Environmental Controls of Brevetoxin Production of the Dinoflagellate Karenia brevis

• Main Author: Hardison, Donnie Ransom
• Format: Dissertation
• Language: English
• Subjects: biology; cellular biology
• ISBN: 9781303547232; 1303547236

Nearly annual blooms of the toxic dinoflagellate Karenia brevis have adversely impacted coastal ecology, human health, and local economies in coastal regions of the Gulf of Mexico. In experiments with several strains of this species, cells increased their toxin content in response to growth limitation by nutrients (nitrogen, phosphorus, and carbon dioxide) in accordance with the carbon:nutrient balance (CNB) hypothesis, originally formulated for terrestrial plants. The CNB states that as algae or plants slow in growth due to nutrient limitation, they will divert a greater fraction of their fixed carbon to defenses, such as toxins. This carbon diversion provides protection from overreduction of the photosynthetic apparatus and decreases grazing mortality rates as growth slows. A corollary to the CNB hypothesis is if the limiting nutrient element is contained in the toxin, then the toxin increase will also be limited. This was observed for the effect of CO 2 limitation on cellular brevetoxins which contain 67% C by weight. Under CO2 limitation, increase in cellular brevetoxins was on average 45% less than observed under P limitation. Field measurements of the toxin content in Karenia brevis cells range from 1-68 pg cell --1, with average values of 8-25 pg cell--1. The toxin per cell values in the N- and P-limitation experiments account for the full range of brevetoxin values observed in the field, with these variations being related to the effects of nutrient limitation and interstrain differences.
Carbon diverted into the production of brevetoxins was documented in each study by presenting toxin values as a percent of total cellular carbon (%C-PbTx). This normalization of brevetoxins is preferred over the commonly used per cell or per cell volume normalizations as these latter values can be affected by changes in cell size or cell carbon:volume ratios and are misleading. Under nutrient limitation, cell volume will increase or decrease depending on the nutrient thus affecting their toxin quotas. Such effects were observed under N- and P-limitation where P-limitation caused a 2-fold greater increase in brevetoxins per cell than N-limitation. However, when these toxins were normalized on a per cell volume basis, the percent toxin increase was similar, and the entire difference in toxin per cell was caused by a two-fold higher volume per cell in the P-limited cultures relative to the N-limited ones.
The results of these studies will benefit coastal managers, who initially judge the toxicity of Karenia blooms from cell count data alone. Our data show the toxin concentrations associated with a given cell can vary by up eight-fold depending on the genetic strain and whether or not the algal growth is limited by N, P or CO2. For a given cell count P-limited cells should have higher toxin contents, and it is useful to know not only whether cell growth is nutrient-limited but also the nutrient element (N, P, or C), that is limiting. Nutrient status indicators thus need to be developed for better prediction of bloom toxicity by coastal managers. These could include simple pH measurements for predicting CO2 limitation, or N:C or P:C of Karenia cells for assessing N- or P-limitation. Also, our data indicate that ongoing anthropogenic increases in atmospheric CO2 may increase the toxicity of Karenia brevis blooms by increasing the CO2 needed to support bloom growth and by increasing the likelihood that high biomass blooms will become limited by N or P rather than by CO2. Using the data from this study, coastal managers should be able to make better predictions of the toxicity of Karenia blooms, allowing them to minimize the adverse effects of these blooms on human and ecosystem health and coastal communities.