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The Interdependence of CO2 and Light

The Interdependence of CO2 and Light
Maximizing Photosynthesis by Balancing CO2 and Light Intensity in Planted Aquariums

One of the biggest planted aquarium myths is that certain plants inherently require a lot of light. In reality, the light requirement of plants (their "light compensation point") varies with the availability of CO2. In an aquarium with CO2 supplementation, even demanding carpet plants (HC Cuba, Glosso, etc...) can be grown beautifully without very much light at all. Rather than blowing your budget (and retinas) on an expensive, energy-guzzling light fixture that causes your plants to melt and algae to get out of control, consider supplementing CO2 instead. It's a lot more effective and a lot less problematic!

The Magic of CO2 Enrichment in Planted Aquariums

In the realm of aquatic gardening, carbon dioxide (CO2) supplementation stands as a pivotal factor that influences the growth, health, and vibrancy of aquatic plants. This importance stems from CO2's central role in photosynthesis, the process by which plants convert light energy into chemical energy, using CO2 and water to produce glucose and oxygen. For aquarists, mastering the art of CO2 supplementation can unlock the full potential of their underwater gardens, transforming them into lush, vibrant ecosystems.

Understanding Photosynthesis in Aquatic Plants

At the heart of the matter is photosynthesis, the biological process that drives plant growth. Aquatic plants, like their terrestrial counterparts, rely on photosynthesis to generate the sugars that fuel their growth and development. Light and CO2 are critical inputs for this process, with water serving as a source of electrons and hydrogen ions. The equation for photosynthesis highlights the role of these elements:

This equation underscores the importance of CO2 in the growth of aquatic plants, making its supplementation in aquariums a key strategy for enhancing plant health and aesthetics.

Photosynthesis is a Two-Step Process

While light energy is required to power the first phase of photosynthesis, it is the creation of organic molecules such as glucose that directly contribute to the growth and health of plants. The biosynthesis of glucose is completely independent of light in a second process known as the "light-independent reactions" or the "Calvin cycle" which relies entirely on CO2. It can even happen entirely in the dark.

CO2 injected low light planted aquarium
A tank showing how well even demanding plants grow with CO2 injection and only basic lighting and substrate


The Light-Indepdendent Reactions

The Calvin Cycle, or the light-independent reactions, is a critical process for aquarium plants, allowing them to fix carbon dioxide (CO2) into organic compounds that can be used for growth and energy. The importance of CO2 in this cycle cannot be overstated as it is the sole ingredient. No amount of light or other tricks can replace it.

CO2 levels in aquariums are artificially low compared to the environments that most aquatic plants are found in nature. Processes such as organic decomposition or groundwater intrusion that enrich natural habitats with CO2 simply cannot be replicated in an aquarium. Many plants growing in an unsupplemented aquarium are working with levels of CO2 hundreds to thousands of times lower than in the wild. As a result, they demand far more light than in nature, fail to grow entirely, and often even melt and die. Supplementing CO2 with a simple system is the only way to restore the environmental conditions that many aquatic plants have evolved in.

After CO2 supplementation, you will find that what you thought were "high light" plants are often more than fine with seemingly modest lighting. We've grown hundreds of aquarium plants for commercial production and not a single one needed anywhere close to the "textbook" recommendation when CO2 concentrations are between 20 and 30 ppm (normal in the wild). There are two reasons why plants simply do not need as much light as most people think once provided with the CO2 levels they require. In scientific terms - why the light compensation point of virtually all aquarium plants decreases with increasing CO2 availability:

Productive Photosynthesis Cannot Happen Without CO2

  1. CO2 Dependent Carbon Fixation: The Calvin Cycle occurs independently of light and uses CO2 from the water to form glucose, which serves as an energy source for the plant and the starting point to all other organic molecules including proteins and DNA. Aquarium CO2 concentration are normally far too low to support many plants and increasing CO2 levels to those found in nature dramatically accelerates the rate of carbon fixation, directly fueling plant growth without requiring more light energy. With more available CO2, plants can more efficiently convert the light energy they already have into sugars, leading to faster growth and healthier plants. Without CO2, even direct sunlight will do nothing as the Calvin cycle cannot proceed and no organic molecules can be created. Photosynthesis stops dead.

