Fragilariopsis kerguelensis response to iron enrichment regarding its growth, uptake of nutrients and trace metals, and changes in CO2, CH4, and N2O

Abstract

We performed laboratory experiments to investigate the response of <em>Fragilariopsis kerguelensis</em>, a predominant diatom species in the Southern Ocean, to different concentrations of dissolved iron in the culture medium to assess changes in nutrients, trace metals, and greenhouse gases-CO₂, CH₄, and N₂O-during growth. <em>F.</em> <em>kerguelensis</em> was cultured in standard f/2+Si media contained in closed chambers at 2°C, which is a typical surface temperature of the Southern Ocean in summer, under continuous irradiation with ~44 μmol photons m^-2 s^-1 for 8 days. The media contained 2.2 nM, 7.0 nM, and 10.6 nM of dissolved iron at inoculation.<em> F.</em> <em>kerguelensis</em> grew faster if the initial dissolved iron concentration was higher. Its production rate was ~40 cells mL^-1 d^-1 with an increase of 10^-18 molar dissolved iron on a single cell basis. Fe and Mo were consumed faster than the growth rate at higher dissolved iron concentrations while Mn and Zn were consumed more slowly taking the mean values into account. Nitrate consumption by single cells increased with an increase of dissolved iron in the media, but phosphate and silicate showed a tendency to decrease. Hence, dissolved iron enhanced uptake of nitrate, but not the other nutrients, on a single cell basis. The carbon uptake per cell decreased with an increase in dissolved iron, which is opposite to the growth rate, suggesting that carbon content in single cells could not keep up with the cell growth. The iron efficacy of carbon uptake by single cells, defined as the ratio of the carbon uptake to the iron uptake, also showed a significant reduction with an increase in dissolved iron. This implies the inefficient usage of iron to absorb carbon at a high dissolved iron concentration. CH₄ uptake by <em>F. kerguelensis</em> occurred in our experiments, but it was trivial in relation to the overall impact. N₂O was consumed at a lower concentration of dissolved iron, but was emitted at a higher dissolved iron concentration, suggesting a facultative response of<em> F. kerguelensis</em> to the available dissolved iron.