Ability of the marine diatoms Pseudo-nitzschia multiseries and P. pungens to inhibit the growth of co-occurring phytoplankton via allelopathy

Diatoms within the genus Pseudo-nitzschia can form near-monospecific blooms in both natural and iron-fertilized high-nutrient, low-chlorophyll (HNLC) regions and can have detrimental impacts on marine ecosystems. Here, we demonstrate the ability of P. pungens isolated from the South China Sea and 2 strains of P. multiseries isolated from the Bay of Fundy, Canada, to produce extracellular compounds capable of lysing and/or inhibiting the growth of multiple phytoplankton species. Since the allelopathic activity was found in both P. multiseries, which produces domoic acid (DA), and P. pungens, which produces little if any DA, the allelopathic effects of Pseudo-nitzschia spp. seem to be unrelated to this compound. Allelopathic inhibition of other phytoplankton was documented during exponential and stationary growth phases of Pseudonitzschia, and the strongest allelopathic effects were obtained from sonicated cultures, suggesting that the sudden release of allelochemicals via processes such as cell lysis or zooplankton grazing may have the strongest effect in an ecosystem setting. Differences in the responses of target species to Pseudo-nitzschia spp. suggest these algae may produce multiple compounds that vary in their allelopathic potency and composition as a function of species, strain, growth stage, and perhaps other factors. Collectively, these results suggest that the allelopathy may affect competition between Pseudo-nitzschia spp. and other phytoplankton and may play an important role in the formation and persistence of natural and iron-fertilized blooms.