George Bowes

Climate change and ocean acidification effects on seagrasses and marine macroalgae

Although seagrasses and marine macroalgae (macro-autotrophs) play critical ecological roles in reef, lagoon, coastal and open-water ecosystems, their response to ocean acidification (OA) and climate change is not well understood. In this review, we examine marine macro-autotroph biochemistry and physiology relevant to their response to elevated dissolved inorganic carbon [DIC], carbon dioxide [CO2 ], and lower carbonate [CO3 (2-) ] and pH. We also explore the effects of increasing temperature under climate change and the interactions of elevated temperature and [CO2 ]. Finally, recommendations are made for future research based on this synthesis. A literature review of >100 species revealed that marine macro-autotroph photosynthesis is overwhelmingly C3 (≥ 85%) with most species capable of utilizing HCO3 (-) ; however, most are not saturated at current ocean [DIC]. These results, and the presence of CO2 -only users, lead us to conclude that photosynthetic and growth rates of marine macro-autotrophs are likely to increase under elevated [CO2 ] similar to terrestrial C3 species. In the tropics, many species live close to their thermal limits and will have to up-regulate stress-response systems to tolerate sublethal temperature exposures with climate change, whereas elevated [CO2 ] effects on thermal acclimation are unknown. Fundamental linkages between elevated [CO2 ] and temperature on photorespiration, enzyme systems, carbohydrate production, and calcification dictate the need to consider these two parameters simultaneously. Relevant to calcifiers, elevated [CO2 ] lowers net calcification and this effect is amplified by high temperature. Although the mechanisms are not clear, OA likely disrupts diffusion and transport systems of H(+) and DIC. These fluxes control micro-environments that promote calcification over dissolution and may be more important than CaCO3 mineralogy in predicting macroalgal responses to OA. Calcareous macroalgae are highly vulnerable to OA, and it is likely that fleshy macroalgae will dominate in a higher CO2 ocean; therefore, it is critical to elucidate the research gaps identified in this review.

Plasticity in the photosynthetic carbon metabolism of submersed aquatic macrophytes

This review details the physiological and biochemical adaptations that enable submersed aquatic macrophytes to manage constraints associated with photosynthesis under water. The constraints are dissolved inorganic carbon (DIC), light and temperature, though pH, O2 and mineral nutrients may be factors. Submersed aquatic macrophytes are characterized by low photosynthetic rates, with low light requirements and very high apparent Km (CO2/HCO3−) values. These features are mainly caused by low DIC diffusion rates. High pH (10) may limit photosynthesis by decreasing free CO2. Temperature responses vary, partly owing to growth adaptations, with photosynthesis reported under ice and at 35°C. Even though the photosynthetic carbon reduction (PCR), and photorespiratory carbon oxidation (PCO) cycles operate in submersed aquatic macrophytes, they possess mechanisms which preclude classifying them with C3 or C4 photosynthesis. These include: utilization of HCO3− ions, C4 acids, and sediment CO2. A major feature of submersed aquatic macrophytes is their plasticity, which is seen in variable CO2 compensation points that indicate the photorespiratory (PR) state. Unlike unicellular phototrophs, submersed aquatic macrophytes usually exist in an O2-sensitive, high-PR (C3-like) state; but under daytime stress conditions of high light, temperature, O2 and low CO2 a low-PR state is induced. The active uptake of HCO3− ions, and/or fixation initially by phosphoenolpyruvate carboxylase in a C4-like system, but without Kranz anatomy, concentrates CO2 internally. The result is a low-PR state, with reduced O2 inhibition of ribulose bisphosphate carboxylase-oxygenase. Not all species use HCO3− ions effectively. Some fix CO2 at night in a CAM-like system, while other, rosette forms take sediment CO2, via roots, and pipe it in lacunae to the leaves. These mechanisms all conserve carbon and improve DIC availability.

