Limits to Microalgal Growth
Our objectives were to determine: 1) the influence of different N forms (NH4 +, NO3, and urea) on the growth rate and on the nitrogen and carbon content of four microalgae, and 2) to relate the N form-induced physiological and biochemical differences to a shortage in energy (presumably reductant for NO3 − reduction), or a rate-limiting step in the N uptake and/or assimilation. The use of NO3 − instead of NH4 + resulted in a decrease in growth rate in Thalassiosira pseudonana and in significantly smaller N quotas and higher C:N ratios in T. pseudonana and Chaetoceros gracilis. Growth on urea instead of NH4 + resulted in a lower growth rate in C. gracilis and Gymnodinium sanguineum and in a lower N quota and a higher C:N ratio in most species tested. Growth on NO3 − rather than on NH4 + often resulted in a smaller (less chlorophyll a cell−1) and less efficient (lower quantum yield for oxygen production, higher chlorophyll a fluorescence yield) photosynthetic system, suggesting that NO3 −-grown cells were not energy-limited. On the other hand, since urea-grown cells have a reductant requirement similar to that of NH4 +-grown cells, the urea-induced decrease in growth rate and/or N quotas cannot be attributed to reductant limitation. The rate-limiting step for growth is more likely the activity of nitrate reductase (NR), nitrite reductase (NiR), or urease. Our results also show that symptoms of N limitation in algae (low N quotas, high C:N ratios) may be introduced by the N form, even at saturating N concentrations.