Algae are made of a surprisingly wide range of chemical compounds, and the exact recipe varies enormously depending on the species. At the broadest level, algal cells contain proteins, carbohydrates, lipids, pigments, minerals, and water, much like plant cells. But the proportions can swing wildly: some microalgae pack more than half their dry weight in protein, while others are dominated by fats or complex sugars. The diversity gets even stranger at the structural level, where different algal groups build their cell walls from entirely different materials, from glass-like silica to thick layers of calcium carbonate to soft polysaccharide gels.
Cell Walls Built from Very Different Blueprints
If you lined up a diatom, a brown seaweed, a red seaweed, and a dinoflagellate under a microscope, you would be looking at organisms that all carry the label “algae” but construct their outer walls from radically different substances. The cell wall is the first thing that sets one algal group apart from another at the chemical level, and it is where some of the most unusual biochemistry in the living world shows up.
Diatoms, the single-celled algae responsible for roughly a fifth of global photosynthesis, encase themselves in rigid walls called frustules made of amorphous silica, essentially a biological version of glass.1PubMed. Beyond micromachining: the potential of diatoms These frustules are intricately patterned and can even incorporate trace metals like aluminum into their structure.2Scientific Reports. Modifying the thickness, pore size, and composition of diatom frustule in Craspedostauros sp. with Al3+ ions Dinoflagellates, another major group, take a completely different approach. They surround themselves with multiple cortical membrane layers called an amphiesma, and many thecate species deposit elaborate cellulosic plates within large vesicles beneath the cell surface.3PubMed Central. Dinoflagellate Amphiesmal Dynamics: Cell Wall Deposition with Ecdysis and Cellular Growth These thecal plates have high cellulose content and are shaped into species-specific forms.4Journal of Nanoscience and Nanotechnology. Mechanical Characterization of Cellulosic Thecal Plates in Dinoflagellates by Nanoindentation
Macroalgae, the large seaweeds you see washed up on beaches, rely heavily on polysaccharides for their cell walls, but the specific polysaccharides differ by color group. Red algae use carrageenans and agars, brown algae use alginates and fucoidans, and green algae produce ulvans, among other sugar-based polymers.5ScienceDirect (Future Foods). Seaweed polysaccharides: Sources, structure and biomedical applications with special emphasis on antiviral potentials One practical consequence of this diversity: macroalgae contain almost no lignin, the tough structural polymer that makes wood rigid. That makes extracting cellulose from seaweed a milder process compared to extracting it from trees or crop residues.6PubMed. From Terrestrial Plants to Marine Macroalgae: A Comprehensive Review of Cell Wall Component-Properties, Extraction, Modification, and Application of Algal Cellulose
Some algae go even further. Coccolithophores, a group of marine microalgae, secrete overlapping plates of calcium carbonate called coccoliths around their cells. The calcification process is so central to their biology that interfering with calcium availability or certain metabolic pathways causes the coccoliths to become grossly malformed.7PubMed Central. The requirement for calcification differs between ecologically important coccolithophore species When these organisms die and their calcium carbonate shells sink, they form a significant part of ocean sediment. The White Cliffs of Dover are largely made from ancient coccolithophore remains.
Proteins and Amino Acids
Protein is often the most abundant organic compound in microalgae by dry weight. Species like Spirulina (technically a cyanobacterium, but commercially grouped with algae) and Chlorella can contain protein levels ranging from about 50% to 70% of their dry weight, with a well-balanced amino acid profile rich in essential amino acids like lysine and leucine.8PubMed Central. Proteins from Microalgae: Nutritional, Functional and Bioactive Properties That is a higher protein concentration than most conventional crops, which is why these species attract so much attention as alternative protein sources.
The amino acid profiles are not identical across species, though. In a study of six freshwater algae, the presence and quantity of individual essential amino acids varied dramatically. Histidine showed up in large amounts in Spirulina but was undetectable in most of the other species tested. Threonine was abundant in one species but absent in Spirulina. Isoleucine was only found in one of the six species examined.9Journal of Agriculture and Food Research. Amino acids profile of six freshwater Algae in the northern region of Bangladesh: A solution for the protein supplement of poultry feed So saying “algae are high in protein” is true in a general sense, but which amino acids you actually get depends heavily on the species.
