A Potential Novel Link Between Organic Nitrogen Loading and Pseudo-nitzschia spp. Blooms

摘要

Recent blooms of the toxigenic microalgae Pseudo-nitzschia spp. have led to mass mortality among dolphins, sea lions and birds along the California coast, due to domoic acid (DA) poisoning. Given the broad range of marine organisms susceptible to DA poisoning, there is considerable concern that Pseudo-nitzschia spp. blooms will have far-reaching and long-lasting negative impacts on coastal fisheries, the diversity and resilience of marine ecosystems and the quality of life in California coastal communities. Since the first reported bloom of Pseudo-nitzschia spp. in Monterey in 1991 along the West coast of the United States, their re-occurrence seems to be increasing in frequency and over a greater geographical range. The simplest potential explanation for these events is that some environmental change has occurred in some regions of the California coastal zone that increases the competitive advantage of Pseudo-nitzschia spp. Based on present literature and our in situ observations of Pseudo-nitzschia spp. sensitivity to solar ultraviolet (UV) radiation, we developed a testable working model. Our working model hypothesizes that relatively high UV-tolerance in surface waters and the ability to augment photoautotrophy with chemoheterotrophy have increased the probability of Pseudo-nitzschia spp. blooms as eutrophication of coastal waters and levels of damaging UV radiation penetrating the ocean surface have increased significantly. Here we report the results of a pilot study to explore the possible competitive advantages that Pseudo-nitzschia spp. may gain from supplementing photoautotrophic growth with the heterotrophic uptake of dissolved organic nitrogen (DON) in the dark. Heterotrophy could prove to be an important strategy for dark survival when Pseudo-nitzschia spp. blooms end and cells settle out of the euphotic zone. While many aspects of their autoecology have been defined in recent years, few studies have explored heterotrophic dark survival strategies of Pseudo-nitzschia spp. that may increase its success due to the eutrophication of coastal waters. Given this heterotrophic ability a higher fraction of cells would remain viable at depth, increasing the probability that surface blooms will be initiated whenever these seed populations are returned to the euphotic zone.