Why start here
The question nobody thinks to ask
You walk past a rock, a puddle of water, and a tree. Two of those three things are not alive. But what is the actual difference? This question — deceptively simple — is where all of biology begins. And the answer is what puts microalgae at the center of enormous commercial opportunity.
Scientists spent centuries trying to pin down a precise definition of life. The answer they landed on is not a single property but a cluster of seven. Living things do all seven of these things. Non-living things fail at least one. Understanding these seven properties will give you an immediate, intuitive framework for understanding microalgae — and for explaining their value to anyone.
Part 1 of 3
The seven properties of life
Each property below is paired with its direct implication for microalgae. As you read, notice that microalgae don't just possess all seven — they perform them with extraordinary efficiency for their size.
01
Made of cells
Every living thing is made of at least one cell — a tiny sealed unit with its own internal machinery. A cell is the smallest thing that can be called "alive."
Microalgae are single cells. The entire organism — eating, growing, reproducing — happens inside one cell.
02
Reproduces
Life makes copies of itself. One organism becomes two, two become four. Without this, a species would eventually vanish.
Under ideal conditions, one algae cell divides every 2–8 hours. A single cell becomes millions in a few days.
03
Uses energy (metabolism)
All living things extract energy from somewhere and use it to do work. The entire set of chemical reactions that run a cell is called metabolism.
Algae capture energy from sunlight. They use it to build proteins, fats, pigments — everything in the commercial diagram you saw.
04
Grows and develops
Living things increase in size and complexity — not by adding material from outside (like a snowball), but by building new material from within using their own chemistry.
A young algae cell absorbs CO₂, water, and light, synthesises new molecules internally, swells in size, then divides.
05
Responds to environment
Living things sense the world and react. Light, temperature, chemicals, touch — these all trigger responses that alter behaviour.
Haematococcus algae swim toward light when it's gentle — and produce protective astaxanthin (turning red) when light becomes intense. This stress response is how producers harvest the pigment.
06
Maintains stability (homeostasis)
Even as the outside world fluctuates wildly, living things keep their internal chemistry within a narrow range. Temperature, pH, salt — all controlled.
Dunaliella salina survives in salt lakes 10× saltier than seawater by actively pumping salt out of its cells. This same hardiness makes it easy and cheap to grow.
07
Evolves and adapts
Over generations, populations change. Individuals better suited to their environment survive and reproduce more. Over millions of years, this produces extraordinary diversity.
Microalgae have been evolving for 2.7 billion years. They now live in boiling hot springs, Arctic ice, hypersaline lakes, and soils. This diversity is an enormous untapped library of novel chemistry.
The commercial insight
That combination of property 2 (rapid reproduction), property 3 (photosynthetic metabolism), and property 5 (stress-response that produces valuable compounds) is the core engine of the microalgae industry. Everything else — cultivation, extraction, markets — is infrastructure built around this biological core.
Part 2 of 3
The great cellular divide
Before we look inside a cell, you need to know about one of the most important splits in all of biology — a divide that happened nearly 2 billion years ago, and that directly determines what kind of products a microalgae species can make.
All cellular life on Earth falls into one of two categories: prokaryotes and eukaryotes. The word roots tell you everything — "pro" means before, "eu" means true, "karyon" means nucleus. Prokaryotes are cells without a proper nucleus; eukaryotes have one.
Simpler · Older · 2.7 billion years old
Prokaryote
Cell membrane
Outer boundary — controls what enters and exits
DNA — floating free
The instruction manual floats loose in the cell fluid. No protective room.
Ribosomes
Tiny machines that build proteins. Present in all cells.
Spirulina (cyanobacterium) is a prokaryote. Easier to grow at massive scale. Good for bulk protein and pigments. Less molecular complexity.
Complex · Younger · 1.5 billion years old
Eukaryote
Cell membrane + wall
Same outer boundary, often with an additional rigid outer wall
Nucleus (sealed room)
DNA is protected inside its own membrane-bound compartment. The defining feature.
Mitochondria
Dedicated power plants. Convert food into usable energy.
Chloroplasts
Solar panels. Only in algae and plants. Where photosynthesis happens.
