Microzones surrounding phytoplankton form the basis for a stratified marine microbial ecosystem

Abstract

Although there are 10⁷–10⁹ bacteria per litre of sea water and up to 50% of marine primary productivity passes through them¹, no direct information exists on small-scale spatial distributions of individual bacteria. It has been proposed that bacteria cluster around participate nutrient sources^2–4, but evidence for this is indirect, based on the presence of chemo taxis in isolates and a broad range—10⁻⁹–10⁻⁴ M—of uptake affinity constants³. These data have led to the suggestion that there exist oceanic microzones having utilizable dissolved organic carbon (uDOC) orders of magnitude above background concentrations⁴. Here, our calculations of size (10⁻¹ cm), utilizable DOC concentrations (10⁻⁹–10⁻⁷ M) and lifetimes of these nutrient gradients suggest that bacteria cluster around phytoplankton in low-turbulence layers and sheets comprising the thermocline. There, phytoplankton may sink slowly enough for bacteria to keep up with them, and mixing times are long enough to allow nutrient gradients to develop and bacteria to track these gradients. However, very favourable conditions are required for micro zone concentrations to be significantly above background.