Dependence of photosynthesis and vitamin B12 uptake on cellular vitamin B12 concentration in the multicellular alga Cladophora glomerata (Chlorophyta)

The influence of vitamin B 12 on the growth potential of Cladophora glomerata (L.) Kütz. was investigated by quantifying the following responses to cellular vitamin B 1 2 concentrations: yield, photosynthetic rate, and vitamin B 12 uptake rate. Yield was expressed as dry weight harvested after a 28‐d growth period; photosynthetic rate was estimated by the 14 C technique; vitamin uptake rates were determined with 3 H‐labeled vitamin B 12 , and cellular vitamin B 12 concentration was estimated by bioassay. Functional dependence of these responses on cellular vitamin B 12 concentration was best described by the Droop cell‐quota model. The critical concentration, defined as the minimal cellular vitamin B 12 concentration needed for a maximal response, was estimated as the concentration range that resulted in a response that was 75–85% of the maximum. Responses consistently resulted in a predicted critical concentration range of 0.3–0.5 ng B 12 (mg dry wt) −1 . This is interpreted as a range of cellular vitamin B 12 concentration below which significant limitation of the growth potential of Cladophora occurs and above which it does not. This critical concentration range can be used to indicate vitamin B 12 limitation in nature. The physiological parameters describing the Cladophora response curves are similar to those of other algae that require vitamin B 12 .