effect of temperature, salinity and irradiance on growth

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Effect of temperature, salinity and irradiance on growth and photosynthesis of Ulva prolifera XIAO Jie1, ZHANG Xiaohong1, 2, GAO Chunlei1*, JIANG Meijie1, LI Ruixiang1, WANG Zongling1, LI Yan1, FAN Shiliang1, ZHANG Xuelei1
1 Key Laboratory of Science and Engineering for Marine Ecology and Environment, The First Institute of Oceanography, State Oceanic Administration, Qingdao 266061, China
2 Qingdao Environmental Monitoring Station, Qingdao 266071, China
Received 31 August 2015; accepted 24 December 2015
©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2016
Intensive Pyropia aquaculture in the coast of southwestern Yellow Sea and its subsequent waste, including disposed Ulva prolifera, was speculated to be one of the major sources for the large-scale green tide proceeding in the Yellow Sea since 2007. It was, however, unclear how the detached U. prolifera responded and resumed growing after they detached from its original habitat. In this study, we investigated the growth and photosynthetic response of the detached U. prolifera to various temperature, salinity and irradiance in the laboratory. The photosynthetic rate of the detached U. prolifera was significantly higher at moderate temperature levels (14–27°C) and high salinity (26–32), with optimum at 23°C and 32. Both low (<14°C) and highest temperature (40°C), as well as low salinity (8) had adverse effects on the photosynthesis. Compared with the other Ulva species, U. prolifera showed higher saturated irradiance and no significant photoinhibition at high irradiance, indicating the great tolerance of U. prolifera to the high irradiance. The dense branch and complex structure of floating mats could help protect the thalli and reduce photoinhibition in field. Furthermore, temperature exerted a stronger influence on the growth rate of the detached U. prolifera compared to salinity. Overall, the high growth rate of this detached U. prolifera (10.6%–16.7% d–1) at a wide range of temperature (5–32°C) and salinity (14–32) implied its blooming tendency with fluctuated salinity and temperature during floating. The environmental parameters in the southwestern Yellow Sea at the beginning of green tide were coincident with the optimal conditions for the detached U. prolifera.
Key words: Ulva prolifera, green tide, photosynthesis, growth rate, temperature, salinity
Citation: Xiao Jie, Zhang Xiaohong, Gao Chunlei, Jiang Meijie, Li Ruixiang, Wang Zongling, Li Yan, Fan Shiliang, Zhang Xuelei. 2016. Effect of temperature, salinity and irradiance on growth and photosynthesis of Ulva prolifera. Acta Oceanologica Sinica, 35(10): 114–121, doi: 10.1007/s13131-016-0891-0
1  Introduction The world’s largest green tides have been proceeding in the
Yellow Sea, China in recent years (Liu et al., 2010a, 2013a, c; Ye et al., 2011). The causative species was confirmed to be Ulva prolif- era, a cosmopolitan fouling green macroalga distributed along the coasts of China and many other countries around the world (Hayden and Waaland, 2002; Hayden et al., 2003; Leliaert et al., 2009). Field surveys and satellite image analyses indicated that macroalgae floating mats were consistently formed in the coastal water of southwestern Yellow Sea where intensive Pyropia (formerly known as Porphyra, Sutherland et al., 2011) aquacul- ture is conducted (Hu et al., 2010; Liu et al., 2010a; Keesing et al., 2011). Large amounts of macroalgal wastes disposed by farmers during Pyropia harvest season, consisting of U. prolifera, were considered to be one of the major sources for floating U. prolif- era mats (Liu et al., 2010a, 2013a; Keesing et al., 2011).
