Questions regarding which form of inorganic carbon is used by marine phytoplankton (CO₂ and/or HCO₃^-) and what is the predominant pathway of carbon acquisition are presently issues of controversial debate. Answers to these questions will inevitably determine the ecological and biogeochemical significance of potential CO₂ effects on growth rate and elemental composition (C:N:P) of marine phytoplankton. Knowledge on inorganic carbon source(s) and mechanism(s) of carbon uptake is also critical in interpreting photosynthetic isotope fractionation and is required in paleoceanographic reconstructions using sedimentary carbon isotopes.  Significant variability is observed in the carbon isotopic composition of marine phytoplankton. Primary factors determining the fractionation of stable carbon isotopes in microalgae are CO₂ concentration, growth rate, cell size and cell shape. Recent experiments performed in our group demonstrated, however, that additional factors have to be taken into account. Light conditions, the factors limiting phytoplankton growth, carbon acquisition mechanisms as well as species specific characteristics strongly influence isotope fractionation. Field observations and laboratory experiments showed that the relationship of isotope fractionation to CO₂ concentration and growth rate changes in the course of an algal bloom. These results severely complicate the interpretation of carbon isotope data in geochemical and paleoceanographic applications.