Endoplasmic reticulum, Golgi
Internal delivery and packaging system for proteins and lipids
Chlorella, Dunaliella, Haematococcus, Nannochloropsis are all eukaryotes. More complex machinery = more sophisticated products: astaxanthin, DHA, pharmaceuticals.
Investment-level insight
This is why two algae companies can look superficially similar but operate in completely different economic universes. A Spirulina producer (prokaryote) is competing on volume and price. A Haematococcus producer (eukaryote) is making astaxanthin worth ~$3,000/kg. The cell type determines the ceiling of what's possible.
Part 3 of 3
Inside an algae cell — the factory tour
Now we go inside a eukaryote algae cell. Think of it as a factory: self-contained, self-replicating, with specialised departments doing very different jobs. Each department below (called an organelle) connects directly to something on the commercial map you're building.
Cross-section · Eukaryote microalgae cell (not to scale)
Cell wall
The outer shell
The outermost rigid layer of the cell. Made of tough carbohydrate molecules — similar in concept to wood or cotton. Gives the cell shape and protects it from bursting when internal pressure builds. Porous enough to let water and small molecules pass through.
Factory analogy: the outer building wall — solid, load-bearing, lets in air, keeps everything inside protected.
In diatoms, the cell wall is literally made of glass (silica). When diatoms die, their glass skeletons accumulate — this is diatomaceous earth, mined commercially for filtration and pest control.
Cell membrane
The intelligent gatekeeper
Just inside the wall: a thin, flexible sheet made mostly of fat molecules (phospholipids). Unlike the wall, this membrane is actively selective — it uses protein channels and pumps to decide what enters and leaves. It maintains the cell's internal chemistry by strictly controlling traffic.
Factory analogy: the customs checkpoint — every molecule trying to enter or exit is inspected and either allowed through or turned back.
The omega-3 fatty acids (DHA, EPA) that algae produce commercially are actually structural components of this membrane. The algae make them not to sell to us — but to build themselves. We harvest a byproduct of the cell's own architecture.
Nucleus
The instruction room
A sealed inner compartment with its own membrane. Inside lives the DNA — the complete instruction manual for building and running the organism. The DNA is organised into chromosomes and protected from the chemical activity happening in the rest of the cell. A smaller structure inside the nucleus (the nucleolus) manufactures ribosomes.
Factory analogy: the locked head office — blueprints are stored here, workers are dispatched with instructions, but the master documents never leave the room.
This is where genetic engineering (CRISPR) acts. The technology enters the nucleus and edits the DNA instructions — changing what the cell builds. Engineering an algae to produce twice as much astaxanthin means rewriting a few lines in this instruction room.
Chloroplast
The solar panel and sugar factory — the most important organelle for your purposes
Where photosynthesis happens. The chloroplast captures sunlight and uses its energy to combine CO₂ and water into sugar — the raw fuel for everything else the cell does. It contains chlorophyll (the green pigment) and also many other pigments (carotenoids, phycocyanin) that absorb different wavelengths of light.
Factory analogy: a solar power plant that also happens to be a food manufacturing line — taking sunlight as input, outputting fuel for the entire operation.
Chloroplasts have their own DNA. Scientists believe they were once free-living bacteria that were absorbed by a larger cell ~1.5 billion years ago — and the partnership was so effective it became permanent. This "endosymbiosis" event is what gave algae and plants their photosynthetic power. Every commercial product in your diagram that comes from algae ultimately begins in this organelle.
Mitochondria
The power plant
Takes the sugar produced by the chloroplast and "burns" it in a controlled way to produce ATP — the cell's energy currency. Nearly everything the cell does costs ATP: building proteins, pumping molecules, dividing. Like chloroplasts, mitochondria also have their own DNA and were once free-living bacteria. Every cell in your body contains hundreds of them.
Factory analogy: the generator room — takes fuel (sugar) and converts it into the electricity (ATP) that powers every other machine.
When algae are nutrient-stressed (low nitrogen), they stop growing and redirect their metabolism — the chloroplast overproduces oil or pigment, the mitochondria shift to processing lipids. Commercial producers deliberately starve algae of nitrogen to trigger this stress response and maximise astaxanthin or oil accumulation.
