They also found that these small diatoms became particularly toxic under low light levels a condition that usually slows the growth of phytoplankton. The species, P. cuspidata, underwent an up to 50 fold increase in toxicity under low light levels compared to the conditions that are thought to normally favor phytoplankton growth.
Scientists already know that in some large-celled species of Pseudo-nitzschia their toxicity increases when the cells grow slower, but in previous studies the slowing of cellular growth was due to the limitation of vital nutrients, such as silicate. However Cochlan's latest study found that the toxicity of these small toxigenic diatoms is affected by the type of nitrogen they consume. He found that under low light levels -- leading to slow growth -- phytoplankton cells that were fed on naturally occurring nitrate were more toxic than cells that were fed on either urea or ammonium caused by pollution.
"Our results demonstrate that the reason for the growth of these specific harmful algal blooms off the coast of North America from British Columbia to California may in fact be due to totally natural causes," Cochlan said.
Such toxic algal blooms may be largely supported by the natural upwelling of nitrogen. However, Cochlan cautions that when the pattern of upwelling is weaker, nitrogen from pollution could play an important role in sustaining a "seed population" of harmful algae a remnant that keeps the bloom going until upwelling resumes and the bloom is able to grow again and perhaps increase their toxic effect on the marine ecosystem.
"This is the first physiological study to look at the environmental conditions that promote both the growth and the toxicity o
|Contact: Elaine Bible|
San Francisco State University