Interaction Of N:P Ratio And Light、CO₂on Photosynthetic Carbon Utilization In Phaeodactylum Tricornutum

Marine diatoms are the most successful group of algae and their photosynthesisin the oceans is known to be an extremely important component of global carboncycles. Photosynthesis is closely related to inorganic carbon utilization which wasaffected by nutrient supply, light and CO₂. Interaction of N:P ratios and light, CO₂onphotosynthetic inorganic carbon utilization in the marine diatom Phaeodactylumtricornutum was investigated in this paper, and the main findings are as follows:The specific growth rate in the algae grown under the medium with N:P=16:1was the largest. Higher or lower than N:P=16:1resulted in a significant decrease,which indicated that the optimum growth was limited by nitrogen or phosphorussupply. Chlorophyll a content decreased in N or P limitation, however the content ofchlorophyll c was significantly affected by N or P limitation. pH compensation point,light-saturated photosynthetic rate and photosynthetic efficiency also reduced in N orP-limited conditions. K0.5CO₂was decreased in P-limited conditions,butunchangeable in N limitation, which indicated P limitation could increase the affinityfor CO₂,Under the different nutrient levels, elevated light intensity from50to250molm-2s-1resulted in a significant increase in the specific growth rate. A decrease inFv/Fm, Yield, qP and an increase of qN were also observed in cells grown under highlight intensity. Moreover, qN in N or P-limited conditions rose with the increasedlight levels more slowly than that of nutrient-replete. High light intensity led to adecrease of Pm, α and Rdand an increase of Ikand IC. With the increase of light levels,carbonic anhydrase activity and plasma membrane redox activity rose significantly.However, K0.5was not affected by high light intensity in N or P-limited conditions.pH compensation point in high light levels had a obvious decline in N limitation, andkept constant in P limitation.The specific growth rate, Pm,, chlorophyll a and c content in N-limited cellswere not affected by CO₂enrichment. Under P-limited condition, Elevated CO₂caused unchange of the specific growth rate and chlorophyll c content, but Pm, andchlorophyll a content increased with the elevated CO₂. CO₂enrichment led to asignificant increase in Fv/Fm, Yield and qP under N or P-limited condition. K0.5declined significantly in N-limited conditions, but the opposite result was observed inP-limited conditions. Extracellular carbonic anhydrase activity and plasma membraneredox activity in N or P limitation increased with CO₂enrichment, but pH compensation point declined.The above results showed that1) The optimum growth in Phaeodactylumtricornutum was limited by nitrogen or phosphorus supply when N:P ratio was higheror lower than=16:1, and this alga grown under conditions of nutrient limitation （N orP） could maintain a moderate growth by adjusting chlorophyll content, the mode ofinorganic carbon utilization.2) Under N or P-limited conditions, high light intensityweakened the growth, and decreased energy for photosynthesis and energy utilizationefficiency. In the meantime, carbonic anhydrase activity was increased to adapt tohigh light conditions.3) The specific grwoth rate in N or P-limited cells was notaffected by CO₂enrichment, however, the effiency of energy utilization increased.The affinity for inorganic carbon and carbonic anhydrase activity in Phaeodactylumtricornutum cultured under N or P limitation were regulated to adapt to the elevatedCO₂condition.