Photosynthetic carbon metabolism of a marine grass

The δ¹³C value of a tropical marine grass Thalassia testudinum is −9.04‰. This value is similar to the δ¹³C value of terrestrial tropical grasses. The δ¹³C values of the organic acid fraction, the amino acid fraction, the sugar fraction, malic acid, and glucose are: −11.2‰, −13.1‰, −10.1‰, −11.1‰, and −11.5‰, respectively. The δ¹³C values of malic acid and glucose of Thalassia are similar to the δ¹³C values of these intermediates in sorghum leaves and attest to the presence of the photosynthetic C₄-dicarboxylic acid pathway in this marine grass. The inorganic HCO₃⁻ for the growth of the grass fluctuates between −6.7 to −2.7‰ during the day. If CO₂ fixation in Thalassia is catalyzed by phosphoenolpyruvate carboxylase (which would result in a −3‰ fractionation between HCO₃⁻ and malic acid), the predicted δ¹³C value for Thalassia would be −9.7 to −5.7‰. This range is close to the observed range of −12.6 to −7.8‰ for Thalassia and agree with the operation of the C₄-dicarboxylic acid pathway in this plant. The early products of the fixation of HCO₃⁻ in the leaf sections are malic acid and aspartic acid which are similar to the early products of CO₂ fixation in C₄ terrestrial plants.     

Differential expression of photosynthesis genes in leaf tissue layers of Peperomia as revealed by tissue printing

Peperomia has species that may be C₃, show Crassulacean acid metabolism (CAM), or CAM-cycling. Species that show CAM progress from C₃ to CAM through CAM-cycling during leaf development. In CAM and CAM-cycling species, CAM metabolism is predominately in the upper multiple epidermis and lower spongy mesophyll, whereas C₃ metabolism is localized mostly in the palisade mesophyll. Using specific protein and cDNA probes prepared from P-enolpyruvate carboxylase (PEPc) and ribulose-1,5-bisphosphate carboxylase (Rubisco), we have now studied the differential distribution of photosynthetic metabolism in Peperomia leaves using the technique of tissue printing. The tissue printing studies detected Rubisco protein in leaves of C₃ P. orba, but not PEPc. Young C₃ leaves of P. scandens and P. camptotricha showed Rubisco protein, but not PEPc; however, the mature leaves of these two species that have CAM showed PEPc protein and RNAs in both the multiple epidermis and spongy mesophyll. In contrast, Rubisco protein and RNAs were present throughout the leaf. The tissue printing data are consistent with our previously published data showing the differential distribution of photosynthetic metabolism in leaves of Peperomia. Although the tissue printing technique is qualitative, coupled with quantitative data it has proven useful for the study of function related to structure.     

Stable Carbon Isotope Ratios of Lipid Biomarkers of Sulfate-Reducing Bacteria

We examined the potential use of natural-abundance stable carbon isotope ratios of lipids for determining substrate usage by sulfate-reducing bacteria (SRB). Four SRB were grown under autotrophic, mixotrophic, or heterotrophic growth conditions, and the δ¹³C values of their individual fatty acids (FA) were determined. The FA were usually ¹³C depleted in relation to biomass, with Δδ¹³C_{(FA − biomass)} of −4 to −17‰; the greatest depletion occurred during heterotrophic growth. The exception was Desulfotomaculum acetoxidans, for which substrate limitation resulted in biomass and FA becoming isotopically heavier than the acetate substrate. The δ¹³C values of FA in Desulfotomaculum acetoxidans varied with the position of the double bond in the monounsaturated C₁₆ and C₁₈ FA, with FA becoming progressively more ¹³C depleted as the double bond approached the methyl end. Mixotrophic growth of Desulfovibrio desulfuricans resulted in little depletion of the i17:1 biomarker relative to biomass or acetate, whereas growth with lactate resulted in a higher proportion of i17:1 with a greater depletion in ¹³C. The relative abundances of 10Me16:0 in Desulfobacter hydrogenophilus and Desulfobacterium autotrophicum were not affected by growth conditions, yet the Δδ¹³C_{(FA − substrate)} values of 10Me16:0 were considerably greater during autotrophic growth. These experiments indicate that FA δ¹³C values can be useful for interpreting carbon utilization by SRB in natural environments.     

