Looking beyond Iron: Dynamics of vitamins, zinc and cobalt and their effect on phytoplankton community composition in a changing Southern Ocean

In large areas of the worlds oceans iron (Fe) limits primary production thus affecting the global carbon cycle.  One such area is the Southern Ocean (SO).  In addition to Fe, other trace metals and organic cofactors may also play a pivotal role in determining the plankton community composition and thus ultimately carbon export.  Other trace metals include zinc (Zn) and cobalt (Co), important in such enzymes as carbonic anhydrase and in the production of vitamin B12, respectively. Some phytoplankton species may be able to replace Zn with Co while others cannot. B-Vitamins are required by more than half of all phytoplankton species and play an important role in methionine synthesis, carbon transfer reactions, DNA synthesis and in the fatty acid cycle. Vitamin B12, in particular has been shown to limit not only biomass but also shape plankton community composition in coastal as well as open ocean waters.  To date, however, the influences of these micronutrients on SO phytoplankton and their biological requirement for growth are poorly known.

The focus of this project will be to identify whether Zn, Co and vitamins are limiting or co-limiting primary production with iron in the Southern Ocean.  We will also address the question how Zn/Co/B12 co-limitation with iron is modulated by ocean acidification. To confirm Zn/Co/B12-limitation or co-limitation with Fe in different parts of the SO, short-term shipboard manipulation experiments with natural phytoplankton assemblages are planned onboard the Polarstern in the austral summers of 2016 and 2017. In addition, in situ uptake rates and measurements of both, dissolved and particulate trace metals and vitamins will better elucidate the cycling of these micronutrients in the SO.

In order to gain a better understanding of the physiological responses of phytoplankton to trace metal and vitamin limitation, laboratory cultures of several key species from the SO will be grown under B12, Co, Zn and Fe limited condition. We will determine cellular uptake and growth kinetics of the various micronutrients under replete and deplete conditions, explore which genes and biochemical pathways may be affected with limitation by the various micronutrients and trace metals and explore how these relationship may change with increasing CO2 and resulting ocean acidification.

This multifaceted approach between laboratory experiments and field studies will shed light on how factors besides iron drive primary production, shape phytoplankton community composition and ultimately affect the biological pump in this important carbon export region.