Response of bacterioplankton to iron fertilization in the Southern Ocean

Abstract

We studied the bacterial response to Fe fertilization over 3 weeks during the second iron-enrichment experiment (EisenEx) in the Southern Ocean. Bacterial abundance in the Fe-fertilized patch increased over the first 12 d following Fe release and remained about twice as high as outside the Fe-fertilized patch until the end of the experiment. Bacterial production peaked a few days after each of the three Fe releases inside the Fe-fertilized patch, reaching rates two to three times higher than outside the patch. Besides the peaks in leucine and thymidine incorporation following Fe release, bacterial production was not significantly higher inside the patch than outside, suggesting direct limitation of bacterial growth by Fe. Bacterial aminopeptidase activity roughly followed the increase in bacterial abundance, whereas cell-specific ␣-and ␤-glucosidase were higher inside the Fe-fertilized patch. The diversity of ␤-glucosidases was determined by capillary electrophoresis zymography. The different ␤-glucosidases showed much higher activity levels inside the patch than in the surrounding waters, and three additional ␤-glucosidases constituting ϳ55% of the total ␤-glucosidase activity were present inside the Fe-fertilized patch from day 9 onward. No major changes in response to Fe fertilization were detected in the phylogenetic composition of the bacterioplankton community, as determined by 16S rDNA fingerprinting, indicating a remarkable adaptation of the bacterioplankton community to episodic iron inputs. This stability on the phylogenetic level is contrasted by the dramatic qualitative and quantitative changes in ectoenzymatic activity.

Experimental test of bacteria-phytoplankton coupling in the Southern Ocean

“…These results are consistent with observations of bacterial development in response to phytoplankton blooms induced by experimental iron additions in the Southern Ocean (Hall and Safi 2001;Arrieta et al 2004;Oliver et al 2004), which show weak bacterial responses to the phytoplankton blooms induced. As the responses are weak in such large-scale experiments, the likelihood of detecting empirically a coupling between bacteria and phytoplankton in unmanipulated Southern Ocean waters is low.…”

“…Mesoscale iron-addition experiments have also shown a much weaker response of bacteria relative to phytoplankton (Hall and Safi 2001;Arrieta et al 2004;Oliver et al 2004). However, bacteria-phytoplankton relationships derived from such individual experiments (e.g., Oliver et al 2004) are confounded because the data are not independent, as they represent a time series.…”

Comparison of SYBR Green I and SYBR Gold stains for enumerating bacteria and viruses by epifluorescence microscopy

“…Noble & Fuhrman (1998) recommended concentrating the bacteria and viruses on 0.02 µm pore-size Anodisc filters and staining them with SYBR Green I for counting. Because of its bright and stable fluorescence, SYBR Green I stain is now widely used in studies of aquatic microbial ecology that employ epifluorescence microscopy (Arrieta et al 2004, Schwalbach et al 2004, Staroscik & Smith 2004, Winter et al 2004) as well as flow cytometry (Marie et al 1997, Marie et al 1999, Brussaard 2004.…”

Current understanding of Phaeocystis ecology and biogeochemistry, and perspectives for future research

“…For example, bacterial degradation of primary production in high-nutrient low-chlorophyll regions can be limited by iron availability (Arrieta et al 2004). Iron supply can either directly stimulate microbial activity, or do so indirectly by enhancing DOC production of phytoplankton upon their release from iron limitation.…”