Microscopic organisms, phytoplankton (single celled photosynthetic algae) and bacteria, which live suspended in seawater form the base of global marine food webs and underpin ecosystem health and fucntion. The Southern Ocean around Antarctica is home to massive phytoplankton populations, and scientists have long considered their growth to be controlled largely by availability of iron and light. However, research conducted through these studies shows that the picture is much more complicated.

The coastal Southern Ocean is a critical climate system component, and is home to high rates of photosynthesis. Research performed here revealed that cobalamin (vitamin B12) and iron availability can simultaneously limit phytoplankton growth in late Austral summer coastal Antarctic sea ice edge communities. Unlike other growth limiting nutrients, the sole cobalamin source is production by bacteria and archaea. By identifying microbial gene expresiion changes in response to altered micronutrient availability, this research revealed the molecular underpinnings of limitation by both cobalamiin and iron and offered evidence that this late season limitation is driven by multiple phytoplankton-bacterial interactions. These results support a growing body of research suggesting that relationships between bacteria and phytoplankton are key to understanding controls of marine primary productivity.

Research was conducted at the US Antarctic Program's McMurdo Station at the south tip of Ross Island. Expeditions were conducted over the froze sea ice to the ice edge where surface water samples were collected and analyzed in the laboratory at McMurdo Station. Although the water appeared teeming with a particular type of phytoplankton called diatoms, the diatoms displayed cellular signatures of malnourishment. Unlike most regions of the global ocean which do not contain sufficient nitrogen or phosphorous for sustained phytoplankton growth, diatoms in the remote waters of McMurdo Sound, adjacent to the Ross Sea, were starving from lack of iron and deficiency of vitamin B12.

A range of different bacteria growing in the water were also identified, but it was not clear what the role of the various types of bacteria might play in driving or alleviating the starvation exhibited by the diatoms. Were they competing for the scarce resources and exacerbating the starvation conditions or were they somehow cooperating to effectively share these resources? By examining how these microbial groups change the expression of their genes in response to shifts in micronutrient availability, these questions were addressed. Research conducted throughout this project confirmed that a large portion of the B12 supply in the Souther Ocean appears to be produced by a particular group of gamma proteobacteria belonging to the Oceanospirllaceae.

Therefore, through a combination of field manipulation experiments and next generation sequencing, a new view of the microbial interactions underpinning a highly productive ecosystem was obtained. Although oceanographers have long recognized that iron fertilization in the Southern Ocean will drive phytoplankton blooms, it is now evident that particular groups of bacteria, perhaps specifically cultivated by the phytoplankton, are important for regulating the magnitude of the blooms as well as sustaining them through supply of the critically limiting micronutrient vitamin B12.

Just like humans, phytoplankton require vitamins, including vitamin B12, to survive. But this precious resource is being competed for by three groups of microbes which, due to the extreme remoteness of the Southern Ocean, is a scarce and consequently invaluable resource. This research illustrates the eye opening sensitivity of marine phytoplankton and bacteria to very minor additions of scarce micronutrients over very short, hourly, time scales.

With this ...

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