Regulation and Localization of Key Enzymes during the Induction of Kranz-Less, C4-Type Photosynthesis in Hydrilla verticillata

Kranz-less, C4-type photosynthesis was induced in the submersed monocot Hydrilla verticillata (L.f.) Royle. During a 12-d induction period the CO2 compensation point and O2 inhibition of photosynthesis declined linearly. Phosphoenolpyruvate carboxylase (PEPC) activity increased 16-fold, with the major increase occurring within 3 d. Asparagine and alanine aminotransferases were also induced rapidly. Pyruvate orthophosphate dikinase (PPDK) and NADP-malic enzyme (ME) activities increased 10-fold but slowly over 15 d. Total ribulose-1,5-bisphosphate carboxylase/oxygenase activity did not increase, and its activation declined from 82 to 50%. Western blots for PEPC, PPDK, and NADP-ME indicated that increased protein levels were involved in their induction. The H. verticillata NADP-ME polypeptide was larger (90 kD) than the maize C4 enzyme (62 kD). PEPC and PPDK exhibited up-regulation in the light. Subcellular fractionation of C4-type leaves showed that PEPC was cytosolic, whereas PPDK and NADP-ME were located in the chloroplasts. The O2 inhibition of photosynthesis was doubled when C4-type but not C3-type leaves were exposed to diethyl oxalacetate, a PEPC inhibitor. The data are consistent with a C4-cycle concentrating CO2 in H. verticillata chloroplasts and indicate that Kranz anatomy is not obligatory for C4-type photosynthesis. H. verticillata predates modern terrestrial C4 monocots; therefore, this inducible CO2-concentrating mechanism may represent an ancient form of C4 photosynthesis.

Photosynthetic Responses to Changing Atmospheric Carbon Dioxide Concentration

When plants made the transition to land, atmospheric CO2 concentration was up to 16-fold higher than today; since then it has fluctuated, but with an overall decline to very low values during the last glacial maximum. Modern-day plants exhibit photosynthetic adaptations to cope with a low [CO2]/[O2] ratio. These include: high specificity and low Km for CO2 of ribulose bisphosphate carboxylase-oxygenase (Rubisco); pathways to recapture photorespiratory C and N; CO2 concentrating mechanisms in some terrestrial and aquatic species based on C4 or HCO 3 − -use systems; improvements in stomatal regulation; and possibly lower ecological compensation points.

Soybean photosynthesis, Rubisco, and carbohydrate enzymes function at supraoptimal temperatures in elevated CO2

Summary Soybean (Glycine max L. Merr. cv. Bragg) was grown season-long in eight sunlit, controlled–environment chambers at two daytime [CO2] of 350 (ambient) and 700 (elevated) µmol mol –1 . Dry bulb day/night maximum/minimum air temperatures, which followed a continuously and diurnally varying, near sine-wave control set point that operated between maximum (daytime, at 1500 EST) and minimum (nighttime, at 0700 EST) values, were controlled at 28/18 and 40/30 uC for the ambient-CO2 plants, and at 28/18, 32/22, 36/26, 40/30, 44/34 and 48/38 uC for the elevated-CO2 plants. The objective was to assess the upper threshold tolerance of photosynthesis and carbohydrate metabolism with increasing temperatures at elevated [CO2], as it is predicted that air temperatures could rise as much as 4–6 uC within the 21st century with a doubling of atmospheric [CO2]. Leaf photosynthesis measured at growth [CO2] and temperature was greater for elevated-CO2 plants and was highest at 32/22 uC, but markedly declined at temperatures above 40/30 uC. Growth temperatures from 28/18 to 40/30 uC had little effect on midday total activity and protein content of Rubisco, while higher temperatures substantially reduced them. Conversely, midday Rubisco rbcS transcript abundance declined with increasing temperatures from 28/18 to 48/38 uC. Elevated-CO2 plants exceeded the ambientCO2 plants in most aspects of carbohydrate metabolism. Under elevated [CO2], midday activities of ADPG pyrophosphorylase and sucrose-P synthase and invertase paralleled net increases in starch and sucrose contents, respectively. They were highest at 36/26–40/30 uC, but declined at higher or lower growth temperatures. Thus, in the absence of other climatic stresses, soybean photosynthesis and carbohydrate metabolism would perform well under rising atmospheric [CO2] and temperature predicted for the 21st century.

