The other half of the energy equation
What cellular respiration is — and why it matters for algae farms
In Weeks 5–7 you learned how algae make sugar from sunlight. Now you learn what they do with that sugar. Cellular respiration is the process by which cells break glucose down, extract the stored energy, and convert it into ATP — the usable energy currency that powers everything else the cell does. It is, in a sense, photosynthesis run in reverse.
Understanding respiration is not just academic. It directly explains some of the most important practical challenges in commercial algae production: why cultures lose biomass overnight, why warm nights reduce productivity, why net yield is always less than gross photosynthesis, and how to design farms that minimise these losses.
The respiration equation — photosynthesis reversed
C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ~36–38 ATP
Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP)
Compare this to photosynthesis: 6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂. The inputs and outputs are exactly swapped. Photosynthesis builds glucose and releases O₂. Respiration burns glucose and releases CO₂. One process stores energy; the other releases it. Every living cell — algae, plant, animal, fungus — runs respiration continuously, day and night.
The immediate commercial implication
Algae photosynthesize only during daylight hours. But they respire 24 hours a day — including all night. This means every night, your algae culture is consuming some of the biomass it built during the day. The net productivity of an algae farm is always: daytime photosynthesis minus round-the-clock respiration. Minimising nighttime respiration losses is one of the key levers for improving algae farm economics.
Part 1 of 3 · The three stages
How respiration works — three linked stages
Like photosynthesis, cellular respiration is not a single reaction. It is a carefully orchestrated sequence of three stages, each happening in a different part of the cell, each producing different amounts of ATP. Click each stage to expand it fully.
1
Glycolysis — splitting glucose
Location: cytoplasm (outside mitochondria) · No oxygen needed
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What happens
The word glycolysis means "glucose splitting." One glucose molecule (6 carbons) is broken in half, producing two molecules of a 3-carbon compound called pyruvate. This happens in the cytoplasm — the fluid of the cell — and does not require oxygen. It is the most ancient energy-producing process in biology, found in every living thing on Earth, including the simplest bacteria.
Inputs and outputs
IN: 1 Glucose (6C)
IN: 2 ATP (to start)
OUT: 2 Pyruvate (3C each)
OUT: 4 ATP (net gain: 2)
OUT: 2 NADH (electron carrier)
ATP yield
A net of just 2 ATP per glucose. The cell actually invests 2 ATP to start the process, then gets 4 back. Tiny compared to what the later stages produce — but crucially, this works without any oxygen. It is the cell's emergency backup power system.
Analogy: glycolysis is cracking open a walnut. You get a small amount of material out, but the real value is inside. You've done the essential first step — splitting the glucose — that makes the next two stages possible.
Algae context: when oxygen is temporarily depleted in a dense, poorly-mixed culture (a common problem), algae can fall back on glycolysis alone. This produces some ATP but generates lactate or ethanol as a byproduct — and is far less efficient. Detecting low-oxygen conditions in a pond is therefore a real operational concern.
2
Krebs Cycle — extracting all remaining energy
Location: mitochondrial matrix (inside the inner membrane) · Requires oxygen indirectly
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What happens
The two pyruvate molecules from glycolysis are transported into the mitochondria and converted into a 2-carbon molecule called Acetyl-CoA, releasing CO₂ in the process. Acetyl-CoA then enters a circular series of reactions — the Krebs Cycle (also called the citric acid cycle) — where the remaining carbon bonds are progressively broken, releasing their stored energy. Each turn of the cycle processes one Acetyl-CoA, so the cycle turns twice per glucose.
Inputs and outputs
IN: 2 Pyruvate → 2 Acetyl-CoA
OUT: 4 CO₂ released
OUT: 2 ATP
OUT: 6 NADH (electron carriers)
OUT: 2 FADH₂ (another carrier)
ATP yield
Only 2 ATP directly — same as glycolysis. But the real output is 8 charged electron carriers (NADH and FADH₂). These are the packed containers of energy that will be used in the next stage to make the bulk of the ATP. The Krebs Cycle is primarily an electron harvesting operation, not a direct power generator.
Analogy: the Krebs Cycle is a disassembly line. The pyruvate arrives like a partially dismantled machine. The cycle strips it down completely, carbon by carbon, handing off the released energy as charged electron-carrier molecules. The CO₂ you breathe out is this stage's exhaust.
Algae context: the CO₂ released by the Krebs Cycle inside algae cells can be immediately recaptured by the Calvin Cycle in the adjacent chloroplast — a form of internal CO₂ recycling. This "photorespiratory" feedback loop affects how efficiently algae use CO₂ at high temperatures, and is one target for genetic improvement.
