Abstract
BackgroundClarifying the relationship between photosynthesis and irradiance and accurately quantifying photosynthetic performance are of importance to calculate the productivity of phytoplankton, whether in aquatic ecosystems modelling or obtaining more economical production.ResultsThe photosynthetic performance of seven phytoplankton species was characterized by four typical photosynthesis–irradiance (P–I) response models. However, the differences were found between the returned values to photosynthetic characteristics by different P–I models. The saturation irradiance (Isat) was distinctly underestimated by model 1, and the maximum net photosynthetic rate (Pnmax) was quite distinct from its measured values, due to the asymptotic function of the model. Models 2 and 3 lost some foundation to photosynthetic mechanisms, that the returned Isat showed significant differences with the measured data. Model 4 for higher plants could reproduce the irradiance response trends of photosynthesis well for all phytoplankton species and obtained close values to the measured data, but the fitting curves exhibited some slight deviations under the low intensity of irradiance. Different phytoplankton species showed differences in photosynthetic productivity and characteristics. Platymonas subcordiformis showed larger intrinsic quantum yield (α) and lower Isat and light compensation point (Ic) than Dunaliella salina or Isochrysis galbana. Microcystis sp., especially M. aeruginosa with the largest Pnmax and α among freshwater phytoplankton strains, exhibited more efficient light use efficiency than two species of green algae.ConclusionsThe present work will be useful both to describe the behavior of different phytoplankton in a quantitative way as well as to evaluate the flexibility and reusability of P–I models. Meanwhile we believe this research could provide important insight into the structure changes of phytoplankton communities in the aquatic ecosystems.
Highlights
Clarifying the relationship between photosynthesis and irradiance and accurately quantifying photosynthetic performance are of importance to calculate the productivity of phytoplankton, whether in aquatic ecosystems modelling or obtaining more economical production
Assuming that the initial slope α was 0.5, increasing values of the light-saturated or photoinhibition parameters decreased μmol O2 mg−1 Cha h−1 (Pnmax) of the curve and increased the magnitude of inhibition in three types (Fig. 1b–f ), which indicated that they could closely reproduce the trend of the photosynthetic rate (Pn)–I curve
The Chlorophyll a (Chl a) contents were 1.647 ± 0.015, 2.778 ± 0.077, 2.297 ± 0.027, 1.320 ± 0.005, 1.739 ± 0.012, 1.318 ± 0.027 and 4.158 ± 0.077 mg L−1 for cultures of I. galbana, D. salina, P. subcordiformis, M. aeruginosa, M. wesenbergii, S. obliquus and Chlorococcum sp., respectively (Table 1), which was used to normalize the photosynthetic oxygen-producing rate
Summary
Clarifying the relationship between photosynthesis and irradiance and accurately quantifying photosynthetic performance are of importance to calculate the productivity of phytoplankton, whether in aquatic ecosystems modelling or obtaining more economical production. Clarifying the relationship between photosynthesis and irradiance is a basis to evaluate the growth performance of phytoplankton. The level of irradiance affects the growth, CO2 fixation efficiency, carbon metabolism, and cell composition of photosynthetic organisms [4,5,6,7,8]. High irradiance causes photoinhibition by the production of reactive oxygen species (ROS) and damages the function of the most light-sensitive complex PSII [5]. Irradiance availability affects phytoplankton community composition and is one of the key factors causing cyanobacteria blooms [12]. Resource competition theory shows that species with lower “critical light intensity” are often superior, such as Microcystis [13]
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