Quantizing photosynthetic performance of phytoplankton 1 using photosynthesis-irradiance response models 2

19 Background: Clarifying the relationship between photosynthesis and irradiance and 20 accurately quantizing photosynthetic performance are of importance to calculate the 21 productivity of phytoplankton, whether in aquatic ecosystems modelling or obtaining 22 more economical production. 23 Results: The photosynthetic performance of seven phytoplankton species was characterized by four typical photosynthesis-irradiance ( P-I ) response models. However, 25 the differences were found between the returned values to photosynthetic characteristics 26 by different P-I models. The saturation irradiance ( I sat ) was distinctly underestimated by 27 model 1, and the maximum net photosynthetic rate ( P nmax ) was quite distinct from its 28 measured values, due to the asymptotic function of the model. Models 2 and 3 lost some 29 foundation to photosynthetic mechanisms, that the returned I sat showed significant 30 differences with the measured data. Model 4 for higher plants could reproduce the 31 irradiance response trends of photosynthesis well for all phytoplankton species and 32 obtained close values to the measured data, but the fitting curves exhibited some slight 33 deviations under the low intensity of irradiance. Different phytoplankton species showed 34 differences in photosynthetic productivity and characteristics. P. subcordiformis showed 35 larger intrinsic quantum yield ( α ) and lower I sat and light compensation point ( I c ) than D. 36 salina or I. galbana . Microcystis sp., especially M. aeruginosa with the largest P nmax and 37 α among freshwater phytoplankton strains, exhibited more efficient light use efficiency 38 than two species of green algae. 39 Conclusions: The present work will be useful both to describe the behavior of different 40 phytoplankton in a quantitative way as well as to evaluate the flexibility and reusability 41 of P-I models. Meanwhile we believe this research could provide important insight into 42 the structure changes of phytoplankton communities in the aquatic ecosystems. 43


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Background 49 Phytoplankton are a key functional component of aquatic ecosystems, play a pivotal 50 role in biogeochemical cycles [1]. In particular, marine phytoplankton, as the principal 51 driving force of ocean carbon cycles and energy flows, fix approximately 50 gigatons of 52 inorganic carbon annually, almost half of the total global primary production [2, 3]. They 53 show higher CO2 fixation rates and higher biomass productivity than any other 54 photosynthetic organism [3]. With increasing concentration of CO2 concentrations in the 55 atmosphere and growing climate warming, an accurate estimate of photosynthetic 56 productivity of phytoplankton becomes ever more important for modelling primary 57 production and structure changes of phytoplankton communities in aquatic ecosystems, 58 especially eutrophic lakes (e.g., Taihu, Erie, Winnipeg lake) and estuaries (e.g., Yangtze 59 River). 60 Clarifying the relationship between photosynthesis and irradiance is a basis to evaluate  crustaceans primarily graze on phytoplankton to build immunity against diseases during 79 their early larval stages [12]. However, large-scale production of phytoplankton has rarely 80 been successful, with no more than 1 g DW L −1 biomass that is mainly limited by the 81 inefficiency of photosynthesis in high-cell density cultivation [11,14,15]. The 82 photosynthetic parameters can be seen as indicators to achieve sustainable carbon 83 assimilation and TAG accumulation in Isochrysis zhangjiangensis [8]. Therefore, 84 accurately quantizing photosynthetic performance is crucial for more economical 85 integration of production management and operation of industrial-scale phytoplankton 86 culture systems [16]. 87 The response curve of photosynthesis to irradiance (P-I) is frequently used to 88 characterize photosynthetic performance by fitting experimental data (measured as 89 oxygen evolution or carbon uptake) with P-I models [17]. Obtained photosynthetic 90 parameters, including the maximum net photosynthetic rate (Pnmax), the optimal intensity 91 of irradiance (Isat), and the dark respiration rate (Rd) can be regarded as indicator to 92 evaluate the response of phytoplankton to meet environmental changes. A variety of P-I 93 models for phytoplankton have been established in the last few decades [18][19][20][21][22][23][24][25][26][27][28][29][30]. Although 94 many recent models are suggested based to "old models" established in the 70s and 80s 95 and have some contributions to improve, the most extensive application are still found in

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The objective of this study was to determine the various relationships between the 106 photosynthetic productivity of phytoplankton and irradiance intensity and investigate the 107 reliability of P-I models to estimate the photosynthetic performance for phytoplankton. 108 We selected the rather extensive range of phytoplankton, including three isolated from day at 26 ± 1 °C.

