Abstract

Nearly one-half of the global primary production of organic carbon compounds occurs in the oceans. Between 30 and 50\% of the newly generated carbon is released into the seawater as dissolved organic matter (DOM), and is almost exclusively accessible to bacteria. When a patch of nutrients is released in a turbulent flow, it is fragmented into a series of filaments whose width is affected by the rates of stirring and diffusion. Chemotactic bacteria are able to move up the resulting nutrient gradients b introducing a bias in their random mobility in response to chemical cues. Here, motile bacteria are simulated using a Keller-Segel model, embedded in direct numerical simulations of a turbulent flow. A spectral analysis of the governing equations shows how both the nutrient and bacteria fields are affected by the flow and chemotaxis. Motility confers an advantage, but distribution functions show that bacterial uptake is strongly discriminatory among the population. Secondly, we present a sensitivity study of the bacterial uptake, showing that for most of the parameter space, motile bacteria benefit from the turbulent flow. We propose a scaling law for the uptake rate, accounting for a wide range of physical and biological parameters.