Heterotrophic and photoheterotrophic utilization of lactate by the diatom,Cylindrotheca fusiformis

The rates of lactate utilization, evaluated in terms of uptake, incorporation into acid insoluble products and respiration, were measured in the light and dark in cells which had been (1) growing autotrophically without lactate in the medium, (2) growing heterotrophically on lactate or (3) growing autotrophically and were then starved in the dark without lactate for 2–4 days. Autotrophic cells have low but significant lactate utilization rates which increase in dark-starved cells and increase further after adaptation to heterotrophic growth. Incubation in the light dramatically increases incorporation rates in dark-starved and heterotrophic cells, an effect that is not due solely to the refixation of respired lactate carbon. Assimilation efficiencies were influenced by the light or dark incubation conditions during the experiment rather than by previous growth conditions of the cells. Our results, including experiments with a photosynthetic inhibitor, support the hypothesis that the induction of lactate utilization systems involves incorporation and respiration pathways as well as lactate uptake systems.