3
Electron Transport Chain — the ATP powerhouse
Location: inner mitochondrial membrane · Requires oxygen directly
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What happens
The NADH and FADH₂ from stages 1 and 2 arrive at the inner mitochondrial membrane and donate their electrons to a chain of protein complexes — the electron transport chain (ETC). Electrons flow "downhill" through this chain, releasing energy at each step. That energy pumps protons (H⁺) from inside the mitochondria to outside the inner membrane, building up a gradient — exactly like the thylakoid proton gradient in photosynthesis. Those protons then rush back through ATP synthase, spinning it and generating ATP in enormous quantities.
Inputs and outputs
IN: 10 NADH + 2 FADH₂
IN: O₂ (final electron acceptor)
OUT: ~32–34 ATP 🔋
OUT: H₂O (O₂ + electrons → water)
ATP yield
~32–34 ATP — the vast majority of respiration's total output. Oxygen is essential here as the final electron acceptor at the end of the chain. Without O₂, the chain backs up, electrons stall, and the whole process stops. This is why oxygen is called essential for life — it is specifically needed to keep the ETC running.
Analogy: the ETC is the hydroelectric dam. NADH and FADH₂ are the water behind it. The electron flow is the water rushing through. ATP synthase is the turbine. O₂ is the drain at the bottom — without it, the water backs up and the turbine stops. The output — ATP — is the electricity powering the rest of the cell.
Algae context: this is why photobioreactor oxygen accumulation matters so much. High dissolved O₂ can actually inhibit the ETC — the electrons cannot offload to a destination that is already saturated. In dense cultures, this creates a counterproductive situation where the algae are photosynthesising vigorously but struggling to respire efficiently.
ATP tally — where the energy comes from across all three stages
Part 2 of 3 · The algae energy balance
Photosynthesis vs respiration — the tug of war that determines farm yield
Algae do both processes simultaneously during the day and only respiration at night. The net productivity of any algae farm is the result of this continuous tug of war between the two processes. Understanding it precisely is the difference between a profitable farm and a break-even one.
☀️ Photosynthesis
WhenDaylight hours only
Location in cellChloroplast
Takes inCO₂ + H₂O + light
ProducesGlucose + O₂
Net effectBuilds biomass ↑
ATPProduces ATP
🔥 Respiration
When24 hours a day, always
Location in cellMitochondria + cytoplasm
Takes inGlucose + O₂
ProducesCO₂ + H₂O + ATP
Net effectConsumes biomass ↓
ATPConsumes glucose to make ATP
Net productivity — the number that actually matters
Gross photosynthesis is how much sugar the algae make during the day. Net productivity is what is left after subtracting respiration losses over 24 hours. In a well-run outdoor pond in full sun, roughly 20–40% of daytime photosynthesis is lost to respiration — mostly at night.
Visualising the daily energy budget — a typical outdoor pond
The compensation point is the light intensity at which photosynthesis exactly equals respiration — net productivity is zero. Below this light level, the culture is losing biomass even during daylight. Getting cultures above the compensation point for as many hours as possible is a primary design goal of outdoor production systems.
Temperature — the hidden lever on respiration
Temperature has a powerful effect on respiration. Every 10°C rise in temperature roughly doubles the rate of enzyme-driven reactions — including respiration. Photosynthesis also speeds up with temperature, but only up to an optimum, after which the photosystems start to degrade.
This creates a critical asymmetry for outdoor algae farms in warm climates:
| Condition | Photosynthesis rate | Respiration rate | Net effect on farm |
|---|---|---|---|
| Cool day 20°C | Moderate — some enzymes slow | Low — slow enzyme rates | Good net productivity; low losses |
| Ideal day 25–28°C | Near maximum for most species | Moderate | Best net productivity window |
| Hot day 35°C+ | Declining — photosystems stressed | Very high — doubling with each 10°C | Net productivity falls sharply |
| Warm night 28°C | Zero — no light | High — fast enzyme rates all night | Large overnight biomass loss |
| Cool night 15°C | Zero — no light | Low — slow enzyme rates | Small overnight loss; good net |
Why tropical locations are not automatically ideal for algae farming
Tropical regions have high solar irradiance — great for photosynthesis. But they also have warm nights, year-round. Warm nights mean high respiration rates all night, consuming the hard-won biomass from the day. Some analyses show that Mediterranean or desert climates with hot days and cool nights can outperform tropical locations on net annual productivity, because the cool nights preserve more of what the day built. This is a counterintuitive but important insight when evaluating algae farm site selection.
Part 3 of 3 · What this means for production
Six practical implications for algae farms
Everything you have just learned about respiration translates directly into farm design decisions, operational choices, and investment risk factors. These are the insights that separate someone who reads about algae from someone who understands the business.
Nighttime biomass loss
Algae consume 10–30% of their daytime photosynthate during the night through respiration. In some species and hot climates, this figure is higher. For high-value species with slow growth rates (like Haematococcus), this is a significant cost factor.
