In the last decade diatoms have been shown to release a wide range of secondary metabolites, such as polyunsaturated aldehydes (PUAs), produced by a wound-activated mechanism. PUAs are highly reactive molecules that in most cases induce a drastic reduction in the reproductive response of predators, such as copepods.
The variable toxicity of diatoms observed in nature suggests a modulation of PUA production by environmental factors. My results indicate a strong dependence of PUA production on culture age, nutrient limitation and other stress factors, such as UV exposure and low light conditions, in cultures of Skeletonema marinoi, suggesting a direct link between toxin production and cell physiological state.
Since it has been hypothesized that PUAs may be released in seawater also as a result of cell mortality, phytoplankton lysis rates were estimated during four oceanographic cruises conducted during periods of diatom blooms in the Northern Adriatic Sea. High lysis rates were observed and preliminary results indicated substantial amounts of PUAs released in seawater. The effects of dissolved PUAs were therefore investigated on organisms other than predators, such as co-occurring bacteria and phytoplankton in culture. PUAs are toxic for several phytoplankton species at concentrations well within the range potentially produced by diatoms. Diatoms can be immunized by sub-lethal concentrations of PUAs. In this case, PUAs may act as signal molecules for bloom termination. PUAs induce also different effects on bacterial growth; some strains are inhibited while others show remarkable resistance to these compounds, and a few of them were even stimulated.
Therefore, PUAs appear to have multiple functions in diatoms, namely as chemical defense against grazers, allelochemicals against phytoplankton and bacteria, and signal molecules within their populations
