The use of NH₄⁺ rather than NO₃⁻ affects cell stoichiometry, C allocation, photosynthesis and growth in the cyanobacterium Synechococcus sp. UTEX LB 2380, only when energy is limiting

Abstract

The assimilation of N-NO requires more energy than that of N-NH . This becomes relevant when energy is limiting and may impinge differently on cell energy budget depending on depth, time of the day and season. We hypothesize that N-limited and energy-limited cells of the oceanic cyanobacterium Synechococcus sp. differ in their response to the N source with respect to growth, elemental stoichiometry and carbon allocation. Under N limitation, cells retained almost absolute homeostasis of elemental and organic composition, and the use of NH did not stimulate growth. When energy was limiting, however, Synechococcus grew faster in NH than in NO and had higher C (20%), N (38%) and S (30%) cell quotas. Furthermore, more C was allocated to protein, whereas the carbohydrate and lipid pool size did not change appreciably. Energy limitation also led to a higher photosynthetic rate relative to N limitation. We interpret these results as an indication that, under energy limitation, the use of the least expensive N source allowed a spillover of the energy saved from N assimilation to the assimilation of other nutrients. The change in elemental stoichiometry influenced C allocation, inducing an increase in cell protein, which resulted in a stimulation of photosynthesis and growth.

Nitrate or ammonium: Influences of nitrogen source on the physiology of a green alga

“…Interestingly, the NR activity in NO − 3 -grown cells under low CO 2 was higher in P i -replete than in P i -limited cells, supporting the assumption that an increased energy demand, in terms of ATP, for NO − 3acquisition (Ruan & Giordano, 2017) was also present in C. acidophila.…”

“…It was previously shown that the influence of the N i ‐source on different physiological parameters might depend on other factors, such as light (Ruan & Giordano, ) and CO 2 (Giordano, ), but to our knowledge there are no studies combining two factors involved in energy and carbon metabolism in comparison with the effect of a different N i ‐source. Therefore, as both C i (Tittel, Bissinger, Gaedke, & Kamjunke, ) and P i (Spijkerman ) have been identified as potential (co‐)limiting factors for C. acidophila in the acidic Lake 111 (Spijkerman, Stojkovic, Holland, Lachmann, & Beardall, ), we included both factors in our setup.…”

“…Therefore, it is quite obvious that interactions between photosynthesis‐related processes and the acquisition of nitrogen occur, which is, for example, shown by a causal relationship between the assimilation of and dark C i ‐fixation under anaplerosis (Giordano, Norici, Forssen, Eriksson, & Raven, ; Vanlerberghe, Schuller, Smith, & Turpin, ) as well as photosynthetic CO 2 ‐fixation (Turpin, Vanlerberghe, Amory, & Guy, ). In Synechococcus sp., the use of over stimulated photosynthesis and growth under a light limitation (Ruan & Giordano, ).…”

Different Nutritional Histories Affect the Susceptibility of Algae to Grazing

“…The possibility of a nutrients remineralization via grazer metabolism was proposed by the authors, but the likelihood of such hypothesis depends on the nutritional and energetic context in which the interspecific interaction occurs. A stimulation of growth by nutrient regeneration can only occur in algae whose growth is limited by the same regenerated nutrient and/or by the energy availability (when regeneration makes a nutrient available in a form that requires less energy for assimilation than the form it was originally assimilated; e.g., NH 4 + vs. NO 3 − ; Ruan and Giordano , Ruan et al. ).…”

“…Also a N and energy co‐limitation can be envisaged: H‐CO 2 cells and LL cells were grown in NO 3 − in semicontinuous cultures and then transferred, at the beginning of the grazing experiments, to a medium (control conditions) containing the same concentration of NO 3 − than the media of origin. The release of an alternative source of N through regeneration by zooplankton, under energy limitation, may have stimulated growth because of the lower cost of NH 4 + than NO 3 − assimilation (e.g., Ruan and Giordano , Ruan et al. ), and the consequent greater availability of energy for growth.…”