Abundant populations of iron and manganese sequestering bacteria in coastal water

Abstract

It has recently been suggested that iron is a limiting factor for phytoplankton production in nutrient-rich seas. Here we report on a group of bacteria in coastal marine surface waters forming extensive appendages with apparently high affinity for iron and manganese (Fe-Mn bacteria). Bacteria and particles were harvested by centrifugation onto grids for Transmission Electron Microscope (TEM) X-ray microanalysis. Four different morphotypes of Fe-Mn bacteria were identified. The total numbers of these bacteria were at the highest in the range of 5.5 × 103 to 1.5 × 104 ml-1. The amount of iron bound was 10 to 110 fg cell-1 including appendages, and the Mn:Fe ratio (w/w) of the metal-encrusted appendages varied between 0.37 and 5.7. In some environments the Fe content of these bacteria was equivalent to a bulk concentration of about 10 nM. The Fe and Mn content per unit biomass of these bacteria is 1000 to 10000 times that of most other microorganisms. These organisms may thus be important for both cycling and sedimentation of Fe and Mn in marine ecosystems and for marine productivity.