Oliver¹, Jacques L., Richard T. Barber², Walker O. Smith¹ and Hugh W. Ducklow¹

¹Department of Biological Sciences, Virginia Institute of Marine Science, College of William and Mary, Gloucester Point, Virginia 23062, Tel: 804-684-7416, Fax: 804-684-7399, E-mail: jloliver@vims.edu and ²Nicholas School of the Environment and Earth Sciences, Duke University, Beaufort, North Carolina, 28516

The heterotrophic bacterial response to iron enrichment during the Southern Ocean Iron Experiment (SOFeX)

We measured the response of heterotrophic bacteria to iron enrichment during the Southern Ocean Iron Experiment (SOFeX). Heterotrophic bacterial production (BP), measured via tritiated thymidine (³H-TdR) and leucine (³H-Leu) incorporation, increased ~40% over an 18 day period in iron-fertilized waters south of the Antarctic Polar Front Zone (APFZ) (South Patch). Also, South Patch BP was 61% higher than in the surrounding non-fertilized waters. Abundance, measured by flow cytometry (FCM) and acridine orange direct counts, also increased in the South Patch from 3 to 5 x 10⁸ cells liter^-1, a 70% increase. Bacterial biomass increased from ~3.6 to 6.3 μg C liter^-1, demonstrating that biomass production exceeded removal rates over the course of 18 days. This pattern is in contrast to SOIREE where no accumulation of biomass was observed. Specific growth rates measured from bottle experiments using iron-fertilized water ranged between 0.08 and 0.15 day^-1 and are comparable to in situ rates from ³H-TdR incorporation. High DNA-containing (HDNA) cells detected by FCM also increased over time in iron-fertilized waters from 20 to 46% relative to the total population and the increase is suggestive of an active subpopulation of heterotrophic bacteria. Primary production (PP) also increased within iron-fertilized waters compared to non-fertilized waters. However, the ratio of BP:PP in iron-fertilized waters was 3-4%, despite increases in PP, compared to 7-12% in non-fertilized waters. Our results show an active bacterial population that responded to newly produced dissolved organic carbon and/or iron, but whose processes were uncoupled from broader autotrophic organic carbon production processes.