Photosynthesis

About: Photosynthesis is a research topic. Over the lifetime, 19789 publications have been published within this topic receiving 895197 citations.

Evidence for anoxygenic photosynthesis from the distribution of bacteriochlorophylls in the Black Sea.

Abstract: The contribution of anoxygenic photosynthesis to carbon cycling in the Black Sea, the world's largest body of anoxic marine water, has been vigorously investigated and debated for over four decades. Penetration of light into the sulphide-containing deep water may result in a zone of anaerobic primary production by photosynthetic bacteria. We report here the results of analyses of photosynthetic pigments in samples of suspended particulate matter collected from two stations in the western basin of the Black Sea. Our data demonstrate high concentrations of a bacterio-chlorophyll at the chemocline, and thus the potential for anoxygenic photosynthesis as a component of primary production in the carbon cycle of the Black Sea. More than 95% of the pigments in the bacteriochlorophyll-maximum are accounted for by a series of aromatic carotenoids and bacteriochlorophylls-e, including a previously unreported geranyl ester of 4-i-butyl bacteriochlorophyll-e. The distribution of pigments is characteristic of the obligate phototrophs Chlorobium phaeobacteroides and C. phaeovibriodes. Total depth-integrated bacteriochlorophyll at one station exceeded total chlorophyll-a in the overlying oxygenated portion of the euphotic zone. We suggest that anoxygenic photosynthesis is a relatively recent phenomenon in the Black Sea initiated by shallowing of the chemocline over the past decade and development of an anoxic layer devoid of O2 and H2S.

Rubisco and Rubisco Activase Play an Important Role in the Biochemical Limitations of Photosynthesis in Rice, Wheat, and Maize under High Temperature and Water Deficit.

Abstract: To understand the effect of heat and drought on three major cereal crops, the physiological and biochemical (i.e. metabolic) factors affecting photosynthesis were examined in rice, wheat and maize plants grown under long-term water deficit (WD), high temperature (HT) and the combination of both stresses (HT-WD). Diffusional limitations to photosynthesis prevailed under WD for the C3 species, rice and wheat. Conversely, biochemical limitations prevailed under WD for the C4 species, maize, under HT for all three species, and under HT-WD in rice and maize. These biochemical limitations to photosynthesis were associated with Rubisco activity that was highly impaired at HT and under HT-WD in the three species. Decreases in Rubisco activation were unrelated to the amount of Rubisco and Rubisco activase (Rca), but were probably caused by inhibition of Rca activity, as suggested by the mutual decrease and positive correlation between Rubisco activation state and the rate of electron transport. Decreased Rubisco activation at HT was associated with biochemical limitation of net CO2 assimilation rate (AN). Overall, the results highlight the importance of Rubisco as a target for improving the photosynthetic performance of these C3 (wheat and rice) and C4 (maize) cereal crops under increasingly variable and warmer climates.

Photoacclimation processes in phytoplankton: mechanisms, consequences, and applications

Abstract: In nature and in the laboratory, phytoplankton cells are exposed to fast and extreme fluctuations in light intensity. These include diel and seasonal changes in irradiance, and changes stemming from vertical mixing over the light field. In algal mass cultures and photobioreactors, sim- ilar changes take place as cultures grow denser and as cells are mixed rapidly in the system. To sur- vive supraoptimal, free-radical-generating irradiance levels as well as prolonged exposure to dim light, phytoplankton species are capable of photoacclimation. Under low light, light-harvesting pig- ments such as phycobilins, chlorophylls, fucoxanthin and peridinin increase all the way to optically becoming black cells. The same pigments decrease under high light, resulting in cells being rather transparent. The opposite takes place with the photoprotective pigments β-carotene and astaxanthin and the elements of the xanthophyll cycle, all of which increase whenever cells are exposed to high irradiance levels, concomitant with enhanced activity of the antioxidant enzymes catalase, superox- ydismutase, and peroxidase. These processes are complemented by up to 5-fold changes in RUBISCO per photosystem unit (PSU) levels, and parallel changes in light-saturated photosynthetic rates. Thus, light-harvesting and utilization efficiencies are maximized under low light, whereas pho- tosynthetic carbon assimilation and throughput rates reach their peak values whenever light is suffi- ciently high. Maximal photosynthesis and growth rates have to be matched by correspondingly high respiration rates. Photoacclimation can be used to optimize biomass and target product yields in biotechnological applications.

Kok effect and the quantum yield of photosynthesis : light partially inhibits dark respiration.

Abstract: The linear response of photosynthesis to light at low photon flux densities is known to change abruptly in the vicinity of the light compensation point so that the quantum yield seems to decrease as radiation increases. We studied this ;Kok effect' in attached sunflower (Helianthus annuus L. cv IS894) leaves using gas exchange techniques. The effect was present even though respiration was constant in the dark. It was observed at a similar photon flux density (7 to 11 micromole photons per square meter per second absorbed photosynthetically active radiation) despite a wide range of light compensation points as well as rates of photosynthesis. The effect was not apparent when photorespiration was inhibited at low pO(2) (1 kilopascal), but this result was complicated because dark respiration was quite O(2)-sensitive and was partially suppressed under these conditions. The Kok effect was observed at saturating pCO(2) and, therefore, could not be explained by a change in photorespiration. Instead, the magnitude of the effect varied as dark respiration varied in a single leaf, and was minimized when dark respiration was minimized, indicating that a partial suppression of dark respiration by light is responsible. Quantum yields measured at photon flux densities between 0 and 7 to 11 micromole photons per square meter per second, therefore, represent the combined yields of photosynthesis and of the suppression of a component of dark respiration by light. This leads to an overestimate of the quantum yield of photosynthesis. In view of these results, quantum yields of photosynthesis must be measured (a) when respiration is constant in the dark, and (b) when dark respiration has been inhibited either at low pO(2) to eliminate most of the light-induced suppression of dark respiration or at photon flux densities above that required to saturate the light-induced suppression of dark respiration. Significant errors in quantum yields of photosynthesis can result in leaves exhibiting this respiratory behavior if these principles are not followed.