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NITROGEN LIMITATION IN ISOCHRYSIS GALBANA (HAPTOPHYCEAE). I. PHOTOSYNTHETIC ENERGY CONVERSION AND GROWTH EFFICIENCIES 1

The effect of steady‐state nitrogen limitation on photo‐synthetic characteristics and growth efficiency was examined in the marine haptophyte Isochrysis galbana Green. Nitrate limited chemostats were maintained at nine dilution rates, ranging from 0.18‐0.96 d−1, under continuous irradiance levels of 175 μmole quanta·m−2·s−1, an irradiance level which saturated photosynthesis at all growth rates. Nitrogen limitation led to an overall reduction in pigmentation and a decrease in the cellular concentration of reaction centers; however, the optical absorption cross section, normalized to Chl a, increased. Moreover, Chl c/a ratios were higher in nitrogen‐limited cells: the change in Chl c/a ratios were correlated with an increase in the functional size of Photosystem II. Both light saturated photosynthetic rates normalized per cell and specific respiratory losses were positively linearly correlated with growth rate. Light saturated photosynthetic rates normalized to Chl a remained relatively insensitive to the rate of nitrogen supply. The minimum quantum requirement for gross photosynthetic oxygen evolution increased from 12.4 to 17.0 quanta/O2. At the growth irradiance, the quantum requirement increased 88%, from 19.9 to 37.5 quauta/O2 Photosynthesis/respiration ratios remained relatively constant at dilution rates greater than 35% of the maximum relative growth rate. Consequently, net growth efficiency, defined as the ratio of the specific growth rate, μ, to specific gross photosynthesis, P, also remained relatively constant over this range of growth rates averaging 85 ± 3%.