The floating U. prolifera, forming green tides in the Yellow Sea of China, exhibited unique features both in morphology and
reproduction compared to the strains in Japan and those previ- ously described in China (Ding and Luan, 2009; Hiraoka et al., 2011). Extremely filamentous, diversified reproduction of the pelagic U. prolifera and high nutrient absorption activity was considered as physiological adaptation to the floating environ- ment, which attributed to the huge biomass accumulation dur- ing floating (Lin et al., 2008; Gao et al., 2010; Wang et al., 2012). It was, however, still unclear how the disposed macroalgae be- came afloat, physiologically adapted to the fluctuated environ- ment, and formed the initial large-scale floating mats. The de- tached U. prolifera could undergo various environmental stresses during the sessile stage on the muddy flats, such as nutrient lim- itation, extreme low and high temperature and salinity (osmotic stress), high irradiance exposure, etc. Therefore, it is the key pro- cess linking detached and floating macroalgae and critical for un- derstanding the green tide formation at early stage that how the detached algae perform under those stressful environmental conditions and then resume growth and proliferation.
Acta Oceanol. Sin., 2016, Vol. 35, No. 10, P. 114–121
DOI: 10.1007/s13131-016-0891-0
Foundation item: The Special Funds for Basic Ocean Science Research of FIO under contract Nos 2012T08, 2014G33 and 2008T30; the National Natural Science Foundation of China—Shandong Joint Funded Project “Marine Ecology and Environmental Sciences” under contract No. U1406403; the National Natural Science Foundation of China under contract Nos 41206162 and 41206161; the National Basic Research Program (973 Program) of China under contract No. 2010CB428703. *Corresponding author, E-mail: [email protected]
2  Materials and methods
2.1  Sample collection and laboratory acclimation In May of 2009, macroalgae were collected from the Pyropia
aquaculture rafts in the coastal water (seawater surface temper- ature 16.2°C, salinity 31) of Rudong, southern Jiangsu province. The samples were transferred to the laboratory in a cooler, and U. prolifera was sorted out based on the morphology (Ding and Luan, 2009) and further identified with a molecular assay (Xiao et al., 2013). The algal thalli were then cleaned and temporarily maintained in the laboratory (salinity 32, temperature 15°C, and 100 μmol photons m–2 s–1 with 12L:12D photoperiod).
2.2  Growth experiment To test the effects of temperature and salinity on the growth of
detached U. prolifera, ten temperature levels (5, 8, 11, 14, 17, 20, 23, 26, 29, 32°C) were fully crossed with four salinity levels (14, 20, 26, 32), which resulted in 40 treatments. The fresh algal thalli were cleaned and dried gently using the paper towel before they were weighted on a balance. For each treatment, approximately (0.50±0.01) g (wet weight) freshly collected algal thalli were weighted and incubated in 500 mL PES (Provasoli’s enriched sea- water medium, Bold and Wynne, 1978) enriched natural seawa- ter with addition of GeO2 (final concentration of 0.50 mg/L) to in- hibit the diatom growing. Each treatment was tested in triplicate. The natural seawater (salinity 32) was collected from Shazikou of Qingdao, filtered and autoclaved before using. Culture medium with salinity lower than 32 was obtained by adding distilled wa- ter to the sterilized natural seawater until the desired salinity. The medium was refreshed every 3 d and gently stirred 3–5 times every day to ensure well-mixed nutrients. Light intensity was set at 90–100 μmol photos m–2 s–1 with 12L:12D cycle, and the beak- ers in each incubator were rotated frequently to ensure the simil- ar light attenuations.
During the 15-day culture period, the algae were harvested every three days and weighted after they were tap dried with pa- per towel. The specific growth rate (SGR) was calculated based on the equation (Brinkhuis, 1985): SGR (% d– 1) = [ln (Wt/W0)]/t×100%, where W0 and Wt were initial and final wet weight of cultured algae, t was the culture period in days.