Measurement of photosynthetic oxygen evolution 127
After seven to ten days of incubation, the photosynthetic oxygen-evolving rate of 128 microalgal cells reaching the exponential growth phase was determined using a bio-129 oxygen metre (Yaxin-1151, Beijing Yaxinliyi Science and Technology Co., Ltd., China).

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Eight-mL cell suspensions of each strain were exposed to increasing orders of irradiance where Pn is the chlorophyll a-normalised net photosynthetic rate at irradiance I; Ps is the 169 parameter reflecting the maximum, potential, light-saturated, rate of photosynthesis; α is 170 the light-limited initial slope; β is the dimensionless parameter reflecting the 171 photoinhibition process; and Rd is the dark respiration rate.

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The Isat is calculated as: The Pnmax can be calculated as: However, the analytic solution of Ic can not be obtained by equation (5). To obtain Ic, 177 the Kok effect must be ignored here, and Ic can be calculated as: The photosynthetic quantum efficiency is calculated as:  Isat is calculated as: Pnmax is given by: When Pn = 0, Ic is given as follows, The photosynthetic quantum efficiency is calculated as:

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Isat is calculated as: Pnmax is obtained by: When Pn = 0, Ic is given as follows,  Goodness of fit of the mathematical models to experimental data was assessed using the 216 adjusted coefficient of determination (R 2 ).

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Comparison of different P-I models of production curves 219 Applying different values of the fundamental parameters to the model, the differences

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The morphological and growth characteristics of phytoplankter 234 The morphology of the cultured cells was observed under a 600x optical microscope.

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Cells were mostly spherical, at 4.3 ~ 10 μm in diameter, and grew singly, except for S. 236 obliquus. Chlorococcum sp., respectively (Table 1), which was used to normalize the    The photosynthetic quantum yield represents the efficiency of carbon dioxide fixation 304 14 or oxygen evolution by a photosynthetic apparatus driven by absorbed photon energy, 305 that is, the conversion efficiency of absorbed solar energy into chemical energy. Fig. 2B   306 shows that the quantum yield calculated by models 2, 3 and 4 for I. galbana, D. salina 307 and P. subcordiformis decreased as I increased, until it was equal to zero at the Isat point.

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Subsequently, it became negative as I increased, which also reveals why Pn decreased as  but the value of Ps among the fitted results was notably higher than the value of Pnmax in 345 seven phytoplankton strains, whether β > 0 or β = 0 (Table 4). However, Pnmax = Ps by 346 Eqn. 7, where there was no inhibition at β = 0, and the fitting curves were similar to model 347 1 (Fig. 6). Additionally, Ps appeared to fluctuate at β > 0, which indicates the presence of  This reveals that P-I models for higher plants are applicable for phytoplankton. Acquiring 371 an accurate and optimal parameter for irradiance intensity is essential to achieve high 372 biomass of phytoplankton in production. Irradiance is rapidly attenuated during high-cell  To meet nutritional requirements, mixed cultures of two or more species of phytoplankton 401 are often fed to larvae in seed faming of aquatic products [49]. It is critical that the 402 photosynthetic productivity of each strain reach as high as possible during production.

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The comparison revealed that, although the Pnmax lay between I. galbana and P. 404 subcordiformis, other photosynthetic characteristic parameters showed great differences.

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The smallest α and highest Ic were found in I. galbana, which meant a low efficiency of Under co-culture conditions, a gradient of irradiance from low to mid to high can be 412 supplied in one photoperiod.

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Our study showed that significant differences were found between the returned values