EAPS

Marine microbial diversity and nitrogen: does species diversity matter?

Marine microorganisms are responsible for half of the biological productivity on Earth, but are reliant on supplies of major nutrients and trace elements. Biological availability of dissolved inorganic forms of nitrogen is one of the most critical factors that constrains productivity and biogeochemical cycling in ocean ecosystems. The nitrogen cycle is being dramatically affected by anthropogenic activities leading to known and undoubtedly as yet unknown impacts on the oceans. Marine microorganisms are key components of ocean ecosystems, and are important in food webs, as well as cycling elements, such as nitrogen, from one chemical form to another. The species of marine microorganisms in the environment are still poorly known, because many of the important species have been difficult to obtain in laboratory culture. Molecular biology and genomics approaches have identified new species, and also have shown that known taxa are comprised of surprisingly great diversity of closely related subspecies that have different ecological niches. Is this diversity important for the nitrogen cycle of the sea?

Only a few types of microorganisms are able to draw on the rich source of nitrogen in the atmosphere (78% of the atmosphere); these organisms, called nitrogen-fixing microorganisms (diazotrophs) are very important components of oceanic ecosystems, are very difficult to study, and are likely to be sensitive to changes in climate and nitrogen perturbations. Technological advances have made it possible to study how the complex microbial communities respond to changes in nitrogen availability, including the use of remotely deployed robotic laboratories. Only within the past few decades, using these cultivation-independent approaches, a novel unicellular symbiosis between 2 single-celled microorganisms was discovered that now appears to be one of the most abundant nitrogen-fixing microorganisms, and plays a key role in global oceanic nitrogen cycling. I will discuss how technological advances are needed to determine how microbial communities respond to global climate change, and whether species diversity is important in how microbial communities respond to nitrogen, using the discovery of a very unusual nitrogen-fixing cyanobacteria called UCYN-A as an example.