Juntian Xu

Rising CO2 and increased light exposure synergistically reduce marine primary productivity

Carbon dioxide and light are two major prerequisites of photosynthesis. Rising CO2 levels in oceanic surface waters in combination with ample light supply are therefore often considered stimulatory to marine primary production(1-3). Here we show that the combination of an increase in both CO2 and light exposure negatively impacts photosynthesis and growth of marine primary producers. When exposed to CO2 concentrations projected for the end of this century(4), natural phytoplankton assemblages of the South China Sea responded with decreased primary production and increased light stress at light intensities representative of the upper surface layer. The phytoplankton community shifted away from diatoms, the dominant phytoplankton group during our field campaigns. To examine the underlying mechanisms of the observed responses, we grew diatoms at different CO2 concentrations and under varying levels (5-100%) of solar radiation experienced by the phytoplankton at different depths of the euphotic zone. Above 22-36% of incident surface irradiance, growth rates in the high-CO2-grown cells were inversely related to light levels and exhibited reduced thresholds at which light becomes inhibitory. Future shoaling of upper-mixed-layer depths will expose phytoplankton to increased mean light intensities(5). In combination with rising CO2 levels, this may cause a widespread decline in marine primary production and a community shift away from diatoms, the main algal group that supports higher trophic levels and carbon export in the ocean.

Effects of solar UV radiation on diurnal photosynthetic performance and growth of Gracilaria lemaneiformis (Rhodophyta)

Previous studies on diurnal photosynthesis of macroalgal species have shown that at similar levels of photosynthetically active radiation (PAR, 400–700nm) the photosynthetic rate is lower in the afternoon than in the morning. However, the impacts of solar ultraviolet radiation (UVR, 280–400nm) have been little considered. We investigated the diurnal photosynthetic behaviour of the economically significant red alga Gracilaria lemaneiformis in the absence or presence of UV-A+B or UV-B with a flow-through system. While UV-A and UV-B, respectively, inhibited noontime Pmax by 22% and 14% on the sunny days, UV-A during sunrise (PAR below about 50Wm−2) increased the net photosynthesis by about 8% when compared with PAR alone. UV-A + PAR also resulted in higher apparent photosynthetic efficiency in the morning than in the afternoon period than PAR alone. Nevertheless, integrated daytime photosynthetic production under solar PAR alone was higher than with either PAR + UV-A+B or PAR + UV-A. Relative growth rate in the long term (9 days) matched the integrated photosynthetic production in that UV-A led to 9–15% and UV-B to 19–22% reduction, respectively. UV-absorbing compounds were found to be higher in the thalli exposed to PAR+UV-A+B than under PAR alone, reflecting a protective response to UVR.

UV-A enhanced growth and UV-B induced positive effects in the recovery of photochemical yield in Gracilaria lemaneiformis (Rhodophyta).

The effects of solar UV radiation (280-400 nm) on growth, quantum yield and pigmentation in Gracilaria lemaneiformis were investigated when the thalli were cultured under solar radiation with or without UV for a period of 15 days. Presence of UV-A (315-400 nm) enhanced the relative growth rate, while UV-B (218-315 nm) inhibited it. The positive effect of UV-A and negative effect of UV-B counteracted to result in an insignificant impact of UVR on growth. During the noon period, both UV-A and UV-B resulted in the decrease of maximum quantum yield (Fv/Fm), but UV-B aided in the recovery of the yield in the late afternoon, reflecting that UV-B might be used as a signal in photorepair processes. UV induced the accumulation of UV-absorbing compounds (UVAC) to defend against the harmful UVR. However, the accumulation of UVAC took a much longer time compared to that previously reported, which was probably due to the lower levels of solar radiation and water temperature in the early spring period. Unknown UV-absorbing compounds (UVAC), which peaked at 265 nm, probably the precursor of MAAs (UVAC(325)), accumulated under moderate levels of solar radiation and were transformed to MAAs under higher solar radiation.

