Efficient zinc/cobalt inter‐replacement in northeast Pacific diatoms and relationship to high surface dissolved Co : Zn ratios

Abstract

The importance of zinc (Zn) as a nutrient and its ability to be substituted for by cobalt (Co) have been characterized in model marine diatoms. However, the extent to which this substitution capability is distributed among diatom taxa is unknown. Zn/Co metabolic substitution was assayed in four diatom species as measured by the effect of free ion concentrations of Zn 2+ and Co 2+ on specific growth rate. Analysis of growth responses found substitution of these metals can occur within the northwest Atlantic isolate Thalassiosira pseudonana CCMP1335, the northeast Atlantic isolate Phaeodactylum tricornutum CCMP632, and within the northeast Pacific isolates Pseudo-nitzschia delicatissima UNC1205 and Thalassiosira sp. UNC1203. Metabolic substitution of Co in place of Zn in the Atlantic diatoms supports their growth in media lacking added Zn, but at the cost of reduced growth rates. In contrast, highly efficient Zn/Co substitution that supported growth even in media lacking added Zn was observed in the northeast Pacific diatoms. We also present new data from the northeast Pacific Line P transect that revealed dissolved Co and Zn ratios (dCo : dZn) as high as 3.52 : 1 at surface (0-100 m) depths. We posit that the enhanced ability of the NE Pacific diatoms to grow using Co is an adaptation to these high surface dCo : dZn ratios. Particulate metal data and single-cell metal quotas also suggest a high Zn demand in diatoms that may be partially compensated for by Co.

Elevated sources of cobalt in the Arctic Ocean

“…These stations correspond to a region of anomalously high dFe and DOC concentrations (Charette et al, 2020), interpreted to be indicative of river inputs carried across the basin in the Transpolar Drift (TPD) (Gascard et al, 2008;Klunder et al, 2012;Middag et al, 2011;Wheeler et al, 1997). This is supported by measurements of 228 Ra, which has been used as a tracer of shelf inputs throughout the Arctic (Kipp et al, 2018;van der Loeff et al, 2018). A similar relationship was also observed with salinity in the North Atlantic, supporting the role of rivers as a source of dCo (Dulaquais et al, 2014a;Noble et al, 2017;Saito and Moffett, 2001).…”

“…The increase in dCo over time in the Arctic is interesting and has been documented for other tracers in the Arctic. Kipp et al (2018) and van der Loeff et al (2018) noted that 228 Ra has increased over time in the central Arctic. They suggest that increases in shelf and/or river inputs from thawing permafrost are the source of this elevated 228 Ra (Kipp et al, 2018;van der Loeff et al, 2018).…”

Seasonal cycling of zinc and cobalt in the south-eastern Atlantic along the GEOTRACES GA10 section

“…The comparison further implies that the presence of Phaeocystis and diatoms in summer STSW may be linked with their metabolic Zn-Co-Cd substitution capability, potentially allowing them to overcome some portion of their Zn deficiency. Largely connected to carbonic anhydrase enzymes, several species of eukaryotic phytoplankton are capable of biochemical substitution of Zn, Co or Cd to maintain optimal growth rates under low-tracemetal conditions (Price and Morel, 1990;Sunda and Huntsman, 1995;Lee and Morel, 1995;Lane and Morel, 2000;Xu et al, 2007;Saito and Goepfert, 2008;Kellogg et al, 2020). For example, metabolic substitution of Co in place of Zn has been observed to support the growth of P. antarctica, T. pseudonana and T. weissflogii in media with Zn 2+ < 3 pM (Sunda and Huntsman, 1995;Saito and Goepfert, 2008;Kel-Figure 6.…”

“…The role for Zn and Co in carbonic anhydrase establishes an interaction between their ocean cycles, whereby biochemical substitutions between the enzyme-bound metals enable a stoichiometric plasticity in their cellular requirements that can negate the effect of limited availability. For example, a number of eukaryotic algae can substitute Zn for Co as well as cadmium (Cd) in carbonic anhydrase when seawater dZn concentrations are low (Price and Morel, 1990;Sunda and Huntsman, 1995;Lane and Morel, 2000;Xu et al, 2007;Saito and Goepfert, 2008;Kellogg et al, 2020). In contrast, the prokaryotic picocyanobacteria Synechococcus and Prochlorococcus appear to have an absolute Co requirement (Sunda and Huntsman, 1995;Saito et al, 2002;.…”

Bioactive trace metals and their isotopes as paleoproductivity proxies: An assessment using GEOTRACES-era data

“…Zinc can be growth-limiting for phytoplankton grown in culture (Anderson et al, 1978;Brand et al, 1983;Morel et al, 1994), but Zn (co)-limitation (with Fe, Co) has only rarely been observed in the open ocean (e.g., Coale, 1991;Coale et al, 2003;Franck et al, 2003;Ellwood, 2004;Lohan et al, 2005). This difference between culture and field may reflect the ability of some phytoplankton to substitute Cd or Co for Zn in some enzyme systems when ambient Zn concentrations are low (e.g., Morel et al, 1994;Lee & Morel, 1995;Yee & Morel, 1996;Kellogg et al, 2020). Nevertheless, Zn availability has been shown to influence species composition and phytoplankton growth, including rates of calcification and alkaline phosphatase activity (Sunda & Huntsman, 1995;Crawford et al, 2003;Schulz et al, 2004;Shaked et al, 2006;Mahaffey et al, 2014).…”