2.3  Photosynthesis experiment Approximately 5 g of freshly collected U. prolifera thalli was
cultured in 250 mL PES enriched natural seawater at salinity 32. Total of nine temperature gradients (1, 5, 11, 14, 20, 23, 27, 32 and 40°C) were set to evaluate the effects of temperatures on the pho- tosynthesis of the detached U. prolifera. Another group of U. pro- lifera thalli were cultured at 20°C, but at different salinity levels (8, 14, 20, 26 and 32) in order to test the photosynthetic response
of U. prolifera to the salinity variations. Culture medium with sa- linity lower than 32 was obtained as described in previous sec- tion. All the treatments were maintained in the incubators with 90–100 μmol photons m–2 s–1 irradiance at 12L:12D photoperiod. The algae were acclimated to the desired experimental condi- tions for 3 days before their photosynthetic performance was evaluated.
The photosynthesis of the acclimated U. prolifera algae was tested by the technique of O2 evolution, in which the photosyn- thesis and respiration rates were determined by the O2 concen- tration changes in light and dark chambers. Initial and final O2
concentrations were measured by an Oxygraph respirometer (Hansatech Instruments Ltd., UK). Specifically, the treated algal thalli was cut into segments, and acclimated in dark under each treatment condition for 1 h. Then algal samples (approximately 100 mg, wet weight) were put in a closed respiration chamber containing 2 mL distilled seawater. Temperature of the chamber was controlled at the pre-acclimated levels by the circulated wa- ter bath. Similarly, the water inside the chamber was at the cul- turing salinity for each treatment and mixed using a magnetic stirring bar during the experiments.
For the salinity experiment, the photosynthesis-irradiance (P/I) curve was constructed using the net photosynthetic rates (Pn) at irradiance levels of 0, 20, 60, 100, 300, 600, 900, 1 200 and 1 450 μmol photons m–2 s–1, respectively. While the irradiance levels were 0, 20, 40, 60, 80, 100, 150, 200, 300, 400, 500, 600, 1 220 and 1 450 μmol photons m–2 s–1 in the temperature experiment. The photosynthetic performance was evaluated in triplicate by subsampling of each treatment.
The oxygen consumption rate in dark was used as the respira- tion rate (Rd), and the net photosynthetic rate at each irradiance level was calculated as the difference in oxygen concentration in the chamber at the beginning and end of incubation. The P/I curve was determined by a nonlinear regression based on the model described by Baly (1935): Pn = (α I Pnmax)/(α I + Pnmax)–Rd, wherein Pn is the net photosynthetic rate (μmol O2 g–1 h–1), α is the slope of P/I curve when I ≤ 200 μmol photons m–2 s–1, Pnmax
is the maximum net photosynthetic rate (μmol O2 g–1 h–1), and Rd
is the respiration rate in dark. Other parameters were calculated according to Binzer and Middelboe (2005): compensation irradi- ance Ic = Rd/α; gross photosynthetic rate Pg = Pn + Rd; light satura- tion Isat = Pgmax/α, where Pgmax is the maximum gross photosyn- thetic rate; the light-use efficiency at high irradiance LUE (μmol O2 g–1 h–1/μmol photons m–2 s–1) = Pgmax/Imax, wherein Imax is the maximum irradiance used in the experiment.
2.4  Statistic analysis Two-way analysis of variance (ANOVA) was performed using
the Origin software package (OriginLab Co., USA) to test the sig- nificant effects of salinity and temperature on growth of the de- tached U. prolifera thalli. The photosynthetic data was tested for normality and homogeneity of variance using a hyperbolic non- linear regression model, and then the photosynthetic paramet- ers were calculated as above after the testing. Figures were drawn by Origin.
3  Results
3.1  Growth of detached U. prolifera under various temperature and salinity treatments The U. prolifera thalli from the Pyropia rafts maintained a
high growth rate of 10.6%–16.7% d–1 at all temperature-salinity combinations. The algal thalli could still grow with SGR of 10% d–1 at the lowest temperature of 5°C, and reached the highest SGR
  XIAO Jie et al. Acta Oceanol. Sin., 2016, Vol. 35, No. 10, P. 114–121 115
(16.7% d–1) at 20°C, salinity 32 (Fig. 1). Statistical analysis indic- ated that growth rate of the detached U. prolifera thalli was signi- ficantly affected by both salinity and temperature, and further- more by the interaction between these two factors (Table 1). However, temperature exerted a stronger influence on the SGR as indicated in Fig. 1. The values of SGR were generally highest at 20°C, and decreased towards both lower and higher temperat- ures. SGR increased by about 4% d–1 from 5°C to 20°C at salinity 26. In comparison, the variation of SGR at different salinity levels was relatively small at the testing range of 14–32. It was highest at high salinities (20–32).