Light-Modulated Responses of Growth and Photosynthetic Performance to Ocean Acidification in the Model Diatom Phaeodactylum tricornutum

Ocean acidification (OA) due to atmospheric CO2 rise is expected to influence marine primary productivity. In order to investigate the interactive effects of OA and light changes on diatoms, we grew Phaeodactylum tricornutum, under ambient (390 ppmv; LC) and elevated CO2 (1000 ppmv; HC) conditions for 80 generations, and measured its physiological performance under different light levels (60 µmol m−2 s−1, LL; 200 µmol m−2 s−1, ML; 460 µmol m−2 s−1, HL) for another 25 generations. The specific growth rate of the HC-grown cells was higher (about 12–18%) than that of the LC-grown ones, with the highest under the ML level. With increasing light levels, the effective photochemical yield of PSII (Fv′/Fm′) decreased, but was enhanced by the elevated CO2, especially under the HL level. The cells acclimated to the HC condition showed a higher recovery rate of their photochemical yield of PSII compared to the LC-grown cells. For the HC-grown cells, dissolved inorganic carbon or CO2 levels for half saturation of photosynthesis (K1/2 DIC or K1/2 CO2) increased by 11, 55 and 32%, under the LL, ML and HL levels, reflecting a light dependent down-regulation of carbon concentrating mechanisms (CCMs). The linkage between higher level of the CCMs down-regulation and higher growth rate at ML under OA supports the theory that the saved energy from CCMs down-regulation adds on to enhance the growth of the diatom.

Use of UV-A energy for photosynthesis in the red macroalga Gracilaria lemaneiformis.

UV radiation is known to inhibit photosynthetically active radiation (PAR)‐driven photosynthesis; however, moderate levels of UV‐A have been shown to enhance photosynthesis and growth rates of some algae. Here, we have shown that UV‐A alone could drive photosynthetic utilization of bicarbonate in the red alga Gracilaria lemaneiformis as evidenced in either O2 evolution or carbon fixation as well as pH drift. Addition of UV‐B inhibited the apparent photosynthetic efficiency, raised the photosynthetic compensation point and photosynthesis‐saturating irradiance level, but did not significantly affect the maximal rate of photosynthetic O2 evolution. The electron transport inhibitor, DCMU, inhibited the photosynthesis completely, reflecting that energy of UV‐A was transferred in the same way as that of PAR. Inorganic carbon acquisition for photosynthesis under UV alone was inhibited by the inhibitors of carbonic anhydrase. The results provided the evidence that G. lemaneiformis can use UV‐A efficiently to drive photosynthesis based on the utilization of bicarbonate, which could contribute significantly to the enhanced photosynthesis in the presence of UV‐A observed under reduced levels of solar radiation.

CO2-driven seawater acidification increases photochemical stress in a green alga

Liu Y., Xu J. and Gao K. 2012. CO2-driven seawater acidification increases photochemical stress in a green alga. Phycologia 51: 562–566. DOI: 10.2216/11-65.1 Increased CO2 and associated acidification in seawater, known as ocean acidification, decreases calcification of most marine calcifying organisms. However, there is little information available on how marine macroalgae would respond to the chemical changes caused by seawater acidification. We hypothesized that down-regulation of bicarbonate acquisition by algae under increased acidity and CO2 levels would lower the threshold above which photosynthetically active radiation (PAR) becomes excessive. Juveniles of Ulva prolifera derived from zoospores were grown at ambient (390 µatm) and elevated (1000 µatm) CO2 concentrations for 80 days before the hypothesis was tested. Here, the CO2-induced seawater acidification increased the quantum yield under low levels of light, but induced higher nonphotochemical quenching under high light. At the same time, the PAR level at which photosynthesis became saturated was decreased and the photosynthetic affinity for CO2 or inorganic carbon decreased in the high-CO2 grown plants. These findings indicated that ocean acidification, as an environmental stressor, can reduce the threshold above which PAR becomes excessive.

Effects of seawater acidification on the growth rates of the diatom Thalassiosira (Conticribra) weissflogii under different nutrient, light, and UV radiation regimes

Effects of ocean acidification (OA) on marine organisms are suggested to be altered by other environmental drivers, such as low nutrient, increased light, and UVR exposures; however, little has been documented on this aspect. Thalassiosira (Conticribra) weissflogii, a marine diatom, was used to examine the OA effects under multiple stressors on its growth. The specific growth rate was inhibited by low nutrient (LN), though it increased with increased sunlight regardless of the nutrient supplies. Presence of UVR reduced the maximal growth rate (μmax) in low CO2 (LC) conditions (both LN and HN) and inhibited the apparent growth light use efficiency (α) in the cells acclimated to LN under both low (LC) and high (HC) CO2 conditions. The HC-grown cells grew faster under HN and low light levels. Conclusively, presence of UVR with high solar radiation, LN and OA acted synergistically to reduce the diatom growth, though, in contrast UVR and OA enhanced the growth under HN.

