MEPS 225:139-146 (2002)  -  doi:10.3354/meps225139

ABSTRACT: Although temperature effects on phytoplankton growth and photosynthesis can be clearly demonstrated in the laboratory, their relevance in the field is much harder to establish. Recently, however, it has been recognized that temperature has a significant influence on nitrogen uptake. In particular, temperate marine diatom species may be limited by their ability to acquire nitrate at temperatures above approximately 16°C. In order to explore this idea, we grew the diatom Thalassiosira pseudonana at 8, 17 and 25°C, and compared cell composition, and rates of growth (µ), ¹⁵N incorporation, calculated nitrate incorporation (the product of µ and cell N content), and the activity of nitrate reductase (NR), a key enzyme involved in nitrate incorporation. Cell N content, protein and volume remained relatively constant across different temperatures, but cell C, chlorophyll a (chl a), and C:N ratio increased with increasing temperature, suggesting that C metabolism was affected more strongly than N metabolism. Classical temperature models suggested that growth and various indices of nitrate metabolism all responded to temperature, with Q₁₀ values of close to 2 over the whole temperature range. However, Q₁₀ values over the interval from 8 to 17°C were higher than 2 and much lower than 2 between 17 and 25°C. Limitations to the Q₁₀ concept are considered. Temperature effects on different measures of nitrate metabolism were very similar, supporting the hypothesis that the effects of temperature on diatom nitrate metabolism are mediated at the level of NR activity. Recent biochemical data for NR also supports this idea.

KEY WORDS: Nitrate reductase · Enzyme activity · ¹⁵N uptake · Phytoplankton ecology · Chemical composition · Temperature adaptation