Red Tide Control and Mitigation Program
Biological control of Karenia brevi s toxicity
Georgia Institute of Technology, Julia Kubanek
Can other organisms break down red tide toxins? This study demonstrated that native Gulf
of Mexico phytoplankton species are capable of detoxifying waters containing red tide. The phytoplankton do not kill Karenia brevis cells, but they remove brevetoxin from the water column.
Growing concern over Florida red tide impacts has motivated researchers to understand how blooms work and how to lessen their effects. There are several forms of brevetoxin (PbTx) produced by Karenia brevis. Researchers at Georgia Institute of Technology have shown the amount of the most reduced 50-90% when competitive phytoplankton species are present. Further understanding how this process occurs is an important step towards developing a biological control for red tide toxicity.
The main goals were to identify which phytoplankton competitors can degrade waterborne brevetoxins and to understand how this degradation occurs. Researchers aimed to determine whether adding live phytoplankton could be a natural biological control of Karenia brevis toxicity and whether this method could also benefit marine life.
Findings and accomplishments
Researchers learned that many phytoplankton competitors (across taxonomic groups, including
diatoms, cryptophytes, and dinoflagellates) can remove waterborne PbTx-2 (see Figure 1). Testing for removal of other brevetoxins by the diatom Skeletonema costatum showed that the detoxifying effect depends on the specific form of brevetoxin (for example, PbTx-2 and -1 were removed, but PbTx-3, -6, and -9 were not removed), suggesting that enzymes play a role in the removal of brevetoxins by competitors.
By adding brevetoxins to killed cultures of Skeletonema costatum and finding no loss of toxin,
researchers learned that live cells are required to remove brevetoxins from the water column, and that the toxin does not simply stick to cellular debris associated with the competitors. However, compounds (probably proteins) exuded by competitors are responsible for some toxin breakdown or removal.
Tests of how much Skeletonema costatum is needed to remove brevetoxins from the water showed that the quantity of competitor cells present has only a small impact on toxin removal. What is most important is the presence of competitor cells.
Experiments with marine invertebrates were also done to learn whether the effects of toxins on
marine life could also be reduced. Tests with brine shrimp (Artemia salina) showed that Skeletonema costatum reduced brevetoxins and removed all toxic effects on the brine shrimp at environmentally realistic concentrations. Tests with the sea anemone Aiptasia pallida included observing physical and behavioral responses as well as toxin levels. Results showed that Skeletonema costatum reduced, and in some cases completely protected against, the physiological damages of brevetoxin exposure.
The finding that Skeletonema costatum can reduce the toxic effects of brevetoxins on marine invertebrates supports using competitor phytoplankton species as control agents for Karenia brevis -- a mitigation strategy that not only will reduce waterborne brevetoxin levels but also could reduce negative impacts on ecosystems and marine wildlife.
Nutrient controls contributing to Karenia brevi s blooms in the Gulf of Mexico
University of South Florida, Jason Lenes
This project addresses one piece of the nutrient puzzle related to red tide. Researchers used computer models and experiments to show that increasing the amount of the nutrient silica in an ecosystem may favor the growth of more beneficial phytoplankton species rather than Karenia brevis.
Understanding how Florida red tide blooms start, grow, and maintain themselves is key to finding ways to stop or reduce their impacts. Trichodesmium, a nitrogen-fixing marine microorganism, and rotting fish killed by brevetoxins are primary food sources for Karenia brevis in the eastern Gulf of Mexico. These nutrient sources provide nitrogen and phosphorus but not silica. Large amounts of silica continually enter the Gulf from Florida’s rivers.
Although Karenia brevis does not need silica to grow, competitive and faster-growing organisms in the Gulf do. In the early stages of a bloom, organisms that are close to sources of silica may be able to compete more effectively for nutrients. This competition may help slow the growth of Karenia brevis and its potential prey.
The project used laboratory and field experiments and computer simulation models to test the role of silica in Karenia brevis growth. Researchers wanted to know how different types and amounts of nutrients available to Karenia brevis may favor growth of more beneficial species. This information can help explain how Karenia brevis blooms grow and maintain themselves in the Gulf, and possibly how altering types and amounts of nutrients might be used to control blooms.
To see whether the presence of silica resulted in competition for food sources, two test cases were run: (1) normal initial silica concentrations and (2) elevated initial silica concentrations.
In case 1, a Karenia brevis bloom began in June (see Figure 1a) in response to the “new” nitrogen provided by Trichodesmium. In late July, the bloom reached levels that would kill fish, which gave it nutrients from the rotting fish. The maximum Karenia brevis level predicted by HABSIM in early October was similar to what was seen in the 2001 bloom. Case 2 showed a similar pattern, but the higher concentrations of silica led to an increase in diatoms, which decreased the predicted overall Karenia brevis concentration by about 50 percent (see Figure 1b).
The project results and HABSIM are great starting points for bloom prediction. Future experiments will test how nutrients with and without silica can alter natural shore samples and will help show competition and dominance among co-occurring Gulf of Mexico phytoplankton species. These results will be used to further test HABSIM as a prediction tool. If models continue to show that increases in silica reduce Karenia brevis concentrations, then ways of changing the nutrient regime to treat and reduce blooms can be considered.