Mix and match : how climate selects phytoplankton

| Climate strongly influences the distribution and diversity of animals and plants, but its affect on microbial communities is poorly understood. By using resource competition theory, fundamental physical principles and the fossil record we review how climate selects marine eukaryotic phytoplankton taxa. We suggest that climate determines the equator-to-pole and continent-to-land thermal gradients that provide energy for the wind-driven turbulent mixing in the upper ocean. This mixing, in turn, controls the nutrient fluxes that determine cell size and taxa-level distributions. Understanding this chain of linked processes will allow informed predictions to be made about how phytoplankton communities will change in the future. nATuRE REvIEWS | microbiology vOLuME 5 | OCTOBER 2007 | 813 PersPeCTives f o c u s o n M a r i n e M i c r o b i o lo g y © 2007 Nature Publishing Group flux of organic matter depletes the surface waters and enriches the ocean interior with inorganic carbon, simultaneously15,19. This phenomenon, which is commonly called the biological pump20, is dependent on cell size and taxa21–23. Larger cells sink faster than smaller cells; diatoms sink faster than flagellated eukaryotes24–26 (FIG. 2). In the upper ocean, the combination of solar heating and the nutrient draw-down by photosynthetic microorganisms of nutrients supplied from the ocean interior sets up two crucial, vertically opposing resource gradients that select for specific phytoplankton groups27. The degree of overlap between these two vertical gradients is mainly determined by the amount of mixing between the upper and deep ocean along density surfaces (isopycnal mixing)28,29, and, to a lesser extent, across the pycnocline (diapycnal mixing)13,30. More mixing results in a deeper pycnocline where cells are exposed to more nutrients and less light, whereas less mixing results in a shallow pycnocline where cells are exposed to less nutrients and more light31. This simple configuration of the ocean water column is a stable system, in which stability is related to the vertical density gradient (see Equation 1).