Macroalgae tend to have lower protein concentrations than microalgae, but they still carry a complete set of essential amino acids. Among the seaweeds, red species like Porphyra (the nori used in sushi) tend to have the highest protein content. Methionine and tryptophan are usually the limiting amino acids across seaweed species, meaning they are present in the lowest amounts relative to human needs.10PubMed Central. Amino Acid Profile and Protein Quality Assessment of Macroalgae Produced in an Integrated Multi-Trophic Aquaculture System Red seaweeds also tend to be rich in free glutamic and aspartic acids, which contribute to the savory umami flavor that makes nori taste so distinctive.
Lipids and Fatty Acids
Under normal growing conditions, lipids typically make up a modest fraction of algal biomass, often somewhere around 5% to 20% of dry weight depending on the species. But that fraction can shift substantially in response to environmental stress, a point covered in more detail below. What makes algal lipids interesting is not just the quantity but the type. Many marine microalgae are rich in the omega-3 fatty acids EPA and DHA, the same long-chain fatty acids that make fish oil valuable for human nutrition.11PubMed Central. Microalgal biofactories: a promising approach towards sustainable omega-3 fatty acid production
In fact, fish do not synthesize EPA and DHA on their own. The omega-3s accumulate up the marine food chain from the microalgae that actually produce them. This is why there is growing commercial interest in cutting out the middlefish entirely and harvesting omega-3s directly from algae. Several heterotrophic microalgae (species that grow on organic carbon rather than sunlight) are already used commercially for DHA production, and autotrophic species that photosynthesize are being developed as well.
Beyond omega-3s, algal lipids include a range of other compounds: membrane phospholipids, glycolipids, and storage triglycerides. The triglycerides are the fraction that biofuel researchers focus on, since they can be converted into biodiesel through a process similar to how vegetable oils are processed.
Carbohydrates and Storage Compounds
Algae store energy and build structural components using various carbohydrates, but the specific types differ from the starches and celluloses familiar in land plants. Red algae, for instance, store their energy as floridean starch and a small sugar called floridoside, both of which are chemically distinct from the starch granules in potatoes or rice.12PubMed Central. Floridean Starch and Floridoside Metabolic Pathways of Neoporphyra haitanensis and Their Regulatory Mechanism under Continuous Darkness Brown algae store laminarin and mannitol. Green algae store true starch, reflecting their closer evolutionary kinship with land plants.
Red seaweed stands out for its overall polysaccharide content. Among the three main color groups of seaweed, red algae have the highest concentration of polysaccharides, ranging from roughly 38% to 74% of dry weight.5ScienceDirect (Future Foods). Seaweed polysaccharides: Sources, structure and biomedical applications with special emphasis on antiviral potentials These polysaccharides are not just passive structural filler. Many have biomedical interest for their antiviral and immunomodulatory properties, and the composition of these sugars can vary depending on the species, the season of harvest, and even the extraction method used.
Some microalgae also accumulate starch granules alongside lipid droplets when stressed. Under nitrogen starvation, for example, Chlorella cells visibly swell with both starch and oil reserves, essentially converting their protein machinery into energy storage when deprived of the nitrogen needed to build new proteins.
The Pigment Palette
All algae share chlorophyll a as their primary photosynthetic pigment, but the accessory pigments they carry on top of that are one of the main reasons different algal groups look so different in color. Green algae contain chlorophyll b alongside chlorophyll a, which is the same pigment combination found in land plants and gives both groups their green color. Brown algae and diatoms carry chlorophyll c and a carotenoid called fucoxanthin, which masks the green of chlorophyll and gives kelp and other brown seaweeds their characteristic olive-brown hue.13PubMed Central. Fucoxanthin, a marine carotenoid present in brown seaweeds and diatoms: metabolism and bioactivities relevant to human health
Red algae owe their color to phycobiliproteins, fluorescent protein-pigment complexes that come in shades of fuchsia, purple-blue, and cyan. These pigments capture light energy in the 450 to 650 nanometer range and funnel it to the photosynthetic reaction centers, allowing red algae to photosynthesize efficiently in deeper water where only blue and green light penetrates.14PubMed Central. Phycobiliproteins: Structural aspects, functional characteristics, and biotechnological perspectives Phycobiliproteins are used commercially as natural food colorants and fluorescent markers in biomedical research.
Fucoxanthin has drawn particular research interest because of its biological properties beyond photosynthesis. It has shown antibacterial activity in laboratory studies and is being investigated for potential roles in fat metabolism and antioxidant defense.15PubMed Central. Fucoxanthin-An Antibacterial Carotenoid Whether those laboratory observations translate into meaningful health benefits for humans is still an open question, but the compound illustrates how algal pigments are more than just light-harvesting machinery.
