INTERACTIVE EFFECTS OF IRRADIANCE AND CO2 ON CO2 FIXATION AND N2 FIXATION IN THE DIAZOTROPH TRICHODESMIUM ERYTHRAEUM (CYANOBACTERIA)1

The diazotrophic cyanobacteria Trichodesmium spp. contribute approximately half of the known marine dinitrogen (N₂) fixation. Rapidly changing environmental factors such as the rising atmospheric partial pressure of carbon dioxide (pCO₂) and shallower mixed layers (higher light intensities) are likely to affect N₂‐fixation rates in the future ocean. Several studies have documented that N₂ fixation in laboratory cultures of T. erythraeum increased when pCO₂ was doubled from present‐day atmospheric concentrations (～380 ppm) to projected future levels (～750 ppm). We examined the interactive effects of light and pCO₂ on two strains of T. erythraeum Ehrenb. (GBRTRLI101 and IMS101) in laboratory semicontinuous cultures. Elevated pCO₂ stimulated gross N₂‐fixation rates in cultures growing at 38 μmol quanta · m^?2 · s^?1 (GBRTRLI101 and IMS101) and 100 μmol quanta · m^?2 · s^?1 (IMS101), but this effect was reduced in both strains growing at 220 μmol quanta · m^?2 · s^?1. Conversely, CO₂‐fixation rates increased significantly (P < 0.05) in response to high pCO₂ under mid‐ and high irradiances only. These data imply that the stimulatory effect of elevated pCO₂ on CO₂ fixation and N₂ fixation by T. erythraeum is correlated with light. The ratio of gross:net N₂ fixation was also correlated with light and trichome length in IMS101. Our study suggests that elevated pCO₂ may have a strong positive effect on Trichodesmium gross N₂ fixation in intermediate and bottom layers of the euphotic zone, but perhaps not in light‐saturated surface layers. Climate change models must consider the interactive effects of multiple environmental variables on phytoplankton and the biogeochemical cycles they mediate.     

SPATIAL SEGREGATION OF CO2 FIXATION IN TRICHODESMIUM SPP.: LINKAGE TO N2 FIXATION POTENTIAL1

The aggregate-forming, nonheterocystous, filamentous blue-green alga (cyanobacteria) Trichodesmium spp. is a widespread and important planktonic N₂ fixer and primary producer in tropical and subtropical oceans. It is unique among nonheterocystous genera because it conducts N₂ and CO₂ fixation (O₂ evolution) simultaneously; a notable achievement, because O₂ is a potent inhibitor of N₂ fixation. Spatial and temporal CO₂ fixation patterns were examined in trichomes and aggregates from natural and cultured populations, utilizing microautoradiographic detection of ¹⁴CO₂ incorporation. Parallel N₂ fixation (acetylene reduction) measurements were also made. Diel N₂ and CO₂ fixation patterns were similar, with co-optimization of both processes near midday. Microautoradiographs revealed several trichome-level ¹⁴CO₂ incorporation patterns: 1)uniform, heavy labeling, 2)uniform, light labeling, 3) heavier labeling in distal as opposed, to proximal regions, and 4) virtually no labeling throughout. Similar patterns were observed in natural and cultured populations. Given previous immunochemical findings that N₂ fixation potential is widespread in Trichodesmium spp. trichomes and aggregates, current results suggest a high degree of individuality, and possibly a “division of labor” in terms of CO₂ fixation, among trichomes comprising active N₂-fixing aggregates. Segregation of photosynthesis within and among trichomes facilitates simultaneous N₂ and CO₂ fixation in Trichodesmium spp. trichomes and aggregates.     

