Golden alga (Prymnesium parvum) is a microscopic, single-celled species of algae. See family tree. It occurs worldwide, and can live in a wide range of water temperatures and salinities. It has a complex life cycle and can form resting cysts under unfavorable conditions. In its vegetative state, golden alga has two hair-like flagella used to swim through the water. There is also a shorter, stiff hair-like structure called a haptonema, which can be used to attach the cell to objects.
The nucleus is clear and cannot be seen under a microscope without adding stains to color it. The cell has a yellow-green, C-shaped chloroplast that wraps around the middle. During a bloom when there are billions of cells, these chloroplasts often lend a yellow or gold tint to the water. This tint has given this alga its common name.
What does it eat?
Golden alga is part of the plankton community that forms the base of the food chain in aquatic environments. Single-celled algae are among the phytoplankton (microscopic plants), which are eaten by zooplankton (microscopic animals). Zooplankton are eaten in turn by aquatic insects and small fish, which provide food for bigger fish. It sounds simple, but in reality the food chain is more like a web, with complex interactions at the low end. Different species of plankton compete for food and often eat each other.
As a plant containing chlorophyll, golden alga can make its own food using sunlight and inorganic nutrients found in the water. However, it's also capable of preying on other organisms. Under certain kinds of stress, golden alga cells release at chemical compounds that combine with minerals in the water to make toxins. These toxins appear to serve several purposes:
- Slow down bacteria and other algae, making it easier for the golden alga to catch and eat them.
- Destroy cells of some organisms, causing them to release dissolved organic matter which then becomes food for the golden alga.
- Repel some of the zooplankton that graze on golden alga.
- Inhibit growth of other algae species.
These survival characteristics appear to provide golden alga with the ability to out-compete other organisms and form high-density blooms that can significantly disrupt aquatic ecosystems.
How does it harm fish?
Golden alga toxins attack cells, allowing excess water and waterborne chemicals inside. Some organisms have protective layers to prevent this, but the exposed cells on fish gills and fins are not so protected. Toxins attack the outside layer of cells, then the next layer is affected, and so on. Soon the gills are so badly damaged that they are unable to function. Blood vessels in the gills leak into the water. At the same time, toxins and other waterborne chemicals are getting into the circulatory system of the fish, damaging internal organs. Fish behave as if there is not enough oxygen in the water. They travel at the top of the water surface or rest on the bottom in edges and shallow areas.
Usually when a toxic bloom lasts one month or less, a partial fish kill occurs, and the water body recovers. Fish will get away from the bloom area if they can, seeking underwater springs and seeps or places where creeks flow in and dilute the toxins.
When a bloom lasts many months and involves a large part of the water body, all populations of fish may be reduced. This means fewer fish to catch or reproduce. Forage fishes such as sunfish, threadfin shad, and most minnows grow fast and produce large numbers of eggs, so they tend to recover quickly. Predators such as catfish and bass need the forage fishes for food and take several years to reach reproductive age. A water body that has suffered a major fish kill may need to be restocked with game fish, and can take years to recover. If there are subsequent fish kills caused by golden alga blooms, recovery can take longer.
What triggers toxic blooms?
The dynamics of bloom formation still aren't well understood. Golden alga can live in a water body for months, even years, without causing a fish kill. Scientists suspect that a number of factors — temperature, water chemistry, rate of stream flow — combine to give golden alga an advantage over other plankton species and lead to a population explosion.
Can fish kills be predicted?
Golden alga cells are quite small, but they can be viewed with a microscope. When biologists examine water samples, they look for two indicators: the number of golden alga cells in a given volume of water, and the percentage of golden alga in comparison to other microorganisms.
- Increased cell counts have generally been found prior to fish kills in Texas. Golden alga can produce enough toxin to cause a fish kill when cell concentrations are as low as 10,000 cells/milliliter, but losses typically do not occur until counts reach at least 20,000 cells/milliliter.
- When golden alga dominates the phytoplankton community, accounting for 50% or more of the total population, there is an increased tendency for a bloom to cause a fish kill.
However, cell counts don't always correlate with toxicity, and a fish kill may not result even when golden alga becomes dominant.
Does it affect other animals?
Unlike toxic red tide blooms on the coast, golden alga toxins appear to have no lethal effect on non-gill breathing organisms. Cattle, predators, scavengers, and birds have been observed drinking water during a bloom, and many eat dead fish from golden alga kills with no apparent effects. The toxins break down in acid conditions, such as those found in the stomach. Also, terrestrial animals have skin layers to protect them; these same layers protect them from the toxins. Officials from the Texas Department of State Health Services have stated that the golden alga is not known to be a human health problem. Still, people should not pick up dead or dying fish to eat.
- Report Kills - If you see a fish kill or suspect golden alga, contact one of TPWD's 24-hour communications centers at 512-389-4848 (Austin) or 281-842-8100 (La Porte).
- Get the Facts - TPWD has collaborated with the Texas Commission on Environmental Quality and other entities to produce a golden alga information card. Download a PDF from the TCEQ website or request a free hard copy from TPWD at firstname.lastname@example.org.