Photosynthetic and photorespiratory responses of the aerial and submerged leaves of Myriophyllum brasiliense

Abstract Submerged and aerial leaves of the amphibious plant Myriophyllum brasiliense Cambess., differed in their CO 2 gas exchange characteristics. Net photosynthesis of the aerial leaves was saturated by an irradiance of 2000 μE/m 2 ·s and a CO 2 concentration of 900 μl CO 2 /l gas phase; whereas the submerged leaves required 250 μE/m 2 ·s, and were not saturated by 2500 μl CO 2 /l gas phase. Aerial leaves had much higher photosynthetic rates than submerged leaves, but the dark respiration rates were similar. Incubation of the submerged leaves under water with a 30°C/14 h photoperiod decreased the CO 2 compensation point (from 73 to 13 μl CO 2 /l gas phase), the O 2 inhibition of photosynthesis (from 25.1 to 17.0%), and the photorespiration rates, as measured by CO 2 evolution into CO 2 -free air in the light, (from 3.3 to 0.5 μmol CO 2 /mg Chl·h). A similar incubation regime, but in air, produced no significant changes in the aerial leaves. Photorespiratory CO 2 release from leaves with high CO 2 compensation points was equivalent to about 15% of their net photosynthetic rate, which is similar to that of C 3 plants; whereas in the submerged leaves with low CO 2 compensation points it was only 2%. Ribulose bisphosphate carboxylase was the predominant carboxylation enzyme in submerged and aerial leaves (77.8 and 153.4 μmol CO 2 /mg Chl·h, respectiviely). Phosphoenolpyruvate carboxylase activity was low in both leaf forms (1.7 to 5.6 μmol CO 2 /mg Chl·h), even in the low CO 2 compensation point state. The apparent Km(CO 2 ) for the photosynthesis of submerged leaves with high CO 2 compensation points decreased from 706 to 122 μM when the plants were measured in air rather than under water. This was still higher than the value determined for the aerial leaves (18 μM), indicating that the submerged leaves possess a large, internal resistance to CO 2 fixation. The low CO 2 compensation point state also decreased the apparent Km(CO 2 ), to 109 μM. The data suggest that the photorespiration-reducing mechanism in Myriophyllum increases the apparent affinity of the photosynthetic process for CO 2 . This mechanism, however, does not rely on increased phosphoenolpyruvate carboxylase activity and therefore probably differs from the photorespiratory-reducing mechanism in certain other aquatic plants. It still appears to involve biochemical, rather than morphological/anatomical, changes in the submersed plant.

Interactions of CO2 enrichment and temperature on carbohydrate accumulation and partitioning in rice

Abstract The objective of this study was to determine the long-term effects of CO 2 concentration and temperature on carbohydrate partitioning and status in rice ( Oryza sativa L. cv. IR-30). The plants were grown season-long in sunlit, controlled-environment chambers with CO 2 concentrations of 330 or 660 μmol mol −1 , and daytime air temperatures of 28, 34 or 40°C. In leaf blades, the priority between partitioning of carbon into storage or into export changed with CO 2 concentration and temperature. Leaf sucrose concentration increased with CO 2 enrichment at all temperature regimes. Over the season, elevated CO 2 resulted in an increase in total non-structural carbohydrate (TNC) concentration in leaf blades, leaf sheaths and culms at all temperature treatments. Elevated CO 2 had no effect on carbohydrate concentration in the grain at maturity, however, grain TNC concentration was significantly lowered by increasing temperature. Under the highest temperature regime, the plants in the 330 μmol mol −1 CO 2 treatment died during stem extension while the CO 2 enriched plants survived but produced sterile panicles. The results suggest that CO 2 -enriched plants could survive and maintain carbohydrate production rates at higher temperatures than the non-enriched plants; however, the optimum temperature for TNC accumulation was 28°C at both CO 2 concentrations.

