Ulva pertusa, a common bloom-forming green alga, was used as a model system to examine the effects of elevated carbon dioxide (CO2) and temperature on growth and photosynthetic performance. To do this, U. pertusa was grown under four temperature and CO2 conditions; ambient CO2 (400 μatm) and temperature (16℃) (i.e., present), elevated temperature only (19℃) (ET; i.e., warming), elevated CO2 only (1,000 μatm) (EC; i.e., acidification), and elevated temperature and CO2 (ET and EC; i.e., greenhouse), and its steady state photosynthetic performance evaluated. Maximum gross photosynthetic rates (GPmax) were highest under EC conditions and lowest under ET conditions. Further, ET conditions resulted in decreased rate of dark respiration (Rd), but growth of U. pertusa was higher under ET conditions than under ambient temperature conditions. In order to evaluate external carbonic anhydrase (eCA) activity, photosynthesis was measured at 70 μmol photons m−2 s−1 in the presence or absence of the eCA inhibitor acetazolamide (AZ), which inhibited photosynthetic rates in all treatments, indicating eCA activity. However, while AZ reduced U. pertusa photosynthesis in all treatments, this reduction was lower under ambient CO2 conditions (both present and warming) compared to EC conditions (both acidification and greenhouse). Moreover, Chlorophyll a and glucose contents in U. pertusa tissues declined under ET conditions (both warming and greenhouse) in conjunction with reduced GPmax and Rd. Overall, our results indicate that the interaction of EC and ET would offset each other’s impacts on photosynthesis and biochemical composition as related to carbon balance of U. pertusa.
Atmospheric concentrations of carbon dioxide (CO2) have rapidly increased since the time of the Industrial Revolution, leading to changes in the chemical composition of seawater. The A1FI scenario of the Intergovernmental Panel on Climate Change (IPCC) projects that the the atmospheric CO2 concentration will have increased to 970 μatm nearly three times the present concentration by the end of the current century (IPCC 2007). Concomitantly, the average pH of seawater is expected to drop by approximately 0.46 units from current levels, with both CO2 and HCO3- concentrations increasing and CO32- decreasing. These changes caused by increasing atmospheric CO2 will also be associated with increased ocean temperature and acidity, sea-level rise, and more frequent natural disasters (IPCC 2007, Doney et al. 2009). There is general agreement that these impacts will be some of the most notable environmental changes in the ocean during the coming decades (e.g., Hansen et al. 2005).
There are several potential biological implications of the expected environmental changes associated with ocean acidification. For example, current CO2 concentrations, in the form of dissolved inorganic carbon (DIC) in seawater, are not high enough to saturate photosynthesis of marine primary producers, and therefore, photosynthesis in macroalgae and phytoplankton could be saturated under current CO2 conditions by using carbon concentration mechanisms (CCMs) (Raven 1997), such as those associated with the enzyme carbonic anhydrase (CA), which catalyzes dehydration of HCO3- to CO2. However, the exact mechanisms by which this occurs may differ at phylogenetic levels, from ecotypes to phyla (Johnston et al. 1992, Mercado et al. 1997, Brading et al. 2011). Indeed, most macroalgae involve CCMs and thus use HCO3- for photosynthesis; however, photosynthesis dose not generally appear saturated under present DIC conditions (Koch et al. 2013). From previous research, several species of
Some metabolic changes including photosynthesis, respiration, and enzyme activity occur as a result of increased temperature (Atkin et al. 2000, Atkin and Tjoelker 2003, Vona et al. 2004), which in turn, alters the physiological responses of the macroalgae to environmental changes (Kübler and Davison 1995, Zou and Gao 2013). Simultaneously, increased atmospheric CO2 will result in ocean acidification and many unpredictable effects (Doney et al. 2009). Considerable research was carried out on the impacts of these factors; however, most has focused primarily on the processes of biological calcification, the general impacts on phytoplankton communities, and only a few have focused on selected macroalgae. Further, few of these studies have considered the combined impacts of these environmental changes on a bloom-forming green alga using ecologically relevant conditions (e.g., Xu and Gao 2012). Here, we consider the impact of these factors on
Considerable research has demonstrated that photosynthetic rates of marine plants are enhanced by increased CO2 concentrations. However, photosynthetic activity depends not only on CO2 concentration but also on nutrient levels, light and temperature conditions (Zimmerman et al. 1997, Gordillo et al. 2001, 2003, Fu et al. 2007, Zou et al. 2011). For example, the marine picocyanobacteria
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Sample collection and incubation
Four treatments were used to measure the individual and combined effects of CO2 and temperature: ambient CO2 (400 μatm) and temperature (16℃) (i.e., present); ambient CO2 and elevated temperature (19℃) (ET; i.e., warming); elevated CO2 (1,000 μatm) and ambient temperature (EC; i.e., acidification); and both elevated temperature and CO2 (ET and EC combined; i.e., greenhouse). Different CO2 concentrations were provided by mixing air with the appropriate amounts of CO2 obtained from a compressed CO2 cylinder, as described by Kim et al. (2008). Carbon dioxide (CO2) concentrations were monitored in the resulting air-CO2 mixtures using a CO2 analyzer (LI-840A; LI-COR, Lincolon, NE, USA). These mixtures were bubbled through the media, and CO2 was monitored daily via measurements of the pH of culture media using a pH-meter (Meterlab PHM210; Radiometer Analytical SAS, Lyon, France). The maximum variation of CO2 in individual cultures was 5%, and CO2 inputs were adjusted daily in order to maintain constant levels in each experimental treatment.
