Different elemental stoichiometries of Fe‐limited <scp><i>Trichodesmium</i></scp> when grown under inorganic and organic phosphorus sources

Abstract Trichodesmium spp. is a colonial diazotrophic cyanobacterium found in the oligotrophic (sub)tropical oceans, where its distribution is strongly regulated by the availability of phosphorus and iron. The bulk carbon : nitrogen : phosphorus elemental composition of phytoplankton has previously been shown to depart from classical “Redfield” values under nutrient‐limitation conditions. We hypothesized that the abundance of intracellular metabolites and the extended Redfield ratios of Trichodesmium , including a range of trace elements, are variable in response to conditions of phosphorus and iron limitation that are found in the ocean. To test this, we grew Trichodesmium under trace metal–controlled conditions, where phosphorus was supplied as either dissolved inorganic phosphorus (DIP) or dissolved organic phosphorus (DOP) from iron depleted to elevated levels. We found that the steady‐state extended Redfield ratios of Trichodesmium under the iron‐depleted condition was (C 106 N 15.82 P 0.62 ) 1000 Fe 2.26 Zn 2.37 Mn 1.68 Cu 0.68 Ni 0.31 Mo 0.42 Co 0.03 for the DIP treatment where Trichodesmium was under iron limitation, and (C 106 N 13.89 P 0.49 ) 1000 Fe 3.38 Zn 2.51 Mn 0.97 Cu 0.52 Ni 0.42 Mo 0.33 Co 0.03 for the DOP treatment where Trichodesmium was under iron–phosphorus co‐limitation. Mean steady‐state cellular iron : carbon in the DIP treatment (iron limited) was only 50% of that in the control treatment, while zinc : carbon was elevated twofold. The average extended Redfield ratios following recovery from iron limitation was (C 106 N 16.8 P 0.7 ) 1000 Fe 4.41 Zn 1.44 Mn 1 Cu 0.52 Ni 0.19 Mo 0.3 Co 0.03 for the DIP and (C 106 N 15.9 P 0.73 ) 1000 Fe 7.36 Zn 2.24 Mn 1.08 Cu 0.71 Ni 0.63 Mo 0.38 Co 0.02 for the DOP treatment. No significant changes were observed in the carbon‐normalized abundance of targeted metabolites produced by Trichodesmium , under the different treatments. These results suggest Trichodesmium employs different strategies to cope with iron/phosphorus limitation, which is reflected in its extended Redfield ratios.