Microbial community response during the iron fertilization experiment LOHAFEX

Area of Science:

• Marine microbial ecology
• Ocean biogeochemistry
• Phytoplankton bloom dynamics

Background:

• High-nutrient, low-chlorophyll (HNLC) regions often exhibit limited phytoplankton growth.
• Iron fertilization is a known method to induce phytoplankton blooms in HNLC areas.
• The microbial community's response to iron-induced blooms remains poorly understood.

Purpose of the Study:

• To investigate the response of bacterial and archaeal communities to an iron-induced phytoplankton bloom.
• To determine if microbial community succession occurs during an artificial phytoplankton bloom.
• To assess the impact of grazing on microbial community dynamics in fertilized waters.

Main Methods:

• Monitoring Bacteria and Archaea during the LOHAFEX experiment over 38 days.
• Measuring microbial production using thymidine and leucine uptake assays.
• Analyzing microbial community composition via 454 tag pyrosequencing of 16S rRNA genes.
• Quantifying specific microbial groups using catalyzed reporter deposition fluorescence in situ hybridization (CARD FISH).

Main Results:

• Microbial production increased significantly (1.6- to 2.1-fold), while total cell numbers showed only a slight increase.
• Bacterial and archaeal community composition remained remarkably constant, lacking typical bloom succession patterns.
• Key bloom-associated groups (Roseobacter, Gammaproteobacteria) showed minimal response.
• SAR11 and SAR86 clades exhibited a slight but significant increase in sequence and cell numbers.

Conclusions:

• Enhanced microbial productivity occurred within the iron-fertilized area.
• Highly effective grazing likely prevented a typical microbial community succession in response to the algal bloom.
• Small-celled microbes, such as SAR11 and SAR86, may have evaded grazing pressure, leading to their numerical increase.