  2. Saturation Point for Light vs. CO2: Both light and CO2 are crucial for photosynthesis, but they have different saturation points. A plant can only use a certain amount of light; beyond this point, additional light doesn't increase photosynthesis but can harm the plant by causing light stress or algae growth. In contrast, increasing CO2 concentration up to a certain level continues to boost the rate of photosynthesis, making CO2 addition a more efficient way to enhance plant growth in the aquarium. Only when CO2 needs are met will an increase in lighting intensity have a beneficial effect on plant growth.

Reduction of Photorespiration

A side effect of the Calvin Cycle that occurs when CO2 concentrations are low relative to oxygen further inhibits productive photosynthesis and in fact, directly harms the plant. This process, called photorespiration not only wastes energy but turns existing organic carbon into CO2 by accident, causing plants to actively destroy themselves. When aquarists refer to plants "melting", they are often succumbing to photorespiration and would more accurately be described as "vaporizing".

  1. CO2 Suppresses Photorespiration: Photorespiration is a process that occurs when plants absorb oxygen instead of CO2, leading to the wasteful consumption of energy, the loss of fixed organic carbon, and a dramatic reduction of photosynthetic efficiency. This typically happens under high light and low CO2 conditions. By increasing CO2 concentration, plants are more likely to absorb CO2 rather than oxygen, thereby reducing photorespiration, reducing the loss of biomass, and increasing the efficiency of photosynthesis.

  2. Optimal Conditions for Photosynthesis: High levels of CO2 relative to oxygen encourage the plant to utilize CO2 more effectively for photosynthesis. This not only boosts growth but also makes the energy production process more efficient by reducing the occurrence of photorespiration, a process that competes with the Calvin Cycle for resources and thus decreases the overall efficiency of photosynthesis.

Practical Implications for Aquarium Plants

Adequate CO2 concentrations not only improves the efficiency of photosynthesis, allowing your aquarium plants to grow with less light, but also actively prevents the extremely harmful process of photorespiration - allowing plants to make use of higher lighting for faster growth. This contradicts the commonly held belief that aquarium plants simply grow slower without CO2 and patience is the solution. Many of the most interesting and vibrant species simply will not grow or not grow well without CO2, no matter how much time you give it. A lack of CO2 relative to oxygen can kill plants that lack the evolutionary adaptations to deal with low CO2 concentrations.


The Impact of CO2 Enrichment on Plant Growth

CO2 enrichment can significantly impact the growth rate, color, and overall health of aquatic plants. By increasing the availability of CO2, aquarists can optimize photosynthetic efficiency, allowing plants to utilize light more effectively and grow more vigorously while keeping photorespiration at a minimum. This is particularly beneficial in aquariums where CO2 levels are extremely low compared to nature, and may be insufficient to support growth (or even life) of all but the fairly limited selection of species that have evolved in very low CO2 environments.

Carpet Plants: Lush Ground Covers with CO2 Supplementation

Glossostigma elatinoides carpet growin in low light with CO2
This dense mat of thriving HC Cuba took over where the Glosso carpet was. This photo was sent to us by a customer using a $40 Amazon light with a CO2ONE system in a 35-gallon "tall". All of this happened in less than a month from when they were planted from our tissue culture cups

Carpet plants, sought after for creating dense, verdant ground covers in aquariums, exhibit remarkable responses to CO2 supplementation - even without the intense lighting they purport to need. Species such as Dwarf Baby Tears (Hemianthus callitrichoides), Monte Carlo (Micranthemum tweediei), and Glossostigma elatinoides are really only viable under CO2-enriched conditions that match their native habitats. CO2 allows these plants to efficiently harness even moderate amounts of light energy, resulting in the dense, healthy growth that is essential for achieving the desired carpet effect. The need for less light and their enhanced growth rates help to outcompete algae, contributing to a cleaner and healthier aquarium environment.