Mitigation: harvest in evening, not morning
Temperature management
Keeping cultures at the species' optimal temperature (typically 20–28°C) maximises the photosynthesis-to-respiration ratio. In hot climates, evaporative cooling of open ponds is used. In cold climates, greenhouse covers or heat exchangers are needed.
Cool nights = less respiration loss
Oxygen management
Dense cultures producing O₂ rapidly can create O₂ supersaturation that inhibits respiration (and photosynthesis). Good mixing and degassing in closed reactors prevents this. Open ponds naturally off-gas O₂ to atmosphere.
O₂ buildup is a reactor design problem
Culture density (biomass)
Denser cultures have more total respiration — but also more total photosynthesis. The optimal density is where light penetration is sufficient for net positive photosynthesis throughout the culture depth. Too dense = self-shading, cells at the bottom only respire.
Compensation point applies culture-wide
Heterotrophic switching
Some algae species can switch to heterotrophic growth — running only respiration, fed organic carbon in the dark, like Schizochytrium. This eliminates light engineering entirely. Lower capex, higher cell density, but requires feeding glucose. Used commercially for DHA production.
Respiration as the sole energy mode
Productivity measurement
Always ask: is the stated productivity figure gross or net? Gross photosynthesis sounds impressive. Net productivity — after 24h respiration — is the commercial reality. A farm reporting areal productivity should specify whether that is measured over the full 24-hour cycle.
Net productivity = the real business metric
The master insight of weeks 8–9
Photosynthesis and cellular respiration are not opposites — they are partners. Photosynthesis converts light energy into chemical energy stored as glucose. Cellular respiration converts that stored glucose into ATP, the currency that actually powers every cellular activity. Algae run both continuously: building biomass with one hand, spending it with the other. The net result — net productivity — is the only number that matters commercially. Every farm management decision, from temperature control to harvest timing to reactor design, is ultimately an attempt to widen the gap between what the algae build and what they burn. Understanding this single idea gives you a lens that most algae industry observers lack entirely.
Quick-reference summary
| Concept | Definition | Algae / commercial relevance |
|---|---|---|
| Glycolysis | Glucose split into 2 pyruvate in cytoplasm. No O₂ needed. Net: 2 ATP. | Works in low-oxygen conditions. Emergency backup. Produces ethanol/lactate as byproduct — a sign of culture stress. |
| Krebs Cycle | Pyruvate fully dismantled in mitochondrial matrix. Releases CO₂. Produces 2 ATP + 8 electron carriers. | CO₂ released can be recaptured by adjacent chloroplast. Electron carriers fuel the ETC — the main ATP generator. |
| ETC + ATP synthase | Electron carriers donate electrons; proton gradient drives ATP synthase turbine. Needs O₂. Produces ~34 ATP. | 90% of all ATP from respiration. O₂ starvation or O₂ supersaturation both impair this — a key reactor management problem. |
| Net productivity | Gross photosynthesis minus 24h respiration losses. The real commercial output. | Always ask if productivity claims are gross or net. Night temperatures and culture density both affect this critically. |
| Compensation point | The light intensity at which photosynthesis exactly equals respiration. Net productivity = zero at this point. | Cultures must be kept well above compensation point during daylight. Poor mixing or shading drops cultures below it. |
| Heterotrophic growth | Algae grown in dark on organic carbon, running respiration only. Used in conventional fermenters. | Schizochytrium, Crypthecodinium: DHA production without light engineering. Different economics from phototrophic systems. |
Self-check — end of week 9
All questions require connecting biology directly to commercial or operational reasoning.
1. A Spirulina farm in India reports annual productivity of 25 tonnes per hectare. A Spirulina farm in southern Spain reports 18 tonnes per hectare. The Indian farm has more sunny days. Why might the Spanish farm's lower gross photosynthesis still result in a more economically efficient operation?
India's tropical climate means warm nights year-round — high respiration rates overnight consuming a larger fraction of the day's photosynthate. Spain's Mediterranean climate features cool nights, especially in spring and autumn, which dramatically reduce nighttime respiration losses. The Spanish farm's net productivity (what remains after 24h losses) may be proportionally closer to its gross productivity than India's. Additionally: warm days in India may push temperatures above the photosynthetic optimum for Spirulina (~35°C), causing photoinhibition and enzyme denaturation, reducing gross photosynthesis even during peak sun. Spain's more moderate summer temperatures may allow sustained photosynthesis at closer to peak efficiency for more hours. Finally, Spain's lower summer humidity may aid evaporative cooling of open ponds, keeping culture temperatures in the optimal range. Productivity comparisons between geographies must always account for the full 24h energy balance, not just peak irradiance.