Vacuole
Storage and pressure control
A large fluid-filled sac. Functions include: storing water, waste, and sometimes pigments or toxins; maintaining internal pressure (a full vacuole pushes against the cell wall, keeping it firm); and digestion (breaking down materials the cell no longer needs). In algae and plant cells, vacuoles can occupy most of the cell's volume.
Factory analogy: the warehouse and waste management department combined.
Haematococcus algae store astaxanthin in lipid droplets near or within vacuoles as a natural sunscreen when light stress is high. This turns the culture visibly red — from green to red — which experienced producers use as a visual cue to begin harvest.
The mental model to keep for every week ahead
A microalgae cell is a self-contained factory, the size of a speck of dust, that takes in sunlight, water, and CO₂ — runs them through the chloroplast — and outputs sugars, proteins, fats, and pigments. It replicates the entire factory every few hours. It has been refined by 2.7 billion years of evolution. Every commercial opportunity in microalgae is a different way of harvesting the output of that ancient, extraordinarily efficient factory.
Quick-reference summary
| Concept | One-line definition | Algae connection |
|---|---|---|
| 7 properties | What all living things share | Algae possess all 7 — the stress-response property is how producers trigger valuable compound accumulation |
| Prokaryote | Simple cell, no nucleus | Spirulina. Bulk market, lower-value products. Very easy to grow at scale. |
| Eukaryote | Complex cell with nucleus | Chlorella, Haematococcus, Dunaliella, Nannochloropsis. Higher-value products: astaxanthin, DHA, pharmaceuticals. |
| Chloroplast | Solar panel organelle for photosynthesis | The engine of the industry. Source of all light-driven production. Contains chlorophyll + carotenoids. |
| Mitochondria | Power plant organelle (ATP) | Burns sugar to power all cell activity. Shifts behaviour under nutrient stress — the trigger for oil/pigment production. |
| Nucleus | DNA storage compartment | Target of CRISPR genetic engineering — editing the instructions changes what the cell produces. |
Self-check — end of week 2
Read each question. Think through your answer. Then click to reveal. If you can answer all 5 confidently, you're ready for Week 3.
1. A virus can reproduce and evolve — yet most scientists don't classify it as "alive." Using the seven properties, explain why.
A virus fails property 1 (made of cells) and property 3 (metabolism). A virus has no cells and no metabolism — it cannot extract or use energy on its own. It must hijack a living cell to reproduce. By the seven-property test, it's disqualified on at least two counts. This is why viruses are described as "on the edge of life" rather than alive.
2. A new algae company tells you they're growing Spirulina for pharmaceutical compounds. What should make you pause?
Spirulina is a cyanobacterium — a prokaryote. Prokaryotes lack the complex internal machinery (organelles, sophisticated metabolic pathways) needed to produce most pharmaceutical compounds. The most valuable pharmaceutical molecules from algae (cryptophycin, cyanovirin, amphidinolides) come from eukaryotes. A Spirulina pharma claim deserves hard scrutiny.
3. Haematococcus algae are green when growing happily, then turn red under stress. Explain this in cell biology terms.
Under stress (high light, low nutrients), the chloroplast redirects its activity — instead of making chlorophyll and growing, it produces astaxanthin, a red carotenoid pigment that acts as a natural sunscreen. This astaxanthin accumulates in lipid droplets. The cell's green colour (chlorophyll) is overwhelmed by the red (astaxanthin). The colour change is a direct readout of the cell's internal metabolic state — and the signal to producers that the compound has accumulated enough to harvest.
4. Why do algae produce DHA (an omega-3 fatty acid)? Did they evolve to supply human nutrition?
No — algae produce DHA for their own structural purposes. DHA is a component of their cell membranes, especially the membrane of the chloroplast. It keeps membranes fluid and functional at cold temperatures, and plays a role in photosynthetic efficiency. We harvest it because it happens to be nutritionally valuable to us. The fish oil in supplements is also algae-derived — fish eat algae and accumulate DHA in their tissues. The algae are the original source.
5. In one sentence, why does a microalgae cell have both a chloroplast AND mitochondria?
The chloroplast makes sugar from sunlight (energy input), and the mitochondria convert that sugar into ATP — the usable energy currency that powers everything else the cell does (energy output). They are two halves of the cell's energy system: one captures energy, the other distributes it.