Copper and light shape a coastal picophytoplankton community via their combined effects on growth limitation and toxicity

Abstract Copper (Cu) and light are resources that limit phytoplankton growth at very low (deficiency) or very high (toxicity) levels. The interactive effects of Cu and light on a coastal California picophytoplankton community were assessed during four incubation experiments using a 7x7 matrix of overlapping Cu and light gradients. Consistent with prior knowledge, sensitivity to Cu was greatest in Prochlorococcus, followed by Synechococcus, and then picoeukaryotes. In September, Prochlorococcus abundance declined with Cu additions >6 nM, whereas Synechococcus showed a toxicity threshold at >10 nM added Cu. An unexpected environmental increase in ambient seawater Cu prior to the October experiments brought Prochlorococcus and Synechococcus close to their toxicity thresholds, potentiating their sensitivity to light during October experiments. Synergistic effects between Cu and light exacerbated toxicity in both taxa, suggesting saturation of shared physiological stress response pathways. Addition of 10 mM nitrate at the start of one experiment did not rescue populations from this synergistic toxicity, eg, by allowing de novo stress response enzyme synthesis. Across all experiments, picoeukaryotes were more resilient to high light and Cu, allowing them to persist or increase under conditions that limited Prochlorococcus and Synechococcus. This robustness combined with relief from resource competition upon decline of the other two taxa, ultimately led picoeukaryotes to dominate the communities despite having very low baseline relative abundances. Selection for different Synechococcus clades and picoeukaryote species likely permitted each of these populations to thrive over a broader range of Cu and light combinations than would be possible for populations with lower diversity.