Other Growth Limitations
The concepts that you gained about growth limitation are based mainly on
a carbonaceous nutrient such as glucose. The situation changes slightly
for other factors such as nitrogen, sulfur, and phosphate. The problem
arises because cell mass may not exactly reflect these limitations. For
example, cells without adequate nitrogenous components will store carbonaceous
ingredients for later use. The storage takes the form of glycogen, starch,
or compounds such as poly-ß -hydroxybutyrate. Some polymers form
inclusion bodies within a cell. A cell needs about one-sixth as much N
as C.
An upturn in cell mass at low dilution rates reflects storage of carbohydrates
(or equivalent poly-ß -hydroxybutyrate) within the cells. Were we
to plot cell nitrogen instead of cell mass, the graph would resemble that
for cell mass in a carbon-limited culture.
Go with the flow to Two Substrates