Fractionation of the stable isotopes of inorganic carbon by seagrasses

The δ¹³C values for seagrasses collected along the Texas Gulf Coast range from −10.9 to −11.4‰. These values are similar to the δ¹³C values of terrestrial C₄ plants, but seagrasses lack bundle sheath cells which are important in determining the δ¹³C values of C₄ plants. This work attempts to explain the reason the δ¹³C values of seagrasses resemble the δ¹³C values of C₄ plants.     

Priming of Soil Carbon Decomposition in Two Inner Mongolia Grassland Soils following Sheep Dung Addition: A Study Using 13C Natural Abundance Approach

To investigate the effect of sheep dung on soil carbon (C) sequestration, a 152 days incubation experiment was conducted with soils from two different Inner Mongolian grasslands, i.e. a Leymus chinensis dominated grassland representing the climax community (2.1% organic matter content) and a heavily degraded Artemisia frigida dominated community (1.3% organic matter content). Dung was collected from sheep either fed on L. chinensis (C₃ plant with δ¹³C = −26.8‰; dung δ¹³C = −26.2‰) or Cleistogenes squarrosa (C₄ plant with δ¹³C = −14.6‰; dung δ¹³C = −15.7‰). Fresh C₃ and C₄ sheep dung was mixed with the two grassland soils and incubated under controlled conditions for analysis of ¹³C-CO₂ emissions. Soil samples were taken at days 17, 43, 86, 127 and 152 after sheep dung addition to detect the δ¹³C signal in soil and dung components. Analysis revealed that 16.9% and 16.6% of the sheep dung C had decomposed, of which 3.5% and 2.8% was sequestrated in the soils of L. chinensis and A. frigida grasslands, respectively, while the remaining decomposed sheep dung was emitted as CO₂. The cumulative amounts of C respired from dung treated soils during 152 days were 7–8 times higher than in the un-amended controls. In both grassland soils, ca. 60% of the evolved CO₂ originated from the decomposing sheep dung and 40% from the native soil C. Priming effects of soil C decomposition were observed in both soils, i.e. 1.4 g and 1.6 g additional soil C kg⁻¹ dry soil had been emitted as CO₂ for the L. chinensis and A. frigida soils, respectively. Hence, the net C losses from L. chinensis and A. frigida soils were 0.6 g and 0.9 g C kg⁻¹ soil, which was 2.6% and 7.0% of the total C in L. chinensis and A. frigida grasslands soils, respectively. Our results suggest that grazing of degraded Inner Mongolian pastures may cause a net soil C loss due to the positive priming effect, thereby accelerating soil deterioration.     

Crassulacean Acid Metabolism in the Succulent C(4) Dicot, Portulaca oleracea L Under Natural Environmental Conditions

Crassulacean acid metabolism (CAM) was examined under natural environmental conditions in the succulent C₄ dicot Portulaca oleracea L. Two groups of plants were monitored; one was watered daily (well watered), while the other received water once every 3 to 4 weeks to produce a ψ of −8 bars (drought stressed). Gas exchange, transpiration rate, and titratable acidity were measured for 24-hour periods during the growing season. CAM activity was greatest in drought-stressed plants during late August which had 13 hour days and day/night temperatures of 35/15°C. Under these conditions net CO₂ uptake occurred slowly throughout the night. Diurnal fluctuations of titratable acidity took place in both leaves and stems with amplitudes of 17 and 47 microequivalents per gram fresh weight, respectively. Transpiration data indicated greater opening of stomata during the night than the day. CAM was less pronounced in drought-stressed P. oleracea plants in July and September; neither dark CO₂ uptake nor positive carbon balance occurred during the July measurements. In contrast, well-watered plants appeared to rely on C₄ photosynthesis throughout the season, although some acid fluctuations occurred in stems of these plants during September.     

Influence of Ozone on the Stable Carbon Isotope Composition, deltaC, of Leaves and Grain of Spring Wheat (Triticum aestivum L.)

