No matter how pure they look, seas, rivers and even puddles are havens for microscopic organisms such as diatoms. These single-celled algae make up a significant proportion of phytoplankton species in many marine and freshwater environments. In fact, they seem to act as the grass of the ocean, providing food for a wide range of animals from zooplankton to larger invertebrates. However, at certain times of the year these apparently benign algae proliferate out of control, forming algal blooms. One explanation for this phenomenon is that a minority of diatoms may be toxic, harming the predators that feast on them and allowing the algae to proliferate unchecked. Serge Poulet from Roscoff, France, explains that these toxic diatoms have evolved sophisticated defences against being eaten by producing aldehydes. Curious to know how these diatomic aldehydes bring about their deadly effects, Poulet and his research team decided to investigate the effects of an aldehyde,decadienal, on one of the diatom's predators, the copepod(p. 2935).
Poulet explains that his initial fascination lay not with the flourishing algal blooms, but with the copepods that dine on them. He knew that copepods that consume toxic diatoms seemed to suffer fertility problems; some of their eggs fail to develop or hatch, while any larvae that develop are deformed. But it wasn't clear how the diatoms wreaked their damage.
Poulet and his team monitored the effects of an aldehyde, decadienal, on a variety of marine and non-marine specimens and found that while most of the organisms were vulnerable to the aldehyde, some bacteria and fungi were resistant to the chemical. Were the yeast and bacteria protected by their tough cell walls? The researchers exposed yeast lacking a key cell wall component to decadienal, and found that the yeast were as vulnerable as the copepods that had originally piqued his interest. Poulet suspects that the yeast and bacteria may be protected from the toxic effects of decadienal by low cell permeability, while higher organisms such as copepods that lack substantial cell walls are vulnerable to the aldehyde's toxic effects.
But how was the toxic aldehyde mediating its effects once it had penetrated the cell wall? The scientists decided to take a closer look at the aldehyde's effects on vulnerable oyster blood cells. A battery of tests revealed that decadienal affects the cytoskeleton and causes apoptosis, as well as inhibiting other cellular processes. Poulet explains, `the biological toxicity of these aldehydes is due to the double bond within them - they have a high affinity for biological molecules.' It is likely that decadienal reacts indiscriminately with enzymes in many physiological pathways within the oyster cells, causing a wide range of noxious effects. The observations from other organisms, such as the copepods' fertility problems, are probably also due to widespread interference with many cellular processes.
These results suggest that under normal circumstances toxic diatoms make up a small but harmful minority of phytoplankton. But Poulet warns that subtle changes in the environment, perhaps due to human activity, can cause dramatic rises in toxic diatom populations, which have knock-on effects for animals higher up the food chain.