The content of carbon, nitrogen, oxygen, phosphorus and sulfur was measured in individual cells from 6 native aquatic samples and 4 samples of cultured bacteria by X-ray microanalysis using a transmission electron microscope (TEM). The molar C N:P ratio for the pooled sample was 50:lO:l From length and width measurements of unfixed air-dried cells we estimated cell volumes over a total range of 0.0026 to 15.8 pm3. and mean C:volume ratios of 30 to 162 fg for the samples included. For the marine samples we found mean N:C ratios of 0.25 to 0.28, while cells from fresh or brackish waters had mean N:C ratios of 0.17 to 0.20, indicating differences in nutrient availability. The P:C ratios for the samples analyzed varied from 0.040 to 0.090, with a pooled mean of 0.052, which is approximately twice that of the Redfield ratio for P C . For 0:C ratios we estimated a pooled mean of 0.37 and a range of 0.22 to 0.77 for all samples. We may conclude that slow-growing or non-growing cells have low 0 -C ratios. The mean S:C ratio for all samples was 0.031, with a range of 0 016 to 0.084 for the sample means. A general conclus~on is that single-cell analyses of elemental composition give important information on the physiological conditions of cells and on possible nutrient limitations. The rationale for this is the assumption that changes in macromolecular composition are due to nutrient availability.

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 rnicroanalysis. Four different rnorphotypes of Fe-Mn bacteria were identified. The total numbers of these bacteria were at the highest in the range of 5.5 X 103 to 1 5 X 104 ml-' The amount of iron bound was 10 to 110 fg cell-' includ~ng 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.