FEEDBACK INTERACTIONS BETWEEN ZINC AND PHYTOPLANKTON IN SEAWATER

In Zn ion-buffered media, oceanic species (Thalassiosira oceanica and Emiliania huxleyi) grew at near-maximal rates at the lowest free Zn ion concentration ([Zn2+] = 10(-12.3) M), whereas coastal species (Thalassiosira pseudonana and Thalassiosira weissflogii) were limited at [Zn2+] < 10(-11) M. The ability of the oceanic species to outgrow coastal ones at low [Zn2+] was due almost entirely to a reduced growth requirement for cellular Zn rather than to an increased capability for uptake. All isolates exhibited similar sigmoidal relationships between cellular Zn:C ratios and [Zn2+] with minimal slopes at [Zn2+] of 10(-10.5) to approximately 10(-9.5) M and increasing slopes above and below this range. The minimal slopes at intermediate [Zn2+] could be explained by negative feedback regulation of a high-affinity Zn uptake system, while increased slopes at high [Zn2+] appeared to be related to uptake by a low-affinity site. Measured relationships between cellular Zn:C ratios and [Zn2+] agreed well with those computed from a modified Redfield model based on depth profiles for Zn and PO4 concentrations and Zn chelation in the nutricline of the North Pacific. This agreement provides evidence that Zn concentrations in the nutricline are controlled by biological uptake and regeneration as occurs for major nutrients.