[P008] Poster

1:00 PM - 1:50 PM JST(4:00 AM - 4:50 AM UTC)

[P04-566] Impacts of changing nutrient availability on CO₂-concentrating mechanism and photosynthesis in the marine diatom, Phaeodactylum tricornutum

○Momoka Amano¹, Ginga Shimakawa², Yusuke Matsuda¹ (1. Grad. Sch. Sci. Tech., Kwansei Gakuin Univ. (Japan), 2. Grad. Sch. Agric., Kobe Univ. (Japan))

Keywords:

photosynthesis,CO2 concentrating mechanism,nutrient deprivation,diatom

Diatoms operate a CO₂-concentrating mechanism (CCM) for the efficient photosynthesis. The diatom CCM is known to be regulated in response to external CO₂ concentrations, while regulation of the CCM by other primary metabolism regulators such as nitrogen (N), phosphorus (P), and sulfur (S) is not clear. Pyrenoid in the chloroplast is a focal point of the CCM, in other words, it is an initiating point for the chloroplastic primary metabolisms. The demand for organic carbon skeleton should greatly be affected by the availabilities of N, P, S as these macro nutrients is essential for amino acids and nucleic acids. In the present study, we investigated the regulation of CCM factors in the marine diatom Phaeodactylum tricornutum under combined N/P/S deprivation and high/low CO₂ (HC/LC) conditions. Cells were pre-cultured under LC/+N conditions, where CCM is fully induced, and then transferred to the LC or HC conditions. Each transferred culture was divided into +N/-N, +P/-P, or +S/-S conditions and allowed to acclimate for 2 days. The effect of N/P/S deprivation on photosynthetic was evaluated by measuring CCM activity and protein accumulations. The photosynthetic DIC affinity in the HC/+N-acclimated cells was less than 10% of the LC/+N cells, revealing a typical suppression of CCM. On the other hand, cells acclimated to LC/-N showed photosynthetic DIC affinity about 10% of the LC/+N, which was comparable to HC/+N cells. Accumulation of proteins required for the maximum induction of the CCM (namely chloroplastic Ptβ-CA1/2 and PtBST1) were also repressed in LC/-N. Similar nutrient-triggered repression on the CCM activity and factors were also observed in LC/-P and LC/-S. These results indicate that P. tricornutum represses LC-induced maximum CCM at the protein accumulation level under N/P/S deprivation, strongly suggesting the occurrence of feedback systems to suppress CCM in response to major nutrient availability presumably by sensing the demand for the carbon skeleton. We have previously reported that CO₂ signals are mediated by cAMP to repress CCM, that is involving transcription factor, Ptbzip11 and its target Cis-element, CO₂-cAMP-responsible elements (CCRE) at the promoter of the LC-inducible CCM factors. We treated cells acclimating to LC/+N, HC/+N and LC/-N for 2 days by a specific inhibitor, H89 for protein kinase A (PKA), a famous mammalian cAMP-downstream transducer, confirming the recovery of protein levels of PtBST1 and Ptβ-CA1/2. These data indicate that PKA plays a role as a part of repression signals of the CCM not only in HC but also in –N. Our study also showed that, in sharp contrast to the LC-induced CCM factors, the protein levels of the core-structural factors to constitute the CO₂ evolving machinery, Pyrenoid Shells (PtPyShells) and luminal carbonic anhydrase (Ptθ-CA1) were resiliently maintained under all conditions, strongly supporting the occurrence of the multiphase CCM in diatoms.