrogen.²⁴ In one set of experiments using a specialized photobioreactor and piggery wastewater as a nutrient source, microalgae achieved CO₂ capture rates of about 0.41 grams per liter per day while producing lipids suitable for biofuel at roughly 53 milligrams per liter per day.²⁵ The appeal is circular: algae eat waste CO₂ and waste nutrients, and the resulting biomass can replace fossil-fuel-derived products.

Macroalgae are also being explored as a feedstock for bioplastics. Seaweeds grow fast, are rich in cellulose and sugars, and do not compete with food crops for arable land or freshwater.²⁶ Some microalgae species also accumulate polyhydroxyalkanoates, a class of naturally produced polymers that can be extracted and processed into biodegradable plastics.²⁷ Neither algal biofuels nor algal bioplastics have reached price parity with petroleum-based alternatives at scale, but the underlying biology is sound, and the economics shift every time fossil fuel costs or carbon prices change.

Reducing Livestock Methane With Red Seaweed

One of the more surprising algal applications involves feeding a tiny amount of red seaweed to cattle to slash their methane emissions. The tropical red macroalga Asparagopsis taxiformis contains bromoform and other halogenated compounds that inhibit the enzymes methane-producing archaea use in the rumen. In a feeding trial with beef steers, supplementing the diet with A. taxiformis at 0.5% of organic matter intake reduced enteric methane yield by up to 80% when animals were on a low-forage finishing diet.²⁸ The reduction persisted over the 147-day trial, and the seaweed did not hurt the animals’ weight gain or meat quality.

Mechanistic work in dairy cattle has since shown that the methane reduction is linked to suppression of methanol-utilizing Methanosphaera in the rumen, suggesting these microbes play a larger role in methane formation than previously appreciated.²⁹ The challenge is scaling production of Asparagopsis to supply the world’s roughly one billion cattle. Wild harvest is not sustainable, and aquaculture of this seaweed is still in early development. But if production costs come down, this could be one of the fastest routes to cutting agricultural greenhouse gas emissions.

Chemical Conversations Between Algae and Bacteria

Algae do not live in isolation. Every microalgal cell is surrounded by a thin zone of influence called the phycosphere, analogous to the rhizosphere around plant roots. Bacteria colonize this zone and engage in chemical exchanges that can either promote or inhibit algal growth. Research on quorum-sensing bacteria in the phycosphere of harmful algal bloom species found that metabolic extracts from these bacteria could either stimulate or suppress microalgae growth depending on the species pairing.³⁰

One particularly striking example involves the bacterium Sulfitobacter pseudonitzschiae H46 and its relationships with different microalgae. This bacterium promotes the growth of diatoms and other algae that produce the sulfur compound DMSP (dimethylsulfoniopropionate), which the bacterium can use as a nutrient. But it kills Chattonella marina, an algal species that does not produce DMSP, by secreting heat-resistant algicidal compounds.³¹ The bacterium essentially favors its trading partners and eliminates non-contributors. These chemical negotiations between algae and bacteria are happening everywhere in the ocean, constantly shaping which species bloom and which fade, and researchers are just beginning to map the complexity of these interactions.

Traces in the Geological Record

Because algae respond so sensitively to environmental conditions, their chemical traces in sediments serve as powerful tools for reconstructing past climates. Alkenones, long-chain organic molecules produced by certain haptophyte algae, change their degree of unsaturation depending on the water temperature in which the algae grew. By measuring alkenone ratios in deep-sea sediment cores, paleoceanographers can reconstruct sea surface temperatures millions of years in the past. Cores from the central Arctic Ocean, for example, revealed a long-term temperature decrease of about 15°C over the interval from 49 to 44.5 million years ago, dropping from roughly 25°C to 10°C as the planet cooled after the early Eocene warm period.³² The idea that microscopic algae living and dying in ancient seas left behind a thermometer readable tens of millions of years later speaks to how tightly woven algae are into the planet’s physical and chemical systems. Their influence extends not just across ecosystems but across deep time.