This study demonstrates differences in ATP levels between attached and planktonic cells of Acidithiobacillus ferrooxidans growing with elemental sulfur. A small fraction of 3.7-14.4% of the bacterial cells was attached to the sulfur particles. The highest cell attachment of 14.4% was at the end of the lag phase, decreasing to 3.7% into the latter part of the active growth phase. Therefore, attached cells and their ATP content made a minor contribution to the total culture biomass in the active growth phase. However, the cellular ATP content was 1.01 amol per attached cell and 0.34 amol per planktonic cell. The significantly (P < 0.01) lower ATP content was attributed to sulfur limitation in the planktonic cells. These results suggest that a negligibly small subpopulation may be a link in cooperative interaction whereby sulfur oxidation by attached cells under boundary conditions provides bioavailable substrates to planktonic cells in the population. (C) 2013 Elsevier Ltd. All rights reserved,