2. An algae startup claims their closed photobioreactor achieves 3× the productivity of an open pond. A skeptic points out that their oxygen management system consumes 40% of the electricity budget. Explain the biological basis of this oxygen management problem and why it doesn't exist in the same way for open ponds.
In a closed photobioreactor, O₂ produced by photosynthesis has nowhere to escape — the system is sealed. As algae photosynthesize vigorously, dissolved O₂ rises rapidly. At supersaturation levels, O₂ begins to inhibit photosynthesis (competing with CO₂ at RuBisCO — photorespiration) and impairs the electron transport chain in mitochondria, where O₂ at the right concentration is needed as the terminal electron acceptor but O₂ excess creates reactive oxygen species that damage cell components. Active degassing — typically using sparging with N₂ or CO₂-enriched air, or through membrane degassing systems — is needed to strip O₂ out continuously. In open ponds, O₂ simply diffuses from the water surface into the atmosphere. The surface-to-volume ratio of open ponds is much higher, and wind aids gas exchange. This passive O₂ removal is free. The 3× productivity claim of the closed system must be evaluated against the total energy input, including that 40% spent on oxygen management — the genuine economic comparison is net productivity per unit of total energy consumed, not just biomass per unit area.
3. A producer harvests their Nannochloropsis culture every morning. Their colleague suggests harvesting in the evening instead. Using your knowledge of respiration and the daily productivity cycle, explain why evening harvest might yield more biomass.
During the day, photosynthesis builds biomass (glucose, proteins, lipids). Respiration runs simultaneously but at a lower rate than photosynthesis in daylight, so net biomass accumulates. By evening, the culture has accumulated the full day's net photosynthate — the maximum biomass of the 24-hour cycle. If harvested then, you capture the peak. If you leave it overnight, respiration continues (but photosynthesis stops at dark), consuming 10–30% of the evening biomass by morning. Morning harvest collects what survived the night — already depleted by overnight respiration. Evening harvest therefore captures more total dry biomass per harvest cycle. The practical complication is that some high-value compounds like astaxanthin may continue accumulating under stress conditions even at night (as a stress response pathway, not photosynthesis-dependent), so timing of harvest must be optimised for the target compound, not just total biomass.
4. Cyanide is a poison that blocks Complex IV of the electron transport chain — the final step where electrons are passed to oxygen. Explain exactly why even a tiny amount of cyanide is lethal, using your knowledge of the ETC and ATP.
Complex IV (cytochrome c oxidase) is the final protein in the ETC — it transfers electrons from the chain to O₂, producing water. When cyanide blocks Complex IV, electrons can no longer be offloaded to O₂. The entire electron transport chain backs up — NADH and FADH₂ can no longer donate their electrons, because there is nowhere for them to go. The proton gradient collapses (no more pumping). ATP synthase stops spinning. ATP production from the ETC falls to zero — which is roughly 90% of all cellular ATP. Without ATP, the cell cannot run virtually any of its processes: ion pumps stop (membranes lose their gradients), proteins stop being synthesised, muscles cannot contract, neurons cannot fire. Glycolysis still produces 2 ATP, which is insufficient to sustain any complex cell for more than seconds to minutes. Death follows rapidly. This is why cyanide poisoning requires immediate treatment — and it is also why the ETC is studied in algae as a potential target for inhibiting harmful algae blooms.
5. You are evaluating two algae companies for investment. Company A grows Chlorella phototrophically outdoors in open ponds in Malaysia. Company B grows Schizochytrium heterotrophically in steel fermenters in Germany. Using your understanding of photosynthesis and cellular respiration, identify two structural advantages and one structural risk for each company's model.
Company A (phototrophic, open pond, Malaysia): Advantage 1 — sunlight is free. No energy cost for the primary input that drives photosynthesis and biomass production. Advantage 2 — lower capital cost per unit volume; open ponds are simple infrastructure. Risk — warm Malaysian nights mean high respiration rates consuming 20–35% of daytime photosynthate. Net productivity is significantly lower than gross photosynthesis, and this loss is structural — it cannot be engineered away without changing location or species. Company B (heterotrophic fermenter, Germany): Advantage 1 — no light engineering needed; fermenters achieve very high cell density (50–100× open ponds) in a small footprint, and process is fully controllable regardless of season or weather. Advantage 2 — Schizochytrium produces very high DHA content (35–50% of oil), a premium product with a large established market (infant formula, supplements). Risk — the carbon feedstock (glucose from corn or sugarcane) is a significant and variable ongoing cost, subject to commodity price fluctuations. The "green" credentials of the product depend on the sustainability of the feedstock supply chain — something regulators and customers increasingly scrutinise. Additionally, Germany's energy costs are high, affecting fermenter operating economics compared to lower-cost geographies.