CHLOROPLAST SPECIALIZATION IN DICOTYLEDONS POSSESSING THE C4-DICARBOXYLIC ACID PATHWAY OF PHOTOSYNTHETIC CO2 FIXATION

The C₄-dicarboxylic-acid pathway of photosynthetic CO₂ fixation found in tropical grasses has recently been demonstrated in members of the Amaranthaceae and Chenopodiaceae. In the tropical grasses this CO₂-fixation pathway is correlated with specialized leaf anatomy and chloroplast structure. This investigation was undertaken to determine if leaf cells of some representatives of these other families had structural features similar to those of tropical grasses. The leaf anatomy of Amaranthus edulis and a variety of Atriplex species is very similar and it resembles that of grasses such as sugar cane. Prominent bundle sheaths are surrounded by a layer of palisade cells. Bundle-sheath cells of Am. edulis have large chloroplasts containing much starch, but the chloroplasts have grana. The palisade cells have much smaller chloroplasts containing very little starch. The bundle-sheath cell chloroplasts of At. lentiformis have grana, their profiles tend to be ovoid, and they contain abundant starch grains. The palisade cell chloroplasts contain little starch and their profiles are discoid. The bundle-sheath cells of both species contain mitochondria which are much larger than those in the palisade cells. The chloroplasts in both types of cells in both species have a highly developed peripheral reticulum. This reticulum is composed of anastomosing tubules which are contiguous with the inner plastid membrane. The leaf anatomy and cell ultrastructure of these dicots are similar to those of the tropical grasses possessing this new photosynthetic carbon-fixation pathway. These morphological features are interpreted as adaptations for the rapid transport of precursors and end products of photosynthesis. A hypothesis is presented stating that the unique morphological and biochemical characters of these plants represent adaptations for efficient and rapid carbon fixation in environments where water stress frequently limits photosynthesis.     

豆科固氮植物对CO2加富的生理响应

 全球CO2浓度升高对植物的影响成为近代植物生理生态学研究的热点,豆科植物因其独特的固氮能力使其研究具有重要价值。从光合生理、固氮特性及碳、氮代谢等几个方面综述了CO2浓度升高对豆科植物生理生态特性的影响,总结出如下结论：在高CO2浓度条件下,豆科植物的光合速率、根     

FORMALIN FIXATION IN THE CYTOCHEMICAL DEMONSTRATION OF SUCCINIC DEHYDROGENASE OF MITOCHONDRIA

A variety of established methods for protecting mitochondria were tested on rat duodenal epithelium during the histochemical assay for succinic dehydrogenase. The use of sucrose at isotonic or hypertonic concentrations, 7.5 per cent polyvinylpyrrolidone, divalent cations, physiological salt solutions, phenazine methosulfate, coenzyme Q₁₀, and menadione failed to improve the quality of the histochemical preparation once fresh frozen sections were prepared. However, preservation of mitochondrial integrity with little diminution in succinic dehydrogenase activity was obtained by fixing tissue slices (less than 1 mm. in thickness) in 8 per cent unneutralized, aqueous formaldehyde from 8 to 16 minutes at from 5° to 10°C. prior to freezing. To offset the inhibition of enzymatic activity it was necessary to extend the incubation period by 10 to 15 minutes. Two-micron-thick sections were easily obtained from the frozen blocks of such fixed tissue and incubated in the unmodified Nitro—BT-succinate medium. Once the optimum conditions for fixation of intestinal epithelium were determined, many other tissues were subjected to the same procedure. From the morphological standpoint the appearance of the mitochondria in these histochemical preparations compares favorably with the results obtained using the classical Regaud iron-hematoxylin staining procedure. With most tissues, the results are superior to those with fresh frozen sections. However, results with muscle, sperm, and kidney tubular epithelium are not as strikingly improved as with gut and liver.     

不同二氧化碳浓度培养对两株栅藻碳固定速率及油脂积累的影响

正人类在利用化石燃料的过程中会导致大量有害温室气体CO_2的排放,促进全球气候变暖。微藻可通过光合作用固定CO_2,同时大量的微藻生物质还能作为生物能源的原料[1],因此,越来越多的研究关注于微藻生物固碳以达到降低碳排放的目的。利用微藻光合作用进行CO_2固定是一种能量节约型和环境友好型技术手段[2]。在利用微藻进行CO_2生物固定以及生物燃料生产时,研究微藻的CO_2固定能力、CO_2对微藻的生长以及油脂积累的影响等都是十分重要的。国内外利用微藻进行生