Phaeocystis globosa is an alga forming harmful blooms in the coastal waters of the North Sea. The decay of algal biomass at the end of the bloom leads to massive release of organic matter, which in turn stimulates the growth of a variety of heterotrophic gamma- and alpha-proteobacteria.
Formation of filamentous star-like structures here visualized with atomic force microscopy.
Max Planck Institute for Marine Microbiology
Formation of filamentous star-like structures here visualized with nanoSIMS
Max Planck Institute for Marine Microbiology
‘An important source of mortality for these algae are lytic P. globosa viruses. We therefore investigated how algal viral infection and subsequent lysis affects the community structure of the associated bacteria,’ explains Dr. Abdul R. Sheik, the lead author of this study.
In control experiments they showed that the bacterial composition of infected algal cultures differed from non-infected cultures after 5 hours. In order to understand the underlying mechanism Dr. Sheik and colleagues monitored the uptake of the released organic material by the bacterioplankton using isotopically-labeled algal biomass (with isotopes of nitrogen and carbon).
Assimilation of the substrate was quantified in single bacterial cells using imaging secondary ion mass spectrometry (nanoSIMS) with a sub-micrometer spatial resolution. ‘Surprisingly, we saw colonization of algal cells and uptake of labeled carbon and nitrogen by Alteromonas cells long before the algal cells lysed’, explains Abdul Sheik. ‘This suggests that infected but still intact algae can already shape the microbial community composition by excretion or leakage of organic matter.’
The bacterial turnover of algal products was so rapid that ca. 40% of the particulate organic carbon was re-mineralized to CO2 within one week after infection, leaving behind refractive material in the form of cellular star-like structures (see Figure).
These results reveal a new pathway in the transfer of algal biomass to the bacterioplankton and, in a larger picture, new mechanism of retaining carbon in the euphotic zone.
Dr. Manfred Schloesser | Max-Planck-Institut
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