Microbial response to a mesoscale iron enrichment in the NE subarctic Pacific: heterotrophic bacterial processes

The response of heterotrophic bacteria to an in situ mesoscale Fe-addition was characterized during the Subarctic Ecosystem Response to Iron Enrichment Study (SERIES), in the high nutrient low chlorophyll region of the Northeast subarctic Pacific, during July 2002. Samples were collected from inside and outside the Fe-enriched patch for the determination of bacterial biomass, and rates of production and growth, and community respiration. The addition of Fe significantly changed the dynamics of the mixed layer heterotrophic bacterial community compared to unfertilized waters. Outside the patch, bacterial dynamics remained relatively constant. Inside the Fe-enriched patch, depth-integrated bacterial biomass decreased 5-fold during the first 12 days after fertilization, after which biomass increased more than 10-fold, to a maximum of 23.3 mg C m−3. Similarly, bacterial production decreased 3-fold over the first 8 days, followed by a 15-fold increase to 5.7 mg C m−3 d−1. Bacterial specific growth rates remained constant for 8 days after the initial Fe-addition and close to values initially observed outside the patch. After day 8, mixed layer specific growth rates inside the patch increased more than 10-fold to a maximum of 1.24 d−1 by day 12, then steadily decreased to 0.22 d−1 by day 16 and remained relatively constant thereafter. Temporal changes in growth were not significantly different inside and outside the patch, suggesting that bacterial growth was not directly limited by Fe availability. The temporal uncoupling of bacterial biomass and production inside the patch, combined with the lack of evidence for direct iron limitation, suggest that inside the patch, bacteria were initially controlled by a combination of moderate bottom-up control, due to the effects of organic substrate limitation of bacterial growth, and strong top-down control, by processes such as microzooplankton bacterivory or viral lysis. Release of bacteria from grazing pressure (around day 12), coupled with an increase in specific growth rate (day 8), resulted in the rapid increase in bacterial biomass observed towards the end of the observation period. Mixed layer bacterial carbon demand ranged from 1.5 to 22.9 mg C m−3 inside the patch and accounted for an average (±st dev) of 25% (±11%) of primary production. Consequently, a high proportion of the Fe-enhanced primary production in the mixed layer during SERIES was channelled through the microbial food web, thus reducing the amount of organic carbon available for export.