3.2  Effect of temperature on photosynthesis of U. prolifera The photosynthesis-irradiance (P/I) responses of U. prolifera
followed the hyperbolic curves with a fast increasing of net pho- tosynthetic rate (Pn) at low irradiance level until Pn was saturated irradiance (Fig. 2). As indicated in Fig. 2, the temperature signi- ficantly impacted the P/I responses of U. prolifera. The photosyn- thesis, in terms of Pnmax, dramatically increased when temperat- ure >14°C, reached maximum of 475.24 μmol O2 g–1 h–1 at 23°C, and then decreased at the highest temperature (40°C, Fig. 2b). Consequently, the light utilization efficiency (LUE) fluctuated similarly as Pnmax, which was significantly higher when temperat- ure was between 14°C and 32°C while much lower at low (<14°C) and highest temperatures (40°C). The LUE values at temperat- ures 14–32°C were about one magnitude higher than those at low temperatures, indicating a high photosynthetic activity and light utilization for the detached U. prolifera at this range (14–32°C).
The other parameters (α, Ic, Isat and Rd) fluctuated with tem- peratures and showed different patterns (Table 2). Ic and Isat were generally increasing with temperature, while the photosynthetic efficiency at low irradiance (α) peaked at 20–23°C. Similar to α, the values of Rd were high at 20–27°C. It was worth noting that an abnormally high value of Rd observed when the algae were treated at the highest temperature (40°C). Given the slight drop of Pnmax and LUE at this temperature (Fig. 2), the highest values for the dark respiration rate at 40°C indicated that U. prolifera was at a critical condition at such a high temperature.
3.3  Effect of salinity and irradiance on the photosynthesis of U. prolifera In the salinity experiment, Pnmax increased significantly with
Fig. 1.  Contour plot of averaged SGR (% d–1) of U. prolif- era over the 15-day culture period as a function of temper- ature and salinity.
Table 1.  Two-way ANOVA tests on the SGRs of U. prolifera among different temperature and salinity treatments (SS means sum of square)
Parameter SS df F P
Temperature 176.9 9 39.2 <0.01
Salinity 23.7 3 15.8 <0.01
Interaction 40.2 27 3.0 <0.01
Fig. 2.  Regressions of photosynthetic rate vs. irradiance (P/I curves) for U. prolifera at various temperature levels (a) and changes of maximum photosynthetic rate (Pnmax) and light utilization efficiency (LUE) of the detached U. prolifera with temperatures (b). Error bars=SD.
116 XIAO Jie et al. Acta Oceanol. Sin., 2016, Vol. 35, No. 10, P. 114–121  
LUE at 32 was approximately one magnitude higher than that at salinity 8 and around 3 times of that at 14, indicating a significant increasing of light utilization when the U. prolifera was in high salinity seawater. In contrast, the two related parameters (Rd and Ic) exhibited a different pattern from that of LUE and Pnmax, which was lowest at salinity 20 and increased at both low and high salinity (Table 3).
Unlike the P/I curves in the temperature experiment, the curves at different salinities were prone to be more linear (Figs 2 and 3). The treatment of salinity 32 and temperature 20°C was tested in both experiments, while the resulting parameters estim- ated (Figs 2 and 3, Tables 2 and 3) were quite different. Values of Pnmax and LUE at the treatment combination of 20°C and salinity 32 were apparently higher in salinity experiment (600 μmol O2 g–1
h–1 and 0.43) than those from temperature experiment (442 μmol O2 g–1 h–1 and 0.35, respectively).