Low Light Plants: Thriving Under CO2 Enrichment

Low light aquarium plants growing with CO2
Even plants that do not need additional CO2 and can survive in low light grow much better with supplemental CO2 and will be able to make use of higher light intensities

Interestingly, CO2 enrichment also benefits low light plants, species that are traditionally considered less demanding in terms of light requirements. Coincidentally they are also able to grow without supplemental CO2 as they have evolved in very variable environments where CO2 concentrations and vary. This is because they are able to use alternate sources like bicarbonate for the Calvin cycle but since it's a slow and inefficient process, they normally require very little light energy to drive their meager photosynthetic rates. In essence, they are as much "low CO2" plants as they are "low light" plants. They're able to "make do" with what they can scavenge from the environment but it is certainly not ideal.

Anubias, Java Fern (Microsorum pteropus), and Cryptocoryne species are exemplary cases, demonstrating improved growth, coloration, and resilience when CO2 levels are optimized. These plants, often valued for their adaptability and aesthetic appeal, can become more robust and visually striking with CO2 supplementation, making them even more versatile components of planted aquariums. Once able to access reasonable CO2 concentrations, they can, naturally, use a lot more light to grow much faster.


Synergistic Effects of Light and CO2

While CO2 enrichment offers considerable benefits on its own, its effects are even more profound when considered in conjunction with light. Light intensity and CO2 availability are intrinsically linked, with each influencing the plant's ability to photosynthesize. Increasing CO2 without adjusting light can lead to improvements, but for many plants, especially those that are both light and CO2 limited, a synergistic increase in both light and CO2 unlocks explosive growth potential. This synergy is particularly noticeable in demanding carpet plants, which can grow rapidly, forming dense mats that are the hallmark of a well-manicured aquatic garden.


Practical Considerations for CO2 Supplementation

Implementing CO2 supplementation used to be quite difficult and expensive, making truly great planted aquariums difficult to acheive. Since then, a variety of solutions (such as our CO2ONE system) have made it as simple, affordable and accessible as any other aquarium accessory - without the steep learning curve associated with building a system from scratch. CO2 systems typically have a CO2 source, either in the form of a compressed gas cylinder (like a SodaStream

canister) or a vessel housing a biological or chemical reaction that produces CO2 (as in DIY systems). CO2 from compressed cylinders (and some DIY products) is released in a slow, consistent manner using a gas "regulator".

CO2 gas from the regulator is passed to a diffuser - essentially a very fine airstone that "diffuses" the gas into the aquarium water. They create very fine bubbles that are too small to float to the top and are instead spread around the aquarium where they either dissolve or end up sticking to the leaves of aquarium plants. Diffusers can be directly placed in the tank, relying on the filter or natural water currents to spread it around or be attached in-line to the outflow of a canister filter. These are sometimes referred to as "atomizers". CO2 concentration is important to keep an eye on as too much CO2 can drop the pH of water and injure your animals. A simple glass "drop checker", which turns color based on the dissolved CO2 concentration in your aquarium water, is the easiest and preferred method. With a high quality regulator, once you've adjusted the it to provide your preferred level of CO2, it should remain consistent and not require additional adjustments. Since plants only use CO2 during the day, respiring like animals during the night, it is a good idea to inject CO2 only when the lights are on. This prevents too much CO2 from accumulating during the dark hours when your plants are typically not using it and saves your CO2 supply, allowing you to go longer between refills. For the typical 8 hour photoperiod of most planted aquariums, shutting your CO2 supply off at night will extent your supply three times longer than running it 24 hours a day. While this can be done manually, a good CO2 system should have a device called a "solenoid" that automatically turns on when powered and off when not powered. This allows you to avoid having to manually turn your CO2 on and off at night and is an extremely important consideration when making a purchase. It can save you a lot of time and hassle. Solenoids are either built into the regulator as in the CO2ONE system or placed inline between the regulator and diffuser - either way, having one plugged into the same timer as your lights makes CO2 a set-it-and-forget-it affair.