Photosynthesis, photorespiration and ecophysiological interactions in marine macroalgae

Abstract Although macroalgae play a significant role in the productivity of marine ecosystems, the marine environment presents constraints to the achievement of maximum photosynthesis and productivity. Many macroalgae have developed strategies to minimize these constraints, which include low saturating irradiance requirements and bicarbonate use for photosynthesis, morphological features to reduce desiccation and morphological and biochemical modifications to enhance photosynthetic carbon fixation. In many marine macroalgae, fixation occurs solely by ribulose bisphosphate carboxylase/oxygenase (RuBPCO) and the photosynthetic carbon reduction cycle, and in some, considerable photorespiration is evident. But, in others photorespiration is suppressed, and studies are only now beginning to elucidate the methods that accomplish this effect. A C 4 -like system based on phosphoenolpyruvate carboxykinase appears to operate in the green alga Udotea flabellum (Ellis & Solander) Lamouroux, while in the red alga, Gracilaria corticata J. Agardh, phosphoenolpyruvate carboxylase is used. Among brown algae CAM-type systems or bicarbonate uptake may reduce photorespiration. Bicarbonate uptake in association with carbonic anhydrase activity may be responsible for low photorespiration in some red algae. Regardless of the mechanism, reduced photorespiration is proposed to be achieved by concentrating CO 2 at the RuBPCO fixation site. However, elevated internal CO 2 levels have yet to be demonstrated in any marine macroalga. Although photorespiration seems suppressed in many marine macroalgae, further studies are needed to comprehend fully the mechanisms involved.

ORGANIC CARBON RELEASE BY DUNALIELLA SALINA (CHLOROPHYTA) UNDER DIFFERENT GROWTH CONDITIONS OF CO2, NITROGEN, AND SALINITY1

Two strains of Dunaliella salina (Dunal) Teod., UTEX 1644 and UTEX 200, were cultured under different growth regimes, including 10 mM NO3− or NH4+, 1.5 or 3.0 M NaCl, and low (0.035%) or high (5%) CO2 in air. The release of 14C‐labeled dissolved organic carbon (DOC), expressed as a rate and as a percentage of photosynthetic 14CO2 assimilation, was subsequently determined. The percentage of DOC released was inversely related to cell density in the assay medium, but photosynthesis on a per‐cell basis was not. Release of DOC was low, in the range of 1–5% of photosynthesis, but during acclimation to growth on NH4+, it rose to 11%. The presence of NH4+ rather than NO3− in the growth medium increased the rate of release by both strains, but the percentage release was stimulated only in UTEX 200 cells, because their photosynthetic rate was depressed by NH4+. For UTEX 1644, high, as compared to low, CO2‐grown cells, had somewhat higher rates and percentages of DOC release, but release from UTEX 200 cells was unaffected by the growth‐CO2. The rate of DOC release by high CO2‐grown cells was not enhanced at a low concentration of dissolved inorganic carbon, indicating that the released material did not originate from the photorespiratory pathway. The effects of NaCl on DOC release varied with strain and growth conditions. For UTEX 200, the cells in NO3−, but not NH4+, exhibited a doubling or more in percentage of release with a doubling in NaCl concentration, irrespective of growth‐CO2. With UTEX 1644 the low CO2‐grown cells showed the greatest enhancement in 3.0 M NaCl. Organic matter accumulated on the external surface of the cell membrane and constituted a well‐defined cell‐coat, which was more dense in NH4+ than in NO3−‐grown cells. Microtubules, which may play a role in maintaining cell shape, were observed just below the plasma membrane.

C02 Enrichment Delays a Rapid, Drought-Induced Decrease in Rubisco Small Subunit Transcript Abundance

Summary Rice ( Oryza sativa L. cv. IR-72) was grown in sunlit chambers at 350 and 700 gmol CO 2 mol -1 under conditions of continuous flooding (control) or drought which was imposed at panicle initiation, to evaluate the effects of C0 2 enrichment and soil water deficit on photosynthesis and Rubisco gene expression. Leaf and canopy photosynthetic rates were enhanced by high [CO 2 ] but reduced by drought. High [CO 2 ] and severe drought both reduced rbcS transcript abundance, along with the activity, activation and protein content of Rubisco, but the K m (CO 2 ) was not affected. The transition from moderate to severe drought caused a rapid decline, within 24 h, in the rbcS transcript abundance. High [CO 2 ], however, delayed the adverse effects of severe drought on rbcS transcript abundance and activities of Rubisco, and permitted photosynthesis to continue for an extra day during the drought-stress cycle.