Biological activity in the world open ocean’s surface is characterized by autotrophic and by heterotrophic processes. Phytoplankton organisms take up dissolved CO2 (dissolved inorganic carbon, DIC) and together with other inorganic nutrients and light they produce biomass (particulate organic carbon, POC) and dissolved organic carbon (DOC).

By these processes marine phytoplankton is responsible for approximately half of the worlds primary production.

These two carbon compounds (POC and DOC) either sink down to the deep ocean (which is basically the biological carbon pump) or they are consumed by other trophic levels.

One important part of the planktonic food web is the microbial community which consists of bacteria (smaller than 3 µm), auto - and heterotrophic flagellates and other protists (larger than three µm).

This community takes up both POC and DOC and by respiration recycles these carbon compounds back into DIC.

Thus in terms of carbon flux the microbial community potentially competes with the biological carbon pump.

In a mesocosm experiment with natural marine plankton Julia Wohlers and her colleagues manipulated future ocean surface warming and measured the carbon flux during the plankton bloom peak.

Whereas in this experiment phytoplankton biomass production (POC of autotrophs) was not affected by warming the authors found that respiration by the microbial community, in particular by organism larger than 3 µm, significantly increased.

This increase in respiration led to a significant decrease in net DIC reduction in the whole planktonic foodweb.

The results are a potential sign for future declining carbon sequestration by biological processes in the world oceans.

Julia Wohlers, Anja Engel, Eckart Zollner, Petra Breithaupt, Klaus Jurgens, Hans-Georg Hoppe, Ulrich Sommer and Ulf Riebesell (2009).

Changes in biogenic carbon flow in response to sea surface warming.

Proceedings of the National Academy of Sciences.

DOI:10.1073/pnas.0812743106

Taken from:evol-eco.blogspot. com

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