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Abstract

Oceans cover ~70% of the Earths surface and are the second largest global carbon reservoir.
Major processes in marine carbon cycling are summarized in the biological carbon pump. Within
the biological carbon pump, pivotal stages are the microbial loop and aggregate related processes.
Within the surface layer and on aggregates, carbon is channelled within a complex food web
based on microbial processes. These processes counteract the biological carbon pump, showing
the importance of the microbial loop for carbon sequestration.
During the iron fertilization experiment LOHAFEX, a phytoplankton bloom of nano- and
picoplankton was induced in the South Atlantic. We used catalyzed reporter deposition
fluorescence in situ hybridization (CARD FISH) and 454 tag pyrosequencing to investigate the
bacterial and archaeal community response to this bloom. The bacterial and archaeal community
was stable over the course of the experiment and only members of the SAR11 and SAR86 clades
showed elevated cell numbers. This led to the hypothesis of a top-down control exerted by a
community of nano- and picoplankton grazers. Consequently, we used the same techniques to
investigate the nano- and picoplankton community during the LOHAFEX experiment. We
discovered a stable community with high but constant abundance of Phaeocystis, the major
bloom forming organism, and a short peak of Micromonas and Pelagophyceae after the second
iron fertilization. This again led to the hypothesis of a strong top-down control and a tight
coupling of the microbial loop.
We investigated the bacterial community on aggregates at different depth from the Canary
Current Upwelling system. A free drifting sediment trap was used to sample aggregates in situ at
100 m and 400 m depth. We used a three dimensional FISH approach to quantify the bacterial
community. Synechococcus dominated the bacterial community on marine snow at both depths,
while Bacteroidetes and Alteromonas abundance significantly decreased with depth. We
hypothesize a change in the bacterial community due to a combined effect of changes in nutrient
quality due to degradation processes, grazing, decreasing temperature and increasing pressure.
In summary, a strong top-down control was exerted on the bacterial and archaeal, and the nanoand
picoplankton community, indicating a tight coupling of the microbial loop during the iron
fertilizing experiment LOHAFEX. Marine snow investigations off Cape Blanc showed that
Bacteroidetes and Alteromonas dominated the bacterial community but decreased with depth,
indicating a nutrient quality, grazer, pressure and temperature dependent community composition.