Total seawater alkalinity (TA) and total DIC were measured with a potentiometric titration system (765 Dosimat; Metrohm AG, Herisau, Switzerland), combined with a ROSS half-cell pH electrode (Orion 8101BNWP; Thermo Scientific, Waltham, MA, USA) and a sure-flow reference electrode (Orion 900200; Thermo Scientific). Proportions of the carbon species in the seawater were calculated from the TA and DIC values using CO2SYS software (Lewis and Wallace 1998). The TA and DIC measurements were checked for accuracy against certified reference materials (distributed by A. Dickson, Scripps Institution of Oceanography) used here as standards. The precisions of the measurement were approximately ±2 μmol kg-1 for TA and ±1.5 μmol kg-1 for DIC. Temperature and irradiance were also monitored using a temperature/light data logger (UA-002-64; Onset, Pocasset, MA, USA) calibrated with a thermometer during the experiment. The experimental temperatures were controlled using aquarium heaters (EHEIM Jager, Deizisau, Germany). Temperatures were maintained to within 0.1℃ of the targeted temperature throughout the experiment.
Photosynthetic rates were measured using an oxygen closed-chamber method consisting of a 2-mm oxygen dipping probe (DP-PSt3) connected to Fibox3 (PreSens, Regenburg, Germany) at eight irradiances (0, 10, 45, 80, 150, 245, and 450 μmol photons m-2 s-1) derived from a halogen lamp (KL2500; SCHOTT, Mainz, Germany). Temperature and CO2 concentrations were maintained at each target level during the photosynthetic measurement. The photosynthesis-irradiance relationships for distinction of photosynthetic traits were fitted to a nonlinear mathematical function that represented the double exponential function (Platt et al. 1980).
In order to assess the external carbonic anhydrase (eCA) activity, photosynthesis was measured with and without acetazolamide (AZ) under 70 μmol photons m-2 s-1. AZ inhibits photosynthesis using HCO3-, hence the results show CO2 utilized during photosynthesis when AZ was added to the seawater media. AZ was prepared in 0.5 N NaOH and added to the medium at a concentration of 60 μM. Experiments with AZ used the same irradiance levels as were used in the incubation conditions.
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Chlorophyll a (Chl a) fluorescence
Chl
ФPSII = (
, where
, with two ambiguous factors (the partitioning of light energy by photosystem [PS] I and PSII, and the absorption factor) were not considered. Chl
μ = ln(A - A0) / (T - T0)
, where A and A0 represent the areas at time T (after 10 days) and T0 (the initial day), respectively.
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Chl a, tissue nutrient, and glucose contents
Chl
Chl
, where
Carbon (C) and nitrogen (N) contents within the
Glucose content within the
All statistical analyses were performed using the SPSS 20.0 statistical software. Data normality and homogeneity of variance were determined using the Kolmogorov-Smirnov normality test and Levene’s homogeneity of variance test, respectively. One-way analysis of variance (ANOVA) and two-way ANOVAs were used to compare the effects of CO2 concentrations and temperatures on photosynthetic parameters, growth rate, Chl
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Photosynthesis and Chl a fluorescence
Gross photosynthesis (GP) in
Photosynthetic parameters of Ulva pertusa obtained from the four temperature and CO2 conditions (n = 3, mean ± SD)
The maximum quantum yields of PSII (
Chlorophyll a fluorescence parameters of Ulva pertusa obtained from the four temperature and CO2 conditions (n = 3, mean ± SD)
Gross photosynthesis (GP) in
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Chl a, tissue nutrient, and glucose contents
The trend for Chl
Chlorophyll a concentration (mg Chl a g-1 FW), tissue elemental contents (%) and glucose content (mg g-1 DW) of Ulva pertusa obtained from the four temperature and CO2 conditions (n = 3-5, mean ± SD)
The carbon and nitrogen contents in
Glucose content within
Growth rates (μ) ranged from 0.493 to 0.681 day-1, with the lowest μ occurring under EC conditions (Fig. 3). Although there were no significant differences among treatments, the μ trended higher under ET conditions, irrespective of CO2 levels (F = 4.319, p = 0.054), and it appeared that EC did not impact the growth rates of
It was shown previously that the growth rates of
The highest growth rates of
Low eCA activity shows that organisms prefer CO2 or both HCO3- and CO2 as primary photosynthetic substrates (Mercado et al. 1997).
In this study, increased CO2 concentrations and temperatures did not enhance photosynthesis, growth rates or biochemical composition, with the exception of glucose content (Table 3). However, photosynthetic pigment composition did change with CO2 concentration in previous studies. Specifically,
It is often argued that global climate change causes more frequent harmful algal blooms (Dale et al. 2006) but our results suggest that this may not be the case with
In recent years, research has focused on the response of whole communities to increased CO2 and temperature. Rodolfo-Metalpa et al. (2011), for example, showed that the rising temperature could aggravate the benthic communities combined with ocean acidification in the Mediterranean Sea. In the case of the phytobenthos communities, the effects of rising CO2 levels singularly or combination with higher temperatures induced a variety of responses from individual or assemblages of macroalgal species, and high CO2 and high temperature were shown to reduce macroalgal assemblage biomass (Olabarria et al. 2013). There may also be a synergistic positive effect of high CO2 and high temperature on algal turfs, but changes in the algal turfs are also exacerbated kelp loss (Connell and Russell 2010). As mentioned above, the combined effects of CO2 and temperature vary and are complex at both the individual and community levels. Our research is relevant to understanding the changes in photosynthetic characteristics and growth responses of a coastal bloom-forming algal species with respect to future climate change as represented by ocean acidification and global warming.