• Received 21 December 1992. Accepted 5 May 1995.

  • We thank Esther Bonneau and Sonia Michaud for assistance in the laboratory and S. Demers, T. Packard, W. Vincent, J-C. Therriault, M. Babin, J A. Berges, and anonymous reviewers for suggesting improvements, to earlier drafts of this paper.


We examined the energetic dependency of the biochemical and physiological responses of Thalassiosira pseudonana Hasle and Heimdal. Chaetoceros gracilis Schütt, Dunaliella tertiolecta Butcher, and Gymnodinium sanguineum Hirasaka to NH4+, NO3, and urea by growing them at subsaturating and saturating photon flux (PF). At subsaturating PF, when energy was limiting, NO3 and NH4+ grown cells had similar growth rates and C and X quotas. Therefore, NO3 grown cells used up to 48% more energy than NH4+ grown cells to assimilate carbon and nitrogen. Based on our measurements of pigments, chlorophyll-a-specific in vivo absorption cross-section, and fluorescence-chlorophyll a−1, we suggest that NO3, grown cells do not compensate for the greater energy requirements of NO3 reduction by trapping more light energy. At saturating PF, when energy is not limiting, the utilization of NO3, compared to NH4+ resulted in lower growth rates and N quotas in Thalassiosira pseudonana and lower N quotas in Chaetoceros gracilis, suggesting enzymatic rather than energetic limitations to growth. The utilization of urea compared to Nh4+ resulted in lower growth rates in Chaetoceros gracilis and Gymnodinium sanguineum (saturating PF) and in lower N quotas in all species tested at both subsaturating and saturating PF. The high C:N ratios observed in all urea-grown species suggest that nitrogen assimilation may be limited by urea uptake or deamination and that symptoms of N limitation in microalgae may be induced by the nature of the N source in addition to the N supply rate. Our results provide new eridence that the maximum growth rates of microalgae may be limited by enzymatic processes associated with the assimilation of NO3, or urea.