Iron chemistry in the Southern Ocean

By Christel Hassler,

Professor of marine and lake biogeochemistry, University of Geneva. Principal investigator, ACE project “Biodiversity and isolation of bacteria and viruses in contrasted regions from the Southern Ocean”.

 

 

During this second leg, oceanographers have sampled highly contrasted water ranging from highly productive polynya waters in the vicinity of Mertz Glacier to Iron (Fe)-limited open waters of the Southern Ocean. The green waters of the polynyas are the most productive ecosystems in the Southern Ocean. In those environments, the retreat and melting of sea-ice allow light and iron to penetrate surface waters whereas the katabatic winds enhance the vertical supply of nutrient-rich deep waters to the sunlit surface ocean, providing a vital cocktail to sustain phytoplankton growth. Sampling contrasting areas is critical to investigate the variability in the biological and chemical parameters associated with high and low productivity waters. Assembling the data acquired from several oceanographic projects will provide a useful dataset to further explore the link between Southern Ocean biogeochemistry and the carbon pump.

One of the aspects in which we are particularly interested in deals with the essential micronutrient iron, an element known to limit the biological carbon pump associated with phytoplankton activity in most of the Southern Ocean. Bacteria and phytoplankton have inherited a high biological iron requirement due to their early evolution in the anoxic ocean where dissolved iron was abundant. However, nowadays, iron is very scarce in these waters, with concentrations usually lower than a tear in the volume of a 25m-long swimming pool (approx. 6 mg Fe).

Moreover, up to 99.9 % of the iron is associated with poorly characterized organic compounds called ligands, which modulate iron accessibility to sustain biological growth. Considering that a single drop of water can contain 1000 cells of phytoplankton and ten times more bacteria, one can easily realize the fierce competition at play to take up dissolved Fe. In order to better understand these complex interactions, we will characterize the iron chemistry as well as organic compounds known to react with iron and relate this information to the distribution and biodiversity of phytoplankton.

Phytoplankton is not isolated from other microorganisms, likewise iron effect on the carbon pump goes beyond its interaction with phytoplankton alone. Bacteria are “trapping” organic carbon in deep water by chewing easily consumable organic carbon, leaving behind dissolved refractory compounds, a process known as the bacterial carbon pump. Because bacteria can also be limited by iron, the connection between iron, organic compounds (and ligands) with phytoplankton and bacteria needs to be elucidated to explore the relation between the biological and the microbial carbon pumps in more details.

During this voyage, data on bacterial abundance and biodiversity using metagenomic tools will complement our current understanding of this link around the South Pole. Furthermore, we are isolating bacterial strains in biogeochemically contrasted waters to further investigate this process once back in our laboratory.

Finally, we investigate for the first time a new biological player with respect to organic compounds and iron biogeochemistry: the viruses. Viruses are more abundant that bacteria and are responsible to up to 30% of daily bacterial death demonstrating their profound impact for the Southern Ocean ecosystemic functioning. Viruses have specific enzymes that they use to infect their host with and can degrade and transform organic compounds with potential effect on iron accessibility to other microorganisms. As a first step to understand how viruses affect Southern Ocean ecosystems, we will characterize their biodiversity and distribution using metagenomic tools.

Whereas parameters for trace elements along the water column (usually 0- 1000 m depth) are collected from a specific rosette (see earlier blog), biological parameters are collected from a regular rosette that is coupled to multiple sensors including conductivity, depth, temperature (CTD) but also salinity, dissolved oxygen and fluorescence. Usually at station, the regular rosette (CTD) goes in the water first and the information from the sensors are used to determine the specific depths of sampling which are replicated in a following trace metal rosette deployment on the same site. In addition, to increase horizontal resolution during this voyage, biologists, microbiologists and virologist are sampling from an underway water supply that is continuously fed from 4.5 m depth. This underway supply is also connected with several instruments that are continuously recording critical information (see further blog).