Investigating zinc toxicity responses in marine Prochlorococcus and Synechococcus

Marine plastic pollution is a growing concern worldwide and has the potential to impact marine life via leaching of chemicals, with zinc (Zn), a common plastic additive, observed at particularly high levels in plastic leachates in previous studies. At this time, however, little is known regarding how elevated Zn affects key groups of marine primary producers. Marine cyanobacterial genera Prochlorococcus and Synechococcus are considered to be some of the most abundant oxygenic phototrophs on earth, and together contribute significantly to oceanic primary productivity. Here we set out to investigate how two Prochlorococcus (MIT9312 and NATL2A) and two Synechococcus (CC9311 and WH8102) strains, representative of diverse ecological niches, respond to exposure to high Zn concentrations. The two genera showed differences in the timing and degree of growth and physiological responses to elevated Zn levels, with Prochlorococcus strains showing declines in their growth rate and photophysiology following exposure to 27 µg l^-1 Zn, while Synechococcus CC9311 and WH8102 growth rates declined significantly on exposure to 52 and 152 µg l^-1 Zn, respectively. Differences were also observed in each strain's capacity to maintain cell wall integrity on exposure to different levels of Zn. Our results indicate that excess Zn has the potential to pose a challenge to some marine picocyanobacteria and highlights the need to better understand how different marine Prochlorococcus and Synechococcus strains may respond to increasing concentrations of Zn in some marine regions.