Additionally, the saturation irradiance (Isat) of the tested U. prolifera varied from 127–809 μmol photons m–2 s–1 for the nine temperature treatments to 29–671 μmol photons m–2 s–1 for the five salinity treatments. Except for the lowest salinity (8), the val-
ues of Isat for the other groups were all larger than 100 μmol photons m–2 s–1. No evident photosynthetic inhibition was ob- served for the tested U. prolifera even at such high irradiance as over 1 000 μmol photons m–2 s–1 (Figs 2 and 3). Except for a few P/I curves at extremely low salinity and temperature (e.g., 1°C in Fig. 2a and salinity 8 in Fig. 3a), the photosynthetic rates did not decrease significantly after the irradiance reached saturation (Isat), but increased slightly.
4  Discussion In this study, we examined the photosynthetic responses of
detached U. prolifera to short-term temperature, salinity and ir- radiance fluctuations in order to understand the underlying physiological mechanism that detached U. prolifera adjusted or acclimated to these environmental changes. We found that U. prolifera could tolerate a wide range of temperature and salinity, and maintained growing with the rate of 10.6%–16.7% d–1 at the tested temperature and salinity ranges. Compared to the relat- ively smaller variation from salinity, the growth rate of detached U. prolifera was more influenced by the temperature with highest SGR at 20°C. The photosynthesis of these detached U. prolifera, on the other hand, was quite sensitive to the environmental changes, and both extreme low or high temperatures and low sa- linity had adverse effects on the photosynthesis. The photosyn- thesis was significantly higher at moderate temperature levels (14–27°C) and high salinity (26–32), with optimum at 23°C and 32. No obvious photoinhibition was observed in the test.
As described above, slightly different shapes of P/I curves were observed in the temperature and salinity experiments. P/I
Table 2.  The photosynthetic parameters of U. prolifera at various temperatures and salinity 32
Photosynthetic parameter
α 0.22 0.44 1.04 1.08 1.43 1.43 1.12 0.62 0.56
Ic 12.87 26.90 22.63 33.28 41.00 38.82 53.13 68.14 115.28
Isat 126.67 282.69 165.46 345.48 349.78 372.18 465.20 808.93 682.78
Rd 2.89 11.83 23.43 35.90 58.66 55.35 59.32 41.99 65.04
Fig. 3.  Regressions of net photosynthesis rate vs. irradiance (P/I curves) for U. prolifera after 3-day acclimation to 5 salinity levels and at temperature 20°C (a). Changes of maximum net photosynthetic rate (Pnmax) and light utilization efficiency (LUE) of detached U. prolifera with different salinity levels (b). Error bars=SD.
Table 3.  The photosynthetic parameters of U. prolifera at different salinities and temperature 20°C
Photosynthetic parameter
α 2.29 1.11 0.64 1.20 0.92
Ic 12.58 21.85 7.06 12.28 16.59
Isat 28.54 215.91 622.37 337.35 671.22
Rd 28.78 24.18 4.53 14.76 15.23
  XIAO Jie et al. Acta Oceanol. Sin., 2016, Vol. 35, No. 10, P. 114–121 117
curves at various salinity were prone to be more linear (Fig. 3). Additionally, compared with the other research showing evident photoinhibition after Isat (Arnold and Murray, 1980; Binzer and Middelboe, 2005; Kim et al., 2011), the photosynthesis in most treatments of this study did not decrease significantly, but slightly increased after Isat (Figs 2 and 3). Unfortunately, the photosyn- thesis of U. prolifera at irradiance higher than 1 450 μmol photons m–2 s–1 was not tested due to the maximum irradiance limit of respirometer used in this study. Therefore, it was not de- termined whether the detached U. prolifera had no photoinhibi- tion or was photo inhibited at an extremely high irradiance. Nev- ertheless, the Isat of U. prolifera in this study was higher than the other macroalgae species, indicating great tolerance of this spe- cies to high irradiance. Previously, Arnold and Murray (1980) re- ported various photosynthetic responses (including saturation and compensate irradiance, maximal photosynthesis and pho- toinhibition at high irradiance, etc.) of the benthic green algae from different locations (littoral and sublittoral) and with differ- ent morphologies. The intertidal, thin sheet-like and tubular sea-
