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.

No abstract

Abstract. The global uptake of CO2 in photosynthesis is about 120 gigatons (Gt) of carbon per year. Virtually all passes through one enzyme, ribulose bisphosphate carboxylase/oxygenase (rubisco), which initiates both the photosynthetic carbon reduction, and photorespiratory carbon oxidation, cycles. Both CO2 and O2 are substrates; CO2 also activates the enzyme. In C3 plants, rubisco has a low catalytic activity, operates below its Km (CO2), and is inhibited by O2. Consequently, increases in the CO2/O2 ratio stimulate C3 photosynthesis and inhibit photorespiration. CO2 enrichment usually enhances the productivity of C3 plants, but the effect is marginal in C4 species. It also causes acclimation in various ways: anatomically, morphologically, physiologically or biochemically. So, CO2 exerts secondary effects in growth regulation, probably at the molecular level, that are not predictable from its primary biochemical role in carboxylation. After an initial increase with CO2 enrichment, net photosynthesis often declines. This is a common acclimation phenomenon, less so in field studies, that is ultimately mediated by a decline in rubisco activity, though the RuBP/Pi‐regeneration capacities of the plant may play a role. The decline is due to decreased rubisco protein, activation state, and/or specific activity, and it maintains the rubisco fixation and RuBP/Pi regeneration capacities in balance. Carbohydrate accumulation is sometimes associated with reduced net photosynthesis, possibly causing feedback inhibition of the RuBP/Piregeneration capacities, or chloroplast disruption. As exemplified by field‐grown soybeans and salt marsh species, a reduction in net photosynthesis and rubisco activity is not inevitable under CO2 enrichment. Strong sinks or rapid translocation may avoid such acclimation responses. Over geological time, aquatic autotrophs and terrestrial C4 and CAM plants have genetically adapted to a decline in the external CO2/O2 ratio, by the development of mechanisms to concentrate CO2 internally; thus circumventing O2 inhibition of rubisco. Here rubisco affinity for CO2 is less, but its catalytic activity is greater, a situation compatible with a high‐CO2 internal environment. In aquatic autotrophs, the CO2 concentrating mechanisms acclimate to the external CO2, being suppressed at high‐CO2. It is unclear, whether a doubling in atmospheric CO2 will be sufficient to cause a de‐adaptive trend in the rubisco kinetics of future C3 plants, producing higher catalytic activities.

No abstract

Light-and C02-saturated photosynthetic rates of the submersed aquatic plants Hydrilla verticillata, Ceratophyllum demersum, and Myriophyllum spicatum were 50 to 60 nLmol OJmg Chl hr at 30 C. At ar levels of C02, the rates were less than 5% of those achieved by terrestrial C3 plants. The The majority of terrestrial plants can be classified as C3 or C. plants, based on specific characteristics associated with their photosynthetic pathways (4). C4 plants typically have higher photosynthetic rates and greater productivity than C3 plants (4). The photosynthetic mechanisms of submersed macrophytes, although basic to their productivity, have received limited attention. For Hydrilla and Ceratophyllum, the photosynthetic pathways and most of the associated characteristics are unknown. Myriophyllum apparently exhibits characteristics of both C3 and C4 plants. The initial product of CO2 fixation is 3-P-glycerate (29), as in C3 plants; but it reportedly also has a high optimum temperature for photosynthesis and a low CO2 compensation point (29), which are characteristics usually associated with C4 plants. In contrast, the submersed macrophytes Egeria densa and Lagarosiphon major, which belong to the same family as Hydrilla, possess CO2 compensation points similar to those of C3 plants, and their photosynthesis is inhibited by 02 (9). Thus, aquatic macrophytes appear to exhibit some diversity in regard to their photosynthetic mechanism.In an aquatic environment, the inorganic carbon can exist in several forms: free C02, H2CO3, HC03-, or C032-, depending on the pH. For both aquatic and terrestrial plants, free CO2 is the form most readily utilized for photosynthesis (25). A number of submersed plants, including Myriophyllum, reportedly can use HCO3-ions in addition to free CO2 for photosynthesis (30). Recent work, however, suggests that several aquatic species are unable to use HC03-ions (9). It has been argued that the ability to use HC03-ions would provide an aquatic plant with a competitive advantage in alkaline waters (21). A further factor that may influence the competitive success of an aquatic plant is its photosynthetic response to light. Egeria, for example, reportedly replaces both Elodea and Lagarosiphon because of its lower light requirement for photosynthesis (9). In this study, we have examined the ability of Hydrilla, Ceratophyllum, and Myriophyllum to use HCO3-ions for photosynthesis and also the photosynthetic responses of these plants to varying irradiance. The possible ecological implications of these factors are discussed.

No abstract

Rice (Oryza sativa L. cv. IR-72) and soybean (Glycine max L. Merr. cv. Bragg), which have been reported to differ in acclimation to elevated CO2, were grown for a season in snnlight at ambient and twice-ambient [CO2], and under daytime temperature regimes ranging from 28 to 40 °C. The objectives of the study were to test whether CO2 enrichment could compensate for adverse effects of high growth temperatures on photosynthesis, and whether these two C3 species differed in this regard. Leaf photosynthetic assimilation rates (A) of both species, when measured at the growth [CO2], were increased by CO2 enrichment, but decreased by supraoptimal temperatures. However, CO2 enrichment more than compensated for the temperatureinduced decline in A. For soybean, this CO2 enhancement of A increased in a linear manner by 32-95% with increasing growth temperatures from 28 to 40 °C, whereas with rice the degree of enhancement was relatively constant at about 60%, from 32 to 38 °C. Both elevated CO2 and temperature exerted coarse control on the Rubisco protein content, but the two species differed in the degree of responsiveness. CO2 enrichment and high growth temperatures reduced the Rubisco content of rice by 22 and 23%, respectively, but only by 8 and 17% for soybean. The maximum degree of Rubisco down-regulation appeared to be limited, as in rice the substantial individual effects of these two variables, when combined, were less than additive. Fine control of Rubisco activation was also influenced by both elevated [CO2] and temperature. In rice, total activity and activation were reduced, but in soybean only activation was lowered. The apparent catalytic turnover rate (Xcat) of rice Rubisco was unaffected by these variables, but in soybean elevated [CO2] and temperature increased the apparent ^^at by 8 and 22%, respectively. Post-sunset declines in Rubisco activities were accelerated by elevated [CO2] in rice, but by high temperature in soybean, suggesting that [CO2] and growth temperature influenced the metabolism of 2-carboxyarabinitol-l-phosphate, and that the effects might be species-specific. The greater capacity of soybean for CO2 enhancement of A at supraoptimal tem- peratures was probably not due to changes in stomatal conductance, but may be partially attributed to less down-regulation of Rubisco by elevated [CO2] in soybean than in rice. However, unidentified species differences in the temperature optimum for photosynthesis also appeared to be important. The responses of photosynthesis and Rubisco in rice and soybean suggest that among C3 plants species-specific differences will be encountered as a result of future increases in global [CO2] and air temperatures.

No abstract