Because aquariums are so small and disconnected from large scale processes, they naturally have extremely low CO2 levels compared to the natural environments of the vast majority of species that are commonly used in planted aquariums. While a limited number of plants have evolved the ability to cope and can grow slowly in a CO2 starved environment, the bulk of aquatic plants cannot and require CO2 to grow normally. Without it, many of these plants will require more light, grow slowly and sparsely or, in the worst case scenarios, melt and die.

Since it is required for photosynthesis, CO2 remediation is transformational for cultivating vibrant, healthy aquatic gardens featuring dense growth and diverse plants. Its ability to enhance photosynthetic efficiency and prevent photorespiration damage opens the door to faster, better growth, vibrant colors, and enhanced plant health. Furthermore, these benefits extend across a wide range of aquatic plant species. From the dense carpets of Hemianthus callitrichoides to the resilient fronds of Java Fern, CO2 supplementation is either required or highly recommended to make the most out of your planted aquarium. It also has the benefit of decreasing light requirements and inhibiting the growth of nuisance algae.

Sparsely growing planted aquarium without CO2
Aquariums without CO2 can support only a limited selection of plants and will usually be much sparser than with CO2 injection

Through careful management and an understanding of the synergy between light and CO2, aquarists can create underwater landscapes that captivate and inspire, showcasing the profound beauty and complexity of aquatic life without inviting excessive algae. Unless you plan on growing only the few species of plants able to make do with emergency sources of inorganic carbon, CO2 supplementation should be very high on your list of priorities, above or equal to lighting upgrades.

Aquatic plants are expensive and some less common species are hard to get. With simple all-in-one systems available these days, why waste your time and money while risking the lives your not just your plants but all of your planted aquarium inhabitants. CO2 supplementation basically guarantees success in aquarium gardening and is something that every aquarist should think carefully about.


References and Further Reading

  1. General Plant Biology and Photosynthesis:

    • Taiz, L., Zeiger, E., Møller, I. M., & Murphy, A. (2015). Plant Physiology and Development. Sinauer Associates, Inc. This textbook provides a comprehensive overview of plant physiology, including detailed chapters on photosynthesis, which is central to understanding the role of CO2 in plant growth.

  2. Aquatic Plant Care and CO2 Supplementation:

    • Walstad, D. (2013). Ecology of the Planted Aquarium: A Practical Manual and Scientific Treatise for the Home Aquarist. Echinodorus Publishing. Diana Walstad's book covers the ecology of planted aquariums, including the benefits of CO2 supplementation and strategies for optimizing plant health.

  3. Aquascaping Techniques and Plant Growth:

    • Amano, T. (2011). ADA Nature Aquarium: Complete Works 1985-2009. Aqua Design Amano Co., Ltd. Takashi Amano's complete works showcase his aquascaping projects and techniques, including the use of CO2 to enhance plant growth and aesthetics in aquariums.

  4. Specific Plants and CO2 Requirements:

    • Barr, T. (2007). The Barr Report. This online forum and publication by Tom Barr discuss various aspects of planted tank management, including the specific CO2 needs of carpet plants and low light plants, offering practical advice for hobbyists.

  5. Practical Guide to CO2 Supplementation:

    • Nielson, G. (2016). Carbon Dioxide in Planted Freshwater Aquaria. Freshwater Aquariums. This guide provides hobbyists with practical information on implementing CO2 supplementation in planted tanks, including diffusion methods and monitoring CO2 levels.

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