Kjell Gundersen, Mikal Heldal, Svein Norland, Duncan A. Purdie, Anthony H. Knap
Limnol. Oceanogr., 47(5), 2002, 1525–1530
Accurate assessments of the true elemental cellular content of carbon (C), nitrogen (N), and phosphorus (P) in bacteria have been a major difficulty in microbial research for some time. This study is the first to present single-cell elemental C, N, and P measurements on natural populations of bacteria from the Sargasso Sea using a transmission electron microscope (TEM) equipped with an X-ray detector unit. Elemental cell content showed a best fit as a power function of the cell size, and smaller bacteria had significantly more elemental C, N, and P per cell volume than the larger ones (scaling factor a < 1). Contrary to coastal, brackish, and freshwater systems, the bacteria collected in this study appeared to have a molar elemental P :N close to the Redfield ratio of 0.063 (N:P = 16). The geometric mean of C per cell volume analyzed in this study (148 fg C µm-3) was higher than any other estimates reported from coastal and brackish water systems. Total amount of bacterial C calculated from abundance estimates requires an average conversion factor, and the choice of a representative bacterial cell volume is therefore critical. We were able to demonstrate that the outcome of biomass assessments is highly dependant on the choice of cell-specific conversion factors. By applying cell volumes previously reported from the BATS site, the average 0–250 m depth integration of bacterial C between 1991 and 1996 ranged between 1.7 and 2.5 times less than other recent biomass estimates for these waters.