weeds showed higher productivity while great photoinhibition at full sunlight (Arnold and Murray, 1980). At the same time, the al- gae with thick, optically dense thallus, such as Codium fragile, had lower productivity but less photoinhibition (Arnold and Murray, 1980). More recently, Binzer and Middelboe (2005) re- ported more linear photosynthesis and irradiance relations for the samples from single- or multi-species communities com- pared to those from a single-species thallus. It was found that compared to the individual thallus, the structure of a whole aquatic plant (positioning of the thalli) could help the distribu- tion of irradiance, and hence greatly increase the photosynthetic production and the irradiance level of photosaturation (Binzer and Middelboe, 2005). The U. prolifera alga used in this study was highly filamentous (Leliaert et al., 2009; Liu et al., 2010c; Hiraoka et al., 2011). The floating small thallus segments in the tests probably partially covered by each other, hence changed the light availability and lowered the photosynthetic inhibition at high irradiance. With stirring, the thalli could be rotated to ex- pose to light, and therefore showed slight increase in photosyn-
Table 4.  Comparisons of maximum daily growth rates (SGRmax) among Ulva spp. from literatures
Taxa1), 2) SGRmax/% d–1 Salinity3) Temperature/°C Laboratory (L) or
on-site (O) Reference
E. compressa 8 15 10 L Taylor et al. (2001)
U. compressa 9 N.D. 15 L Wang et al. (2010)
U. curvata 13 27 20 L Taylor et al. (2001)
U. expansa 7.5 35 N.D. L Fong et al. (1996)
E. intestinalis 4.5 15 N.D. L Fong et al. (1996)
U. lactuca 21–28 N.D. 15–20 O Nielsen and Sand-Jensen (1990)
U. lactuca 16 N.D. N.D. L Ale et al. (2011)
U. linza 6.7 32 20 L Kim et al. (2011)
E. linza 12 27 15 L Taylor et al. (2001)
U. linza 12 N.D. 15 L Luo et al. (2012)
E. linza 4 33 20 L Gao et al. (2012)
U. linza 9.6 27–31 >15 O Wang et al. (2015)
U. pertusa 3.2 20 N.D. L Choi et al. (2010)
U. pertusa 12 30 20 L Liu and Dong (2001)
U. pseudocurvata 25 N.D. 10 L Lüning et al. (2008)
U. rigida 13 23 15 L Taylor et al. (2001)
Enteromorpha 14–26 N.D. 15–20 O Nielsen and Sand-Jensen (1990)
Ulva 25 N.D. 15 O de Paula Silva et al. (2008)
E. prolifera 7.3 20–28 18–25 L Wu et al. (2000)
E. prolifera 3.7 24 25 L Wang et al. (2007)
E. prolifera 3 33‰ 20 L Gao et al. (2012)
E. prolifera (F) 10 N.D. 25 L Fu et al. (2008)
U. prolifera (F) 1.5 25–29 23 L Xin et al. (2009)
U. prolifera (F) 8 30 20 L Li et al. (2009)
U. prolifera (F) 10 30 20 O Wu et al. (2010)
U. prolifera (F) 20 30‰ 16 L Luo and Liu (2011)
U. prolifera (F) 17 30 15 L Luo et al. (2012)
U. prolifera (G) 37 N.D. 20 L Hiraoka and Oka (2008)
U. prolifera (G) 36 N.D. 20 L Liu et al. (2010c)
U. prolifera (G) 27 N.D. 15 L Liu et al. (2010b)
U. prolifera (A) 38 26 20 L Zhang et al. (2012)
U. prolifera (A) 23 N.D. N.D. O Zhang et al. (2013)
U. prolifera (A) 26 27–31 >15 O Wang et al. (2015)
U. prolifera (A) 16 20–32 20 L this study
Note: 1) Enteromorpha was synonymized with Ulva in Tan et al. (1999) and Hayden et al. (2003); 2) Letters in parentheses indicate types of the Ulva (Enteromorpha) prolifera samples: F is floating, G germling, and A attached. 3) Unless otherwise specified, the unit of salinity is psu. N.D. means not determined.