Comparative research on inorganic carbon acquisition by the macroalgae Ulva prolifera (Chlorophyta) and Pyropia yezoensis (Rhodophyta)

The red macroalga Pyropia/Porphyra is one of the most important marine crops in the world; Pyropia cultivation is severely affected by the green macroalga Ulva spp., a fouling organism. In this study, growth competition and acquisition of dissolved inorganic carbon (Ci) by Ulva prolifera and Pyropia yezoensis were investigated to understand the physiological characteristics of the two species. Competition experimental results showed that the relative growth rate of U. prolifera was not affected, whereas the growth rate of P. yezoensis was significantly inhibited. U. prolifera exhibited a competitive advantage when these two species were cultured together. The two algal species displayed high pH compensation points, suggesting that U. prolifera and P. yezoensis can photosynthesize by using HCO3−; the utilization ability of U. prolifera is stronger than that of P. yezoensis. The net photosynthetic rates of P. yezoensis and U. prolifera were significantly inhibited by the carbonic anhydrase (CA) inhibitors acetazolamide and 6-ethoxyzolamide indicating that CA is implicated in carbon-concentrating mechanisms (CCM). 4,4′-diisothiocyano-stilbene-2,2′-disulfonate (DIDS), another inhibitor that prevents direct HCO3− uptake, did not significantly affect U. prolifera. Conversely, DIDS can sharply decrease the photosynthetic rate of P. yezoensis, particularly at high pH. U. prolifera and P. yezoensis also showed CO2-limited photosynthesis by which the half-saturating concentration of Ci exceeds that of seawater. U. prolifera did not show active HCO3− uptake; instead, U. prolifera used HCO3− via extracellular CA for photosynthetic carbon fixation. U. prolifera also utilizes extracellular CA-mediated HCO3− absorption to a higher extent than P. yezoensis. By contrast, P. yezoensis shows active HCO3− uptake and extracellular CCMs for photosynthetic carbon fixation. Therefore, these two algal species can survive in changing environments with high pH and low Ci concentration because of the action of CCMs.

Photosynthetic Performance of the Red Alga Pyropia haitanensis During Emersion, With Special Reference to Effects of Solar UV Radiation, Dehydration and Elevated CO2 Concentration.

Macroalgae distributed in intertidal zones experience a series of environmental changes, such as periodical desiccation associated with tidal cycles, increasing CO2 concentration and solar UVB (280–315 nm) irradiance in the context of climate change. We investigated how the economic red macroalga, Pyropia haitanensis, perform its photosynthesis under elevated atmospheric CO2 concentration and in the presence of solar UV radiation (280–400 nm) during emersion. Our results showed that the elevated CO2 (800 ppmv) significantly increased the photosynthetic carbon fixation rate of P. haitanensis by about 100% when the alga was dehydrated. Solar UV radiation had insignificant effects on the net photosynthesis without desiccation stress and under low levels of sunlight, but significantly inhibited it with increased levels of desiccation and sunlight intensity, to the highest extent at the highest levels of water loss and solar radiation. Presence of UV radiation and the elevated CO2 acted synergistically to cause higher inhibition of the photosynthetic carbon fixation, which exacerbated at higher levels of desiccation and sunlight. While P. haitanensis can benefit from increasing atmospheric CO2 concentration during emersion under low and moderate levels of solar radiation, combined effects of elevated CO2 and UV radiation acted synergistically to reduce its photosynthesis under high solar radiation levels during noon periods.

Photosynthetic contribution of UV-A to carbon fixation by macroalgae

Abstract: Previous studies showed that energy of ultraviolet light A (UV-A) (315–400 nm) could be used for photosynthesis by some macroalgae; however, little has been documented on the extent of such photosynthetic contribution among different macroalgal taxa. We selected 11 macroalgal species, representing red, brown and green phyla, and investigated their ability to utilize UV-A for photosynthesis. Our results showed that, in the absence of photosynthetically active radiation (PAR), UV-A alone triggered photosynthetic carbon fixation rates in all the selected species. The gross photosynthetic rates of the tested macroalgae when exposed to UV-A ranged from 6.5 ± 0.3 to 52.3 ± 1.8 μmol C g (fresh weight)−1 h−1, with the highest rate found in the green alga Ulva lactuca Linnaeus. The ratio of gross photosynthesis driven by UV-A alone to that by saturating PAR varied from 14% to 22%. The present work demonstrated that macroalgae are capable of utilizing UV-A irradiance to drive photosynthetic carbon fixation, and this was consistent for all the species tested across green, red and brown algae.