Photosynthetic response of lake plankton to combined nitrogen enrichment
Lean, D.R.S.; Murphy, T.P.; Pick, F.R.
Journal of Phycology 18(4): 509-521
ISSN/ISBN: 0022-3646 DOI: 10.1111/j.1529-8817.1982.tb03217.x
The complex interplay between photosynthesis and the uptake of N was investigated in samples from 5 lakes [Canada] of different size and trophic state. When enriched with 15NH4+, the photosynthetic rate was often reduced for 4-5 h in samples believed to be N deficient. This implies that energy was reallocated from photosynthesis to the uptake and assimilation of N. Stimulation in C uptake at low levels of NH4+ enrichment was followed by a progressive decline with further NH4+ enrichment. On other occasions when ambient NH4+ was undetectable, nutrient regeneration by zooplankton supplied a significant fraction of the required N. At these times and when the plankton had sufficient available N, there usually was no change in photosynthetic rate with either NH4+ or NO3- enirchment. Typically, little NO3- was taken up and no photosynthetic response was observed. On 2 occasions the uptake of NO3- was significant due to high NO3- and low NH4+ levels early in the season. At 1 of these times there was a reduction in photosynthesis with NO3- enrichment. A further complication was observed when photosynthesis decreased with NH4+ enrichment but increased with NO3- enrichment despite negligible NO3- uptake. The complex metabolism of these 2 N sources is not fully understood. AT optimum light intensity, C:N uptake ratios, even under NH4+ enrichment, are only sufficient to maintain the cellular C:N ratio unless much of the fixed C is respired or excreted. Apparently, photosynthesis and N uptake are not coupled. Photoinhibition of C uptake, but not N uptake was observed when low light adapted populations are exposed to high light conditions. The light intensity for maximum N uptake was slightly less than that for carbon. Dark N uptake was always near 50% of the maximum rate in the light whereas the C uptake was near 2% of Popt [optimum photosynthetic rate]. Certainly, there is an interconnection because dark C uptake was enhanced by NH4+ enrichment.