Zinc stimulation of coastal productivity in low carbon dioxide environments

Zinc (Zn) is a key micronutrient used by phytoplankton for carbon (C) acquisition, yet there have been few observations of its influence on natural oceanic phytoplankton populations. In this study, we observed Zn limitation of growth in the natural phytoplankton community of Terra Nova Bay, Antarctica, due to low (∼220 μatm) pCO2 conditions, in addition to primary iron (Fe) limitation. Shipboard incubation experiments amended with Zn and Fe resulted in significantly higher chlorophyll a content and dissolved inorganic carbon drawdown compared to Fe addition alone. Zn and Fe response proteins detected in incubation and environmental biomass provided independent verification of algal co-stress for these micronutrients. These observations of Zn limitation under low pCO2 conditions demonstrate Zn can influence coastal primary productivity. Yet, as surface ocean pCO2 rises with continued anthropogenic emissions, the occurrence of Zn/C co-limitation will become rarer, impacting the biogeochemical cycling of Zn and other trace metal micronutrients. Significance Statement Zinc has tremendous nutritional importance in life, and due to its central role in carbon acquisition in major phytoplankton species, is directly connected to ocean carbon cycling. As a result, it has been hypothesized that co-limitation by zinc and CO2 could influence primary productivity, but a connection between Zn and CO2 had not yet been observed in field experiments. The present study demonstrates that Zn can be a limiting nutrient in natural waters, particularly in coastal environments where CO2 is diminished by bloom events, and explores the implications for Zn-C co-limitation in the context of climate change: the oceans may have decreased susceptibility to Zn-C co-limitations in the future due to the rapid input of anthropogenic CO2 to the atmosphere. ### Competing Interest Statement The authors have declared no competing interest.