Comparing pulse amplitude modulated (PAM) fluorometry with radiocarbon technique for determination of inorganic carbon fixation in <i>Chlorella vulgaris</i> (Trebouxiophyceae, Chlorophyta)

Integration of pulse amplitude modulated (PAM) fluorometry and conventional methods for estimating carbon assimilation in microalgae is important for physiological, ecological and economic purposes. In this study, we compared PAM fluorometry and carbon-14 (¹⁴C) uptake techniques to estimate the carbon fixation rate in <i>Chlorella vulgaris</i> under controlled laboratory conditions. The key parameter for this comparison was the electron yield for carbon fixation (<i>Ф_e</i>), commonly assumed when converting electron transport rate (<i>ETR</i>) values into the chlorophyll-specific carbon fixation rate (<i>P^B</i>). Additional analyses of maximum (<i>Φ_M</i>) and effective (<i>Φ’_M</i>) quantum efficiency of photosystem II, photochemical (<i>qP</i>) and non-photochemical (<i>NPQ</i>) quenching, and photosynthesis-irradiance response curves demonstrated that the photophysiology of <i>C. vulgaris</i> did not change after a 2-h incubation with NaH¹⁴CO₃ and Na₂CO₃ (control). The association of <i>P^B</i> obtained through the ¹⁴C method (151 ± 8.77 µmol C [mg chl <i>a</i>]^–1 h^–1) with <i>ETR</i> (411 ± 3.91 µmol e⁻ [mg chl <i>a</i>]^–1 h^–1) resulted in an average <i>Ф_e</i> of 0.37 ± 0.02 µmol C [µmol e⁻]^–1, which is higher than theoretical <i>Ф_e</i> values usually reported in the literature (e.g. 0.20 and 0.25). We attributed this discrepancy to a possible inaccuracy in <i>ETR</i> due to underestimated values of chlorophyll-specific absorption cross-section (<i>a*</i>) and the common assumption that only 50% of total light is absorbed by photosystem II. We here demonstrate the importance of associating chlorophyll fluorescence with other primary production techniques, so that adjustments to calculation procedures can be made in accordance to species-specific physiological traits and particularities regarding culturing conditions.将脉冲振幅调制（pulse amplitude modulated, PAM）荧光法与常规方法结合以估算微藻的碳同化量，在生理学、生态学与经济学领域均具有重要意义。本研究在受控实验室条件下，对比了PAM荧光法与碳-14（¹⁴C）摄取技术，以估算普通小球藻（<i>Chlorella vulgaris</i>）的固碳速率。本次对比的核心参数为固碳电子产率（Ф_e），该参数是将电子传递速率（electron transport rate, ETR）转换为叶绿素特异性固碳速率（<i>P^B</i>）时的常用假设参数。本研究还对光系统II的最大量子效率（Φ_M）与有效量子效率（Φ’_M）、光化学淬灭（<i>qP</i>）与非光化学淬灭（NPQ），以及光合作用-光响应曲线进行了额外分析，结果显示，普通小球藻经NaH¹⁴CO₃与Na₂CO₃（对照组）孵育2小时后，其光生理特性未发生改变。通过¹⁴C方法获得的<i>P^B</i>为151 ± 8.77 µmol C [mg chl <i>a</i>]^–1 h^–1，结合电子传递速率（411 ± 3.91 µmol e⁻ [mg chl <i>a</i>]^–1 h^–1），可计算得到平均固碳电子产率Ф_e为0.37 ± 0.02 µmol C [µmol e⁻]^–1，该值高于文献中通常报道的理论固碳电子产率（如0.20与0.25）。研究将这一差异归因于电子传递速率的计算可能存在误差：这源于叶绿素特异性吸收截面（<i>a</i>^*）的取值被低估，以及一个通用假设——总光照中仅有50%被光系统II吸收。本研究证实了将叶绿素荧光法与其他初级生产力测定方法结合的重要性，以便能够根据物种特异性生理特征与培养条件的特殊性，对计算流程进行调整。