The pH scale measures the acidity or alkalinity of an aquatic environment. Algae are diverse, photosynthetic organisms that form the base of many aquatic food webs. The pH of the surrounding water is a controlling environmental factor that dictates whether an algae species will survive, thrive, or fail to grow. This chemical characteristic influences both the internal workings of the algae cell and the external availability of necessary resources.
How pH Directly Influences Algae Metabolism
The internal processes of an algae cell are regulated by specialized protein molecules called enzymes, which catalyze all metabolic reactions, including photosynthesis and respiration. Each enzyme functions best within a narrow optimal pH range. This is because pH directly affects the electrical charges on the amino acids that form the enzyme’s structure.
When the water’s pH deviates too far from the enzyme’s optimum, the change in hydrogen ion concentration causes electrical charges to shift. This alters the enzyme’s three-dimensional shape, leading to denaturation. Denaturation severely slows or completely halts metabolic activity.
The pH also impacts the integrity and function of the algae’s cell membrane, which controls the exchange of substances. If the pH is too high or too low, membrane permeability can be compromised, disrupting the cell’s ability to take in nutrients and expel waste. Maintaining a stable internal pH becomes energetically taxing when the external environment is highly acidic or alkaline, diverting energy away from growth.
pH and Nutrient Availability in Water
The ambient pH of the water alters the availability of resources algae need to photosynthesize and build new cells. A significant effect is on the form of inorganic carbon available for photosynthesis. Dissolved carbon dioxide (CO2) in water creates a chemical equilibrium with carbonic acid, bicarbonate ions, and carbonate ions.
In acidic conditions (lower pH), the equilibrium favors dissolved CO2, which most algae readily absorb. As the water becomes alkaline (higher pH), the dissolved CO2 converts into bicarbonate and then carbonate ions. Many algae species are less efficient at utilizing these carbon forms. Therefore, high pH can make carbon biologically unavailable even if the total amount is high.
The solubility of phosphorus, a nutrient required for cellular energy transfer, is also dependent on pH. In freshwater systems, high alkalinity can cause phosphorus to bind with calcium and precipitate out of the water column. This removes the phosphorus from the dissolved state, making it inaccessible to the algae. Similarly, the availability of nitrogen forms, such as ammonium and nitrate, and micronutrients like iron, are chemically regulated by the water’s pH.
Optimal pH Ranges for Algae
Algae growth is maximized when the environmental pH aligns closely with the metabolic optimum of the dominant species. For most common freshwater algae, the optimal growth range spans from slightly acidic to slightly alkaline, generally between pH 6.5 and 8.5. In this range, favorable forms of carbon and other nutrients are present, and cellular machinery functions effectively.
The specific optimum varies significantly between different groups of algae. Cyanobacteria, for instance, can tolerate and thrive in conditions too high for other algae. This tolerance provides a competitive advantage, leading to a change in the dominant species when environmental conditions fluctuate. A shift in water pH acts as a selective pressure, favoring species adapted to the new chemical state.
The Dynamic pH-Algae Feedback Loop
The relationship between pH and algae growth is a dynamic feedback loop, as algae can actively change the water’s pH. This is most noticeable during rapid algae growth, such as a bloom. During the daytime, algae actively photosynthesize, consuming dissolved CO2 from the water.
The removal of CO2, which forms carbonic acid, reduces the water’s acidity. This consumption causes the pH to rise, making the water more alkaline during daylight hours. Conversely, at night, photosynthesis ceases, and algae perform cellular respiration, releasing CO2. This influx of CO2 forms carbonic acid, causing the pH to drop, resulting in a daily, or diurnal, fluctuation.