PHYSIOLOGICAL ACCLIMATION OF MARINE PHYTOPLANKTON TO DIFFERENT NITROGEN SOURCES¹

Abstract

ABSTRACT 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 NH 4 + , NO 3 − , and urea by growing them at subsaturating and saturating photon flux (PF). At subsaturating PF , when energy was limiting, NO 3 − and NH 4 + grown cells had similar growth rates and C and X quotas. Therefore, NO 3 − grown cells used up to 48% more energy than NH 4 + 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 NO 3 − , grown cells do not compensate for the greater energy requirements of NO 3 − reduction by trapping more light energy. At saturating PF, when energy is not limiting, the utilization of NO 3 − , compared to NH 4 + 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 Nh 4 + 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 NO 3 − , or urea.