Trace metal limitations (Co, Zn) increase PIC/POC ratio in coccolithophore <em>Emiliania huxleyi</em>

International audienceThe effect of cobalt (Co), zinc (Zn) or cadmium (Cd) availability on the growth and particulate inorganic (PIC) to organic (POC) carbon ratio of the cosmopolitan coccolithophore Emiliania huxleyi was examined using batch cultures. Growth co-limitation by Co and Zn occurred at low free metal ion concentrations below 10–12 pM. The two metals could replace one another in biochemical functions: full replacement of Zn by Co was possible, but the 20% higher growth rate observed with high Co levels in the absence of Zn than observed in the presence of high Zn but no added Co suggested that there was an absolute growth requirement of Co that could only partly be met by Zn. In contrast, Cd had no effect on growth in the absence of Co and Zn, indicating that this strain cannot use Cd under these conditions. The co-limitation by inorganic Co and Zn led to highly calcified cells containing a large quota of nitrogen, but diminished organic carbon content, likely due to a lower photosynthetic production rate. Deficiency of Zn and Co cofactors could have dramatically decreased the activity of carbonic anhydrase, lowering CO2 availability for photosynthesis. In contrast to the large decreases in carbon fixation rates at low Zn and Co ion concentrations, calcification rates were unaffected at low Zn levels and were only minimally decreased by low Co. As a result the PIC:POC ratios in E. huxleyi increased by up to 8-fold under Zn limitation and 3–5-fold under Co limitation to values where coccolithophores act as a source of atmospheric CO2. As a result, low Co and Zn availability in seawater could affect the efficiency of the biological carbon pump, and atmospheric CO2 levels. Thus, the impact of Co and Zn limitation may need to be considered to improve assessments of the impact of E. huxleyi and other biogenic calcifiers on oceanic sequestration of CO2