Algae are diverse photosynthetic organisms, ranging from microscopic, single-celled organisms to large seaweeds. Like terrestrial plants, carbon dioxide (CO2) is fundamental for algae’s survival and growth through photosynthesis. This process converts light energy and CO2 into chemical energy, forming the base of many aquatic food webs. Whether CO2, a life-sustaining nutrient, can become lethal depends entirely on its concentration in the water. While low or moderate CO2 levels are necessary for thriving algae, excessively high concentrations can transition this nutrient into a potent toxin, with the resulting chemical changes proving fatal.
CO2 as an Essential Nutrient for Algae Growth
Algae, as photoautotrophs, rely on carbon dioxide as their primary carbon source for photosynthesis. Carbon fixation is facilitated by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), which catalyzes the first major step of the Calvin cycle. Because the aquatic environment naturally contains low concentrations of dissolved CO2 compared to air, many algae have evolved sophisticated Carbon Dioxide Concentrating Mechanisms (CCMs). These specialized systems actively transport and accumulate inorganic carbon, often as bicarbonate (HCO3-), inside the cell.
CCMs ensure a high concentration of CO2 is delivered directly to the RuBisCO enzyme, maximizing photosynthetic efficiency. This adaptation allows microalgae to sequester carbon efficiently, sometimes 10 to 50 times more efficiently than terrestrial plants. Consequently, CO2 is often a growth-limiting factor under normal conditions, meaning increased availability promotes algal growth and proliferation. Low or normal levels of dissolved CO2 are necessary for healthy algal populations.
The Lethal Effect of High Carbon Dioxide Concentrations
While low concentrations stimulate growth, carbon dioxide becomes lethal to algae only at extremely elevated concentrations. The threshold for toxicity is highly dependent on the algal species, with some strains displaying remarkable tolerance. For example, the growth of the species Dunaliella salina can be completely blocked when CO2 concentrations reach around 25% by volume in the gas supply.
Other species, such as Chlorella vulgaris, exhibit a much higher tolerance, maintaining their growth rate even when exposed to CO2 concentrations of 50% or more. This inhibitory effect is not caused by the CO2 molecule acting as a direct poison, but rather through the profound environmental changes it induces. These toxic concentrations are far above the natural dissolved CO2 levels found in the atmosphere-equilibrated water. High CO2 stress causes a decline in the viability of sensitive strains, observed as a significant reduction in biomass productivity. This suppression of growth is a consequence of the internal and external acidification caused by the excess dissolved gas.
The Role of pH and Carbonic Acid in Algae Suppression
The mechanism by which high CO2 concentrations suppress or kill algae is rooted in the chemistry of carbon in water. When CO2 dissolves, a portion of the gas reacts with water (H2O) to form carbonic acid (H2CO3). This chemical reaction is the initial step in a cascade that leads to the acidification of the water. Carbonic acid is a weak acid that quickly dissociates, releasing hydrogen ions (H+) into the solution.
The influx of hydrogen ions causes the water’s pH level to drop rapidly, making the environment highly acidic. This drop in pH is detrimental to the physiological functions of most algae, as cellular life requires a tightly controlled pH range. The low external pH disrupts the proton gradient across the cell membrane, which is essential for energy production and nutrient transport.
Acidic conditions lead to an intracellular pH drop, inhibiting the activity of critical enzymes. Many metabolic enzymes, including those in the photosynthetic pathway, are sensitive to pH fluctuations and can become denatured or non-functional in an overly acidic environment. The inability to maintain stable internal pH homeostasis ultimately leads to the suppression of growth, metabolic failure, and, at extreme concentrations, the death of the algal cell.
Applying CO2 Management in Aquatic Environments
The understanding of CO2’s dual nature—nutrient versus toxin—is applied practically in controlled aquatic environments, such as planted aquariums and commercial aquaculture. In planted aquariums, CO2 injection systems promote the growth of desirable aquatic plants, not to kill algae directly. Boosting plant growth allows plants to outcompete algae for limiting nutrients like nitrates and phosphates, serving as an effective indirect control strategy.
Aquarists must carefully monitor the CO2 injection rate. The resulting pH drop, while beneficial for higher plants, can be lethal to fish and invertebrates if it falls too low. Periodic pH swings are also utilized; injecting CO2 during the day and turning it off at night stresses many types of algae. Since most algae thrive in stable pH conditions, this daily cycling prevents them from establishing a foothold.
In commercial settings, the toxicity threshold is sometimes exploited for species that are highly susceptible to CO2 stress. However, the common application is finding the optimal balance where CO2 maximizes microalgae growth for biofuel or bioproduct production. This management requires constant monitoring and adjustment to ensure the environment remains productive without inducing the toxic effects of excessive acidification.