118 XIAO Jie et al. Acta Oceanol. Sin., 2016, Vol. 35, No. 10, P. 114–121  
thesis at high irradiance. In field, the numerous tubular branch- ing of U. prolifera often forms dense algal mat, especially during floating. This floating algal mat with complex structure could par- tially protect the algae at lower part, and maintain high pro- ductivity and growth rate during floating even with high sunlight in the southwestern Yellow Sea. Furthermore, a higher genetic diversity was observed for the attached U. prolifera population compared to the floating one (Zhang et al., 2011; Zhao et al., 2011; Liu et al., 2012) indicating multiple genotypes of U. prolif- era existing on the Pyropia rafts. Our laboratory tests also indic- ated distinct photosynthetic activities of different tissue parts of the algae (thalli, root, germlings, etc.). Here in our study, al- though a single species (U. prolifera) was used for both salinity and temperature experiments, the thalli samples tested might enclosed different genotypes or different tissues parts. A single- species community consisted of multiple genotypes or tissue types might be used for this salinity experiment. Thus a higher saturation irradiance and photosynthetic rate would be expected for the salinity experiment due to more efficient distribution and photosynthetic use of light (Binzer and Middelboe, 2005). It was confirmed by the observation in our study that Isat and Pnmax at salinity 32 and 20°C were higher in the salinity experiment than those in temperature experiment (Figs 2 and 3, Tables 2 and 3).
The maximum photosynthetic rate of the detached U. prolif- era in this study was relatively higher compared to the other Ulva species and U. prolifera at other stages. A number of researches investigated the photosynthesis of floating ulvoid algae at differ- ent stages in the Yellow Sea, and found variable photosynthetic performances (Lin et al., 2009, 2011; Kim et al., 2011). By using the averaged Chl a content for U. prolifera (about 0.20–0.68 mg Chl a g–1; Lin et al., 2011), the photosynthetic rate (Pnmax, 300–600 μmol O2 h–1 g–1) of the detached U. prolifera under optimum con- dition from this study was higher than that of floating Ulva at early stage (72–245 μmol O2 g–1 h–1) and at late stage (29–100 μmol O2 g–1 h–1) in the eastern coast of Yellow Sea (Kim et al., 2011). We also tested the photosynthesis of the floating U. prolif- era collected from the coast of Shandong Province, and con- firmed the higher photosynthetic performance of the detached U. prolifera relative to those floating ones (Gao et al., unpublished data). Compared to the floating algae which had limit potential for growth and reproduction (Lin et al., 2011; Kim et al., 2011), the high photosynthesis of the detached U. prolifera indicated its high growth rate and proliferation once they were at optimum conditions, which was confirmed by our growth experiments.
