COS 162-4 - Bottom-up regulation of phytoplankton community diversity by a mechanism linking limiting nutrient and allelopathic compounds

Abstract: The role of allelopathy in phytoplankton community structure is completely unknown. Since decades ago, theoretical works have shown the potential of this phenomenon in promoting phytoplankton diversity. However, this principle was only recently demonstrated experimentally by our research group in a simple two-species system. Our more recent advances in this system revealed a direct positive link between nitrate availability and allelochemical production, which mechanistically explains the dynamics of the system. This link is explained by the high nitrogen content of the allelochemicals involved (small cyclic peptides known as portoamides). Species coexistence and hence increased diversity, was observed at intermediate effects of allelopathy, whereas extinction of each different competitor was observed at extreme (low or high) effects of allelopathy. Our aim in the present work was to test if this principle, driven by the same bottom-up mechanism would explain community structure and diversity in more complex communities. The approach consisted in bringing natural phytoplankton communities to equilibrium in continuous cultures established in the laboratory. These communities were inoculated (except the controls) with Phormidium sp. the cyanobacterial species producing the allelochemical portoamide. A treatment consisting in manipulating nitrate availability was set with three levels: low, intermediate and high. These systems were run until equilibrium and during a total of 73 days. Allelochemical and nitrate concentrations were directly monitored. Community composition was estimated at different time points by microscope counts (the eukaryote community) and sequencing in the ribosomal 16S RNA (the prokaryote). Our results revealed a completely different community composition at each of the three nitrate levels during equilibrium. The communities were dominated by 3-6 species, being the expected strong competitors (diatoms) dominant at low levels, chlorophytes and diatoms at intermediate levels, and chlorophytes and Phormidium sp. at high nitrate levels. Community diversity was significantly higher, as predicted, at the intermediate effects of allelopathy (intermediate nitrate and presence of Phormidium sp.). Our results are among the first demonstrations of the role of allelopathy in phytoplankton dynamics, and the first in a complex community, with the relevant added value of a mechanistic description.