Resource Colimitation Drives Competition Between Phytoplankton and Bacteria in the Southern Ocean

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. Plain Language Summary In large areas of the Southern Ocean, phytoplankton growth is controlled by the availability of iron and light while bacterial growth is controlled by the availability of iron and labile dissolved organic carbon (LDOC). We developed a mechanistic model to examine how phytoplankton and bacteria compete for iron under different light levels and LDOC supply. We find that phytoplankton dominate as iron and light increases while increasing LDOC and decreasing light favors bacterial dominance. If enough LDOC is present, bacteria can outcompete phytoplankton for iron. More broadly, we find that while bacteria are a major consumer of iron in the Southern Ocean, seasonal changes in light drive phytoplankton dominance over bacteria, unless LDOC is supplied by other sources such as viral lysis and excretion, which strengthen competitive interactions. Key Points Competition for iron between phytoplankton and bacteria is modulated by the colimiting effects of light and dissolved organic carbon When enough labile dissolved organic carbon is present, bacteria can outcompete phytoplankton for iron Phytoplankton exudation may be insufficient to stimulate bacterial dominance in the absence of other dissolved organic carbon sources