The change of growth rate with temperature and salinity was relatively small, but significant. We summarized the maximum growth rates (SGRmax) of Ulva spp. from the literatures in Table 4, including U. prolifera at different stages (germling, floating and attached). Considering the different species, culturing methods and physio-chemical conditions at every test, it is not surprising that SGRs were quite variable (1.5%–38% d–1) even for the single species (e.g., U. prolifera and U. linza). However, the SGRs of U. prolifera from most research (67%) were higher than 10% d–1, in which, the values for “germling” and “attached” were evidently higher than the floating ones and the other Ulva species (Table
4). The high SGR of U. prolifera indicated the great ecological ad- vance for this species under the favored environmental condi- tions. High nutrient absorption and assimilation was considered to contribute greatly to the rapid growth rate and consequent ecological success of this species over the other co-occurring ul- void species, especially in the eutrophic waters (Luo et al., 2012; Wang et al., 2012). The U. prolifera strain bloomed in the Yellow Sea was found to be extremely filamentous (Liu et al., 2010c; Hiraoka et al., 2011), and such an increased surface/volume ratio could assist the nutrient absorption and light utilization, and fur- ther enhance the photosynthesis and rapid growing (Arnold and Murray, 1980; O’Brien and Wheeler, 1987; Nielsen and Sand- Jensen, 1990). The field growth rate of U. prolifera was estimated to be 23%–26% d–1 based on the on-site and mesocosm tests un- der local natural environments (surface seawater temperature >15°C, salinity 27–31; Zhang et al., 2013; Wang et al., 2015). Based on the numerical modeling, such high growth rates were well enough supporting the rapid floating biomass proliferation at the initial stage of the green tide in the Yellow Sea (Wang et al., 2015). Besides, the high growth rates in field indicated the optimal local environmental conditions favoring blooming of U. prolifera as discussed in the following.
Local environmental conditions in the southwestern Yellow Sea during April–May favored the blooming of the detached U. prolifera, especially the salinity, irradiance and sea surface tem- perature (Table 5). Daily irradiance varied from 0–1 800 μmol photons m–2 s–1 which did not show any evidence limiting the productivity of U. prolifera. As shown in Figs 1 and 3, no evident decline of SGR and photosynthesis at highest salinity indicated that the salinity range tested in this study probably did not cover the optimal level at which maximum SGR and photosynthetic re- sponse could reach. It needs further investigation whether the detached U. prolifera could tolerate the higher salinity and per- form a normal or even better biomass accumulation and photo- synthesis. However, the salinity range in this study (8–32) covered the real salinity variation in the coastal water of Subei Shoal, where the U. prolifera bloom initiated (Table 5, Huo et al., 2013; Shi et al., 2015; Wang et al., 2015). Salinity varied little and had less effect on the growth of the detached U. prolifera as shown above (Fig. 1). The sea surface temperature, on the other hand, increased rapidly from 9.9°C to 20.3°C (Wang et al., 2015),
Table 5.  Physico-chemical parameters during April to May in and around the Subei Shoal, southwestern Yellow Sea of China
Environmental parameters Range (Mean)
Salinity1) 27–32 (30)
Phosphate2)/μmol·L–1 ≤0.6 (0.2) N:P ratio2) 5.1–230.2 (69.1)
Irradiance3)/μmol photons m–2 s–1 0–1 800
Although the algae in this study (Kim et al., 2011) was named as U. linza, the type specimen was same as green tide-forming U. prolifera in the Yellow Sea based on morphology and molecular data (Kang et al., 2014). So the growth rate data from Kim et al. (2011) was cited. Gao Chunlei, Fan Shiliang, Xiao Jie. Photosynthesis of Ulva prolifera in field. The words “attached” in Table 4 and “detached” in the text have the similar meaning, referring to the U. prolifera materials separated from its original substrata.
  XIAO Jie et al. Acta Oceanol. Sin., 2016, Vol. 35, No. 10, P. 114–121 119
and could be one of the important triggers for U. prolifera bloom- ing. Fan et al. (2015) observed that the macroalgal assemblage on the Pyropia rafts was sensitive to the seawater temperature and U. prolifera biomass was prone to rising with temperature. Our multiple field surveys also implied a consistent sea surface tem- perature (≥15°C) when the floating U. prolifera started to prolif- erate rapidly and the large-scale floating mats were firstly spot- ted in the Yellow Sea (Fan et al., 2015; Wang et al., 2015).
Acknowledgements The authors thank Song Wei and Xu Jintao, Wang Xiaona for
their assistance with sample collection.
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