Minerals and Ash Content
Seaweeds are mineral sponges. They absorb elements from seawater and concentrate them to levels that can be surprisingly high. In wild-collected edible seaweeds from New Zealand, the ash content (a measure of total mineral matter) ranged from about 20% to 27% of dry weight.16New Zealand Journal of Crop and Horticultural Science. Nutrient and heavy metal content of edible seaweeds in New Zealand That is far higher than almost any land vegetable.
The specific minerals present include potassium, calcium, magnesium, copper, zinc, cobalt, and molybdenum, among others. A comparative study of 15 seaweed species from Sri Lanka found considerable concentrations of potassium and calcium across all groups, with the green alga Ulva lactuca recording the highest concentrations of copper, zinc, cobalt, molybdenum, and magnesium.17Scientific Reports. Comparative analysis of proximate compositions, mineral and functional chemical groups of 15 different seaweed species Iodine is another mineral seaweeds are well known for concentrating, which is why kelp supplements are marketed for thyroid support but also carry a risk of excessive iodine intake if consumed carelessly.
The mineral story has a less appealing side too. Seaweeds can accumulate heavy metals like arsenic and cadmium from their environment. In the New Zealand study, total arsenic was high in some species (up to 97 mg/kg), though the dominant forms were organic arsenic compounds considered non-toxic, and inorganic arsenic levels were well below regulatory limits.16New Zealand Journal of Crop and Horticultural Science. Nutrient and heavy metal content of edible seaweeds in New Zealand Cadmium levels in the Sri Lankan species did not exceed safe thresholds for food supplements made from dried seaweed.17Scientific Reports. Comparative analysis of proximate compositions, mineral and functional chemical groups of 15 different seaweed species Still, the potential for heavy metal accumulation is something regulators and consumers keep an eye on, especially for seaweeds harvested from polluted waters.
Bioactive Secondary Metabolites
Beyond the major building blocks of protein, fat, carbohydrate, and pigment, algae produce a variety of secondary metabolites, compounds that are not strictly necessary for basic growth but serve ecological or defensive roles. Some of these are beneficial from a human perspective, and some are decidedly not.
On the beneficial side, brown algae produce phlorotannins, a class of polyphenols built from repeating phloroglucinol units.18PubMed Central. A Bioactive Substance Derived from Brown Seaweeds: Phlorotannins These are chemically distinct from the tannins and polyphenols in tea, wine, or berries. Phlorotannins exist as complex mixtures of different structural types, and their composition varies with species, tissue, season, and habitat.19PubMed Central. Brown Seaweed Phlorotannins: Chemical Diversity, Sustainable Extraction, Selective Quantification, Ageing-Related Bioactivities, and Phlorotannin-First Biorefinery Potential In the brown alga Fucus vesiculosus, the polyphenol fraction consists mainly of high molecular weight phlorotannin polymers with demonstrated antioxidant capacity in laboratory tests.20PubMed. Antioxidant capacities of phlorotannins extracted from the brown algae Fucus vesiculosus
On the harmful side, certain diatom species in the genus Pseudo-nitzschia produce domoic acid, a potent neurotoxin that targets nervous system receptors and can cause amnesic shellfish poisoning in humans who consume contaminated shellfish.21PubMed Central. Domoic Acid: A Review of Its Cytogenotoxicity Within the One Health Approach Dinoflagellates produce their own suite of toxins responsible for paralytic and other forms of shellfish poisoning. These toxic metabolites are a reminder that the chemical complexity of algae can be dangerous as well as useful.
How the Environment Reshapes Algal Chemistry
One of the most striking things about algal composition is how dramatically it shifts in response to growing conditions. Algae are not locked into a fixed chemical recipe the way a mineral crystal is. Change the light, the temperature, or especially the nutrient supply, and the internal proportions of protein, fat, and carbohydrate can swing in ways that would be startling in most organisms.
Nitrogen availability is the most powerful lever. When nitrogen runs low, microalgae can no longer build new proteins (since amino acids require nitrogen), so they redirect their metabolism toward carbon-rich storage compounds: lipids and starch. In Chlorella vulgaris, nitrogen limitation and starvation increase lipid content while decreasing biomass productivity, pigment concentration, and protein content. Under a microscope, nitrogen-starved Chlorella cells are visibly different, filled with larger and more numerous lipid droplets and starch granules.22PubMed Central. Biochemical and Morphological Changes Triggered by Nitrogen Stress in the Oleaginous Microalga Chlorella vulgaris
The magnitude of these shifts can be enormous. In one study comparing multiple microalgae species under nitrogen starvation, carbohydrate content increased by 59% in Dunaliella tertiolecta, while lipid levels jumped by 139% in the diatom Phaeodactylum tricornutum, both compared to nutrient-replete controls.23PubMed. Effects of nitrogen starvation on growth and biochemical composition of some microalgae species This metabolic flexibility is what makes algae attractive for biofuel and nutraceutical production: by manipulating growth conditions, you can push the cells to accumulate more of whatever compound you want. The trade-off is that stressing the algae usually slows their growth rate, so there is always a tension between accumulating the target compound and growing enough biomass to make harvesting worthwhile.
