Phytoplankton are microscopic plants, or microalgae, that form the foundation of most aquatic food webs. These single-celled organisms convert light energy into biomass, making them rich in essential fatty acids, vitamins, and proteins. Culturing phytoplankton at home provides a continuous, high-quality food source for marine organisms. Hobbyists primarily cultivate them for feeding filter-feeding invertebrates, such as clams and corals, and for raising zooplankton like rotifers and copepods. A successful home culture ensures a steady supply of this nutrition-packed food, supporting the growth and health of a reef tank ecosystem.
Necessary Equipment and Environment
The cultivation process begins with a simple, sterile setup, typically utilizing clear plastic or glass containers. Two-liter beverage bottles or dedicated culture vessels are popular choices because their narrow neck helps limit airborne contamination. Sterilizing these containers, often by rinsing with a dilute bleach solution or isopropyl alcohol followed by thorough rinsing with reverse osmosis (RO) water, ensures a clean start.
Phytoplankton require light for photosynthesis, which is best supplied by an artificial source set on a timer. LED or fluorescent bulbs with a color temperature between 4000 and 6000 Kelvin (K) are effective, as this range mimics the light spectrum these organisms use most efficiently. While a continuous 24-hour light cycle maximizes growth, a 16-hour light period followed by 8 hours of darkness is often recommended to reduce heat buildup and conserve energy. The culture temperature should be maintained between 70 and 85 degrees Fahrenheit (21–29 degrees Celsius) for common species like Nannochloropsis or Tetraselmis.
A constant stream of air is supplied using an aquarium air pump, air line tubing, and a rigid air stone placed at the bottom of the culture vessel. This aeration serves two purposes: providing the necessary carbon dioxide for photosynthesis and keeping the microalgae suspended in the water column. Without sufficient mixing, the cells can settle out of the solution, leading to cell death and culture collapse. The bubbling should be gentle, creating a soft rolling motion rather than a violent churn, to prevent physical damage to the cell walls.
Formulating the Nutrient Medium
The growth medium requires a carefully prepared water base enriched with specific chemical nutrients. For marine species, the foundation is saltwater mixed to a salinity between 1.015 and 1.025 specific gravity, using only RO or deionized (DI) water and a quality marine salt mix. Sterilization of this base water is achieved by boiling, microwaving, or using a chemical sterilant like bleach, followed by neutralization and rinsing. This step eliminates competing bacteria and other microalgae that could quickly overrun the desired culture.
Once the water is sterilized, it must be fortified with nutrients that support rapid cell division. The primary requirements are nitrogen and phosphorus, typically supplied as nitrates and phosphates. Nitrogen is used for synthesizing amino acids and proteins, while phosphorus is essential for energy transfer molecules like ATP and DNA structure.
Trace minerals are a necessary component of the medium, with iron being important for cellular enzyme systems involved in photosynthesis. Commercial phytoplankton fertilizers, such as Guillard’s f/2 medium, provide a balanced delivery of these macro- and micronutrients. The culture’s pH will naturally increase during the day as the phytoplankton consume dissolved carbon dioxide, but the buffering capacity of the saltwater base keeps this change within a tolerable range.
Establishing and Monitoring Growth
The culture process begins with inoculation, introducing a starter culture into the freshly prepared nutrient medium. A common practice uses a ratio of one part starter culture to four or five parts of new medium, making the inoculum 10 to 20% of the final volume. This high initial cell density helps the desired species quickly establish dominance and reduces the risk of contamination.
After inoculation, the culture enters a short lag phase as the cells acclimate to the new environment before entering exponential growth. Maintaining the air pump 24 hours a day ensures consistent gas exchange and prevents the cells from settling. The light cycle should be kept consistent to provide the energy needed for rapid photosynthesis and cell division.
Visual monitoring is the simplest way to track the health and density of the culture. A healthy culture transitions from a light, translucent green or brown to a dark, opaque color over 7 to 10 days. A technique called “candling” involves shining a flashlight through the container; when the light beam is obscured or the air line tubing inside is blurry, the culture has reached a harvestable density. A sudden change to a clear or light-yellow color, or the development of an offensive smell, usually indicates a culture crash due to bacterial contamination or nutrient depletion.
Harvesting and Scaling Cultures
A phytoplankton culture is ready for harvest once it reaches peak density, indicated by the dark, opaque color and the inability to clearly see through the container. Harvesting at this stage ensures the maximum concentration of cells before nutrient exhaustion causes the culture to enter the decline phase. The most efficient method for home use is the semi-continuous harvest technique.
This method involves drawing off 25 to 50% of the mature culture and immediately replacing the harvested volume with fresh, sterile nutrient medium. This process keeps the culture in the exponential growth phase, allowing for frequent, sustained harvests without needing to restart a batch. If the culture is maintained this way, it can produce a steady supply for several weeks until a crash necessitates a full restart.
The harvested phytoplankton can be used immediately or stored for short periods. Refrigeration is the most common storage method, as cooler temperatures slow the cells’ metabolism and reduce their consumption of nutrients. Stored cultures should be kept in a clean, sealed container and shaken daily to prevent the cells from settling. While refrigeration can extend viability up to two months, a portion of the healthy culture should always be retained to inoculate a new batch or serve as a backup starter culture.