Allelopathy of Synechococcus increases diversity of coexisting phytoplankton communities

Picocyanobacteria of the genus Synechococcus are a widespread component of marine planktonic communities and seasonally dominant in the Baltic Sea. Many Synechococcus strains have allelopathic properties, but the role of allelopathy remains unknown. Recent studies in 2-species systems provided experimental evidence for increased diversity at intermediate levels of allelopathy, while extreme levels (low or high) led to reduced diversity. Here, we studied the role of Synechococcus allelopathy in structuring coexisting phytoplankton communities. We brought natural phytoplankton communities from the Baltic Sea to equilibrium in semi-continuous cultures under nitrate limitation. These communities were inoculated with 2 Baltic Synechococcus strains (BA-124 and BA-132) known to have allelopathic properties. We used low, medium, and high cell densities as a proxy for allelopathy strength. With this design, according to classical resource competition theory, the dominant species at equilibrium should be the same irrespective of the initial inoculum of any species. Our equilibrium communities were dominated by 2-4 diatom genera at all Synechococcus levels. Community diversity was significantly higher only at the medium inoculum of strain BA-124, consistent with allelopathic effects shown in 2-species studies. There is no other clear factor that could produce such an effect, considering the experimental design. Therefore, we state that allelopathy is the best hypothesis to explain these results. In strain BA-132, the weaker allelopathic effect did not alter community diversity, with constant abundance in equilibrium indicating exploitative competition as the main driver of structure. These results suggest that allelopathy may be a driving factor of phytoplankton succession in the Baltic Sea.