Abstract
Across the Southern Ocean, phytoplankton growth is governed by iron and light, while bacterial growth is regulated by iron and labile dissolved organic carbon (LDOC). We use a mechanistic model to examine how competition for iron between phytoplankton and bacteria responds to changes in iron, light, and LDOC. Consistent with experimental evidence, increasing iron and light encourages phytoplankton dominance, while increasing LDOC and decreasing light favors bacterial dominance. Under elevated LDOC, bacteria can outcompete phytoplankton for iron, most easily under lower iron. Simulations reveal that bacteria are major iron consumers and suggest that luxury storage plays a key role in competitive iron uptake. Under seasonal conditions typical of the Southern Ocean, sources of LDOC besides phytoplankton exudation modulate the strength of competitive interactions. Continued investigations on the competitive fitness of bacteria in driving changes in primary production in iron‐limited systems will be invaluable in refining these results.
Highlights
Phytoplankton growth in large areas of the Southern Ocean is regulated by the micronutrient iron (Fe) (Martin et al 1994, Blain et al 2007, Boyd et al 2007), with seasonal co-limitation by light (Mitchell et al 1991, Strzepek et al 2012) due to the deep mixed layers and ice cover that are prevalent in early spring, autumn and winter
While the Fe-PAR simulation suggests that bacteria are poor competitors for available Fe when they rely solely on phytoplankton exudation as a source of dissolved organic carbon (DOC), we find that bacteria are more effective competitors for Fe as total DOC increases
Our analysis finds that bacteria can outcompete phytoplankton leading to a decrease in phytoplankton biomass if sufficient labile DOC is available which can be found in regions of high recycling (Ruiz-Halpern et al 2011, Arístegui et al 2014) and ice melt (Norman et al 2010, Underwood et al 2010), where there is concurrent release of Fe, a concept that has previously not been considered in our understanding of the Fe cycle in the Southern Ocean (Tagliabue et al, 2014)
Summary
Phytoplankton growth in large areas of the Southern Ocean is regulated by the micronutrient iron (Fe) (Martin et al 1994, Blain et al 2007, Boyd et al 2007), with seasonal co-limitation by light (Mitchell et al 1991, Strzepek et al 2012) due to the deep mixed layers and ice cover that are prevalent in early spring, autumn and winter. While low surface dissolved Fe and DOC is thought to restrict heterotrophic bacterial (hereafter bacteria) growth in the Southern Ocean (Church et al 2000, Obernosterer et al 2015), studies from Fe-limited regions have demonstrated that bacteria may be significant consumers of dissolved Fe and can exhibit greater cellular Fe quotas than phytoplankton (Tortell et al 1996, 1999, Boyd et al 2012, Fourquez et al 2015, 2020). Colimitation can lead to stable coexistence (Burson et al 2018) or alternative stable states, depending on ambient environmental factors (e.g. resource supply and lability, temperature, light and mortality) and the traits of the competing species (Tilman 1982, Brauer et al 2012) that affect resource uptake and growth (Hutchinson 1953, Titman 1976). While labile DOC is often rapidly utilised within hours (Fourquez et al 2014, Obernosterer et al 2015), seasonal accumulation of DOC has been observed in the Bermuda
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