Ferredoxin and flavodoxin as indicators of iron availability in Antarctic sea ice microalgal communities

The availability of the micronutrient iron is known to exert control on phytoplankton growth and community composition in much of the Southern Ocean. The role of this trace element in regulating primary production in Antarctic pack ice is however, largely unknown. To investigate the availability of iron to microalgae in Antarctic pack ice, immunoassays were developed for the proteins ferredoxin and flavodoxin. In pervious studies these proteins have been shown to be regulated by iron availability in many temperate marine phytoplankton and have the potential to be used as indicators of iron availability in situ. Antibodies generated towards ferredoxin and flavodoxin purified from a temperate diatom were found to have good cross-reactivity with the proteins from a range of sea ice diatoms. The putative iron stress protein flavodoxin was found to be expressed constitutively in several sea ice diatoms from both the Antarctic and Arctic. Along with constitutive flavodoxin expression some sea ice diatoms were observed to never express ferredoxin suggesting that these organisms have lost the ability to produce this protein, possibly as a means to reduce cellular iron quotas. The effects of iron availability on Fragilariopsis curta and Fragilarzopszs cylzndrus were determined in laboratory cultures of these organisms. Growth rates, ferredoxin and flavodoxin expression and photophysiological parameters determined by pulse amplitude modulation (PAM) fluorometry were investigated in relation to iron supply. Half saturation constants for growth were similar for both F. cylzndrus and F. curta and photosynthetic parameters showed quantitatively similar reductions for both organisms in response to reduced iron supply. Different patterns in the expression offerredoxin and flavodoxin were observed in these two organisms. Iron replete F. curta only expressed flavodoxin (without ferredoxin) and cellular levels of this protein were not regulated by iron availability. In F. cylmdrus ferredoxin was expressed under iron replete conditions and was replaced completely by flavodoxin as a very early response to iron stress, prior to reductions in growth rate or decreases in photosynthetic parameters such as Fv/Fm. Ice cores collected from Southern Ocean pack ice north of the Adelie Land coast were analysed for both ferredoxin and flavodoxin. Flavodoxin was detected in the majority of core sections and the concentration of this protein was significantly correlated with chlorophyll concentration. Ferredoxin was less widely distributed, being detected in approximately half of all core sections examined, and flavodoxin was always detected along with this protein. These results are consistent with constitutive expression of flavodoxin in many sea ice diatoms and in combination with the culture experiments demonstrate that flavodoxin cannot be used as a stand alone marker for iron stress in this environment. High concentrations of ferredoxin were associated with the top half of ice floes and were never observed in bottom communities which were the most highly productive. The observed distribution of this protein suggests that differences exist in the ability of these communities to access iron, however iron availability did not regulate the distribution of biomass within these floes.