Physiological and biochemical responses of Thalassiosira punctigera to nitrate limitation

Abstract

Marine diatoms play an important role in the global primary production and contribute significantly to carbon export in upwelling regions. However, their carbon fixation is frequently limited by nutrient availability, which varies regionally and temporally. In general, as a result of global warming, coastal waters are predicted to become more eutrophic, while nutrient limitation in the open ocean would be further enhanced. For a better understanding on how diatoms respond to nitrogen supply in the future ocean, we cultivated Thalassiosira punctigera at different levels of nitrate (from 1 to 100 µmol L−1). Results showed that the half-saturation value for growth rate (Kµ = 3.06 µmol L−1) corroborates the values obtained from a size-dependent model (3.03 µmol L−1), suggesting that nutrient storage in this large diatom has equilibrated with the bulk medium added during culturing. Measured physiological parameters, including growth rate, cellular Chl a concentration, and photosynthesis, were all significantly reduced as the extent of nitrogen limitation increased. However, the responses of elemental composition did not vary linearly with respect to the different degrees of nitrate limitation. Specifically, the elemental ratios of C:N and Si:N all increased under moderate nitrate limitation (2–10 µmol L−1), but further decreased when nitrate became extremely limited (1 µmol L−1). The particulate organic nitrogen production rate was sensitive to nitrate limitation, showing a significant decrease when nitrate was below 10 µmol L−1. However, particulate organic carbon production rate remained stable until nitrate was reduced to 2 µmol L−1. The diverse responses of physiological processes and elemental ratios to various nitrate levels indicate the fact that there is a large uncertainty about the biogeochemical consequences of changes in the future ocean.