The Importance of Algae
The Role of Algae in Oyster Development
Most species of algae are microscopic unicellular organisms and are found in water. Similar to plants, algae use photosynthesis to produce energy. Many species of algae can be found in the Chesapeake Bay and its tributaries. Algae cultured in the hatchery are very small (2-30 microns). One milliliter of water can hold millions of algae cells.
Certain species of algae are known to be more beneficial due to size and nutrition for oyster larvae and newly settled oyster spat. Several of these species are grown at the Horn Point Oyster Hatchery to feed oyster larvae and young spat. In general, adult oysters and older spat are not fed due to the large quantities of algae needed, however, broodstock oysters are sometimes given supplemental algal feedings to enhance gonad production.
Algal Culture Techniques
Different volumes of algae are grown at the hatchery for different purposes. Small scale stock cultures are grown to provide a supply of starter cultures to inoculate larger volumes of water. Intermediate size cultures are grown to inoculate the large scale cultures. The large scale cultures are then used to feed the oyster larvae. When growing algae for the hatchery, cultures at all scales require sterile water, f/2 nutrient media, light, air, and CO2.
Stock cultures are small monospecific cultures started with small cultures that have been obtained from culture collections at reputable institutions. The hatchery has two sizes of stock cultures- 125mL and 500mL. The 125 mL flasks are only used to inoculate new stock cultures every three weeks. The larger 500 mL stock cultures are used to inoculate the intermediate scale of cultures. Since these cultures are used to start all other levels of cultures, they need to be kept sterile and the risk of contamination needs to be minimized.
Carboys are larger cultures used to inoculate the mass algal tanks. Our hatchery uses 20 liter glass containers.
In our greenhouse, 18- 5000L mass algal tanks are used to provide large scale cultures that can be used to feed the oysters, primarily larvae. After being inoculated with several carboys, the algae blooms for approximately 4-6 days. Densities may reach 1-4 million cells/mL. These large scale cultures can be batch cultures or semi-continuous cultures. Batch cultures are completely harvested. Once the tank is drained, it can be sterilized and inoculated with a new culture. Semi-continuous cultures are only partially harvested before being refilled with sterile water and nutrient media and the same culture is allowed to rebloom.
In order to produce high density cultures, new technologies have been implemented including Solatubes and a new pH sensor system. Solatubes allow light to be concentrated and directed to each of the mass algal tanks and help the cells get sunlight even on overcast days. The pH sensors are a part of an accurate feedback system that allows the computer to direct carbon dioxide into the algae cultures in order to help control the pH among other factors.
Since algae is the primary food source of oysters, algal culture is essential to the success of most hatchery operations. In order to meet the nutritional needs of the oyster larvae grown in the hatchery, large quantities of certain types of algae need to be produced continuously through the hatchery season. The species of algae grown are selected based on research and industry standards and what type of algae can be grown in dense cultures at our facility.
A mixed algal diet of flagellates and diatoms is needed for better oyster larval growth and development. Currently the Horn Point Oyster Hatchery feeds the oyster larvae with a mixture of Isochrysis sp. Thalassiosira pseudonana, and Chaetoceros sp. Once the larvae are larger and older, they are also fed Tetraselmis chui, which is a larger celled algal species.