Where All That Diversity Came From
The reason algae show such remarkable chemical diversity is partly evolutionary. The chloroplasts inside algal cells, the compartments that carry out photosynthesis, trace back to a single ancient event in which a non-photosynthetic cell engulfed a cyanobacterium. This primary endosymbiosis gave rise to the common ancestor of three major lineages: green algae and land plants, red algae, and glaucophyte algae.24PubMed Central. Primary endosymbiosis and the evolution of light and oxygen sensing in photosynthetic eukaryotes But the story did not end there. Descendants of those primary lineages were themselves engulfed by other organisms in secondary endosymbiosis events, passing their photosynthetic machinery along to diatoms, dinoflagellates, and other groups.
Each of these evolutionary handoffs introduced new combinations of host cell and chloroplast, and each lineage then spent hundreds of millions of years adapting to different ecological niches. The result is organisms that share the fundamental ability to photosynthesize but diverge wildly in everything else: their cell walls, their storage compounds, their pigment cocktails, and their secondary metabolites. When you ask “what is algae made of,” you are really asking about a patchwork of lineages that are no more closely related to each other than you are to a mushroom.
Extracellular Substances and Biofilms
The chemical story of algae does not stop at the cell surface. Many microalgae secrete extracellular polymeric substances (EPS), a sticky matrix made mainly of polysaccharides, proteins, nucleic acids, and lipids. These EPS glue cells to solid surfaces and to each other, forming biofilms.25PubMed. A methodological review on the characterization of microalgal biofilm and its extracellular polymeric substances If you have ever felt the slippery green film on a submerged rock, you have touched an algal biofilm held together by EPS.
Biofilms serve multiple purposes for the algae. They protect against desiccation, help retain nutrients in nutrient-poor environments, and offer some defense against grazing. From a human perspective, algal biofilms are both a nuisance (fouling ship hulls, clogging water treatment systems) and a resource (biofilm-based cultivation systems are being explored for wastewater treatment and biomass production). The composition of the EPS itself varies by species and conditions, mirroring the same environmental plasticity seen in the cells’ internal chemistry.
Commercial Hydrocolloids and What They Are Made Of
Three seaweed-derived polysaccharides, agar, alginate, and carrageenan, are among the most commercially important compounds extracted from algae. These hydrocolloids serve as gelling and thickening agents in food, pharmaceuticals, and biotechnology.26PubMed Central. Seaweed hydrocolloid production: an update on enzyme assisted extraction and modification technologies Their functional properties depend on specific structural features. The degree and position of sulfate groups on the sugar chains, the presence of chemical bridges within the molecules, and the molecular weight all determine whether a given polysaccharide forms a firm gel, a soft gel, or just thickens a liquid.
Agar, extracted from red algae like Gelidium and Gracilaria, is the compound that gives agar plates their firm gel in microbiology labs. Carrageenan, also from red algae, shows up in dairy products, plant-based milks, and processed meats as a stabilizer. Alginate, from brown algae like kelp, is used in everything from wound dressings to the spherified “caviar” pearls in molecular gastronomy. Each of these products exists because of the particular polysaccharide chemistry that red and brown algae evolved for structural purposes in the ocean. Humans figured out how to extract and exploit those properties, turning seaweed cell wall material into a multi-billion-dollar industry.
Inside some green algae and diatoms, another structural detail worth noting is the pyrenoid, a specialized compartment within the chloroplast that concentrates carbon dioxide around the enzyme Rubisco to boost photosynthesis. In the model green alga Chlamydomonas, the pyrenoid matrix behaves like a liquid-like condensate, mixing internally on a timescale of roughly 20 seconds and capable of dissolving and re-forming during cell division.27Oxford Academic (The Plant Cell). The pyrenoid: the eukaryotic CO2-concentrating organelle The pyrenoid is one of the features that makes algal photosynthesis more efficient than that of most land plants under certain conditions, and researchers studying how to improve crop yields are investigating whether pyrenoid-like structures could be engineered into terrestrial plants.