Shape-shifting protein channel

mechanism of oceanic control of climate. They constitute a repository for heat, fresh water and gases such as carbon dioxide, all of which are exchanged at the ocean–atmosphere interface.On the largest scale, the water masses spread and fill the ocean basins. These huge bodies of water are also the engines of large-scale ocean circulation, one that is primarily driven by so-called North Atlantic Deep Water (NADW). As its name suggests, this is water that forms at high latitudes in the North Atlantic, sinks to depth and flows southwards. Since the initial recognition of this circulation, oceanographers have busied themselves with finding out how NADW is resupplied to the Northern Hemisphere. A complex pattern of circulation pathways has become evident, with the Southern Ocean (Fig. 1) apparently having an important role in the production and interchange of water masses. Subantarctic Mode Water (SAMW) is a large water mass created by exchange of heat and fresh water with the atmosphere over much of the Southern Ocean. It sinks below the ocean surface and moves northwards at depths of about 200–600 m. These regions of the Southern Ocean can be thought of as windows to the deep ocean that allow water of particular heat, freshwater and nutrient content to enter the subsurface ocean. The surface regions where SAMW is formed are characterized by low levels of silicic acid and high concentrations of nitrate. Sarmiento et al. apply a newly designed ‘conservative’tracer that captures this nutrient signature as a characteristic of SAMW. Their analysis of its distribution shows that SAMW reaches most of the world’s upper ocean — that is, much of the global marine environment receives nutrients from the surface of the Southern Ocean. Another water mass, known as North Pacific Intermediate Water,has a subordinate role only:its nutrient delivery is limited to the upper ocean of the North Pacific. Sarmiento et al. also performed several computational experiments. They used a model of the global ocean that couples physical and biological processes, and came up with a reason why diatoms, a major component of the phytoplankton,do not reach their full productive potential in much of the global ocean. By varying the strength of the nutrient source in their models,Sarmiento et al. found that the ratio of silicic acid to nitrate in SAMW is less than ideal for diatom growth, and is therefore a primary cause of diatoms’widespread low productivity. The conclusion that a physical process,and one operating in only a tiny part of the world’s oceans, has such a huge influence on productivity is startling in itself. But there are more lessons to be learned from the new work.