Resource patch formation and exploitation throughout the marine microbial food web

Exploitation of microscale (microm-mm) resource patches by planktonic microorganisms may influence oceanic trophodynamics and nutrient cycling. However, examinations of microbial behavior within patchy microhabitats have been precluded by methodological limitations. We developed a microfluidic device to generate microscale resource patches at environmentally realistic spatiotemporal scales, and we examined the exploitation of these patches by marine microorganisms. We studied the foraging response of three sequential levels of the microbial food web: a phytoplankton (Dunaliella tertiolecta), a heterotrophic bacterium (Pseudoalteromonas haloplanktis), and a phagotrophic protist (Neobodo designis). Population-level chemotactic responses and single-cell swimming behaviors were quantified. Dunaliella tertiolecta accumulated within a patch of NH4(+), simulating a zooplankton excretion, within 1 min of its formation. Pseudoalteromonas haloplanktis cells also exhibited a chemotactic response to patches of D. tertiolecta exudates within 30 s, whereas N. designis shifted swimming behavior in response to bacterial prey patches. Although they relied on different swimming strategies, all three organisms exhibited behaviors that permitted efficient and rapid exploitation of resource patches. These observations imply that microscale nutrient patchiness may subsequently trigger the sequential formation of patches of phytoplankton, heterotrophic bacteria, and protozoan predators in the ocean. Enhanced uptake and predation rates driven by patch exploitation could accelerate carbon flux through the microbial loop.