Microbial community response during the iron fertilization experiment LOHAFEX
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Iron fertilization experiments in high nutrient-low chlorophyll areas are known to induce phytoplankton blooms. However, little is known about the response of the microbial community upon iron fertilization. As part of the LOHAFEX experiment in the Southern Atlantic Ocean, Bacteria and Archaea were monitored within and outside of an induced bloom, dominated by Phaeocystis-like nanoplankton, during the 38 days of the experiment. The microbial production increased 1.6-fold (thymidine uptake) and 2.1-fold (leucine uptake) while total cell numbers increased only slightly over the course of the experiment. 454 Tag-pyrosequencing of partial 16S rRNA genes and catalyzed reporter deposition fluorescence in situ hybridization (CARD FISH) showed that the composition and abundance of the bacterial and archaeal community in the iron fertilized water body was remarkably constant without developing typical bloom-related succession patterns. Members of groups usually found in phytoplankton blooms, like Roseobacter and Gammaproteobacteria, showed no or only minor response towards the bloom. However, sequence numbers and total cell numbers of the SAR11 and SAR86 clade increased slightly but significantly towards the end of the experiment. It seems that although microbial productivity was enhanced within the fertilized area, a succession - like response of the microbial community upon the algal bloom was averted by highly effective grazing. Only small-celled members like the SAR11 and SAR86 clades could possibly escape the grazing pressure, explaining a net-increase of those clades in numbers.
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