Variability in the in situ bioavailability of Fe to bacterioplankton communities in the eastern subtropical Pacific Ocean

It is now established that iron (Fe) availability controls phytoplankton productivity and community structure in ca. 50% of the Pacific Ocean's surface waters and that heterotrophic bacterio- plankton may also be either directly or indirectly Fe-limited. Proxy indicators of Fe-stress are avail- able for the phototrophic community (e.g. ferredoxin/flavodoxin ratios) but are lacking for the hetero- trophic bacterioplankton. While current analytical tools provide valuable information with regard to micronutrient chemistry and speciation, they do not provide insight into the relative bioavailability of different Fe sources. We present the results of a field trial in an oceanic system of a tool that allows for the assessment of Fe bioavailability in natural systems: the Fe-responsive bioluminescent hetero- trophic bacterial reporter. Fe bioavailability was monitored with this tool at the scale of the Eastern Pacific Basin during the mature phase of the El Nino event of 2002. The results demonstrate signifi- cant spatial variance, highlighted by regions of decreased Fe availability at equatorial stations along the transect. Using this tool in combination with radiotracer studies of bacterial growth and commu- nity Fe uptake, we provide insight into system Fe chemistry and the status of the heterotrophic bac- terial community. Our results indicate that different environments with similar concentrations of total Fe can demonstrate different Fe bioavailabilities. Moreover, the small particulate size fraction (0.2 to 0.8 µm) appears to buffer artificially induced variations in Fe bioavailability, implying that studies of Fe bioavailability need to be extended beyond those in the dissolved (<0.2 µm) size class.