In this study we have quantified the ionic content and volume of native aqu
atic, and two cultured bacteria, by X-ray microanalysis (XRMA) in the trans
mission electron microscope (TEM). The cellular concentrations of magnesium
(means of 630 and 710 mM) were more than an order of a magnitude higher th
an the outside concentrations. The internal concentrations of sodium were o
n average 50-180 mM, and the [K+]/[Na+] ratios were in the range of 0.1-0.5
; lowest for apparently nonactive bacteria. Magnesium and chloride probably
act as the major components of cell turgor, since no other inorganic ions
were present in comparable amounts. Our carbon and nitrogen measurements in
dicated that organic solutes are not likely to be present at significant co
ncentrations. The estimated charge of inorganic ions (Na, Mg, P, Cl, K, and
Ca) gave a positive net internal charge for most cells. However, in cultur
es of Vibrio natriegens, the high internal chloride concentration made the
net inorganic charge negative in these cells. Our results suggest that grow
ing marine bacterioplankton have an internal environment in which magnesium
is the dominating cation. These results suggest that actively growing mari
ne bacteria are physiologically adapted to high internal concentrations of
both magnesium and chloride.