The relative composition of stable carbon isotopes, δ¹³C, was determined in flag leaves and grain of spring wheat (Triticum aestivum L. cv Albis) grown in open-top field fumigation chambers and exposed to different O₃ levels during the growing season. The aim of the study was to establish exposure-response relationships for the radiation-weighted seasonal mean O₃ concentration and δ¹³C (relative deviation of the ¹³C/¹²C ratio) values of the two plant parts. Samples were collected at harvest in 1986, 1987, and 1988. With increasing O₃ concentration, δ¹³C values increased (became less negative) proportionally. Year to year δ¹³C differences at equivalent O₃ concentrations were small. The shift in δ¹³C caused by O₃ was more pronounced in grain than in leaves. According to models of ¹³C discrimination in C₃ plants, these results indicate increasing limitation of photosynthesis by CO₂ diffusion relative to limitation by carboxylation with increasing O₃ exposure. This conclusion is not in agreement with results from gas exchange analysis. Water use efficiency in green flag leaves tended to decrease with increasing O₃, indicating a dominating effect of O₃ on CO₂ carboxylation.     

Isotope Fractionation in Photosynthetic Bacteria during Carbon Dioxide Assimilation

The δ _PDB¹³C values have been determined for the cellular constituents and metabolic intermediates of autotrophically grown Chromatium vinosum. The isotopic composition of the HCO₃^- in the medium and the carbon isotopic composition of the bacterial cells change with the growth of the culture. The δ _PDB¹³C value of the HCO₃^- in the media changes from an initial value of −6.6‰ to +8.1‰ after 10 days of bacterial growth and the δ _PDB¹³C value of the bacterial cells change from −37.5‰ to −29.2‰ in the same period. The amount of carbon isotope fractionation during the synthesis of hexoses by the photoassimilation of CO₂ has a range of −15.5‰ at time zero to −22.0‰ after 10 days. This range of fractionation compares to the range of carbon isotope fractionation for the synthesis of sugars from CO₂ by ribulose 1,5-diphosphate carboxylase and the Calvin cycle.     

Autotrophy in maize husk leaves : evaluation using natural abundance of stable isotopes

The natural abundance of carbon and hydrogen isotopic composition, expressed as a δ¹³C value of plant dry matter and cellulose in the hypsophylls (husk leaves) of maize (Zea mays L.) was measured and compared with that of leaves and cobs. The δ¹³C values of outer hypsophylls were usually 2 to 3%‰ more negative than leaves or other tissues, and became more negative with increasing chlorophyll content, indicating significant local C₃ pathway fixation of CO₂ in the outer hypsophylls. The δD values indicated a significant part of hypsophyll cellulose was derived from heterotrophic sources (sucrose from C₄ photosynthesis in other tissues). Isotopic mass balance calculations allowed quantitative estimation of these carbon sources and, in the samples examined, about 16% of hypsophyll cellulose was derived from local C₃ photosynthesis, about 62% from local C₄ photosynthesis, and about 22% from sucrose imported from other leaves.     

Carbon isotope ratios demonstrate carbon flux from c(4) host to c(3) parasite

Carbon isotope ratios of mature leaves from the C₃ angiosperm root hemiparasites Striga hermonthica (Del.) Benth (−26.7‰) and S. asiatica (L.) Kuntze (−25.6‰) were more negative than their C₄ host, sorghum (Sorghum bicolor [L.] Moench cv CSH1), (−13.5‰). However, in young photosynthetically incompetent plants of S. hermonthica this difference was reduced to less than 1‰. Differences between the carbon isotope ratios of two C₃-C₃ associations, S. gesnerioides (Willd.) Vatke—Vigna unguiculata (L.) Walp. and Oryza sativa L.—Rhamphicarpa fistulosa (Hochst.) Benth differed by less than 1‰. Theoretical carbon isotope ratios for mature leaves of S. hermonthica and S. asiatica, calculated from foliar gas exchange measurements, were −31.8 and −32.0‰, respectively. This difference between the measured and theoretical δ¹³C-values of 5 to 6‰ suggests that even in mature, photosynthetically active plants, there is substantial input of carbon from the C₄ host. We estimate this to be approximately 28% of the total carbon in S. hermonthica and 35% in S. asiatica. This level of carbon transfer contributes to the host's growth reductions observed in Striga-infected sorghum.     

Carbon Isotope Composition of Nighttime Leaf-Respired CO2 in the Agricultural-Pastoral Zone of the Songnen Plain,Northeast China

Variations in the carbon isotope signature of leaf dark-respired CO₂ (δ¹³C_R) within a single night is a widely observed phenomenon. However, it is unclear whether there are plant functional type differences with regard to the amplitude of the nighttime variation in δ¹³C_R. These differences, if present, would be important for interpreting the short-term variations in the stable carbon signature of ecosystem respiration and the partitioning of carbon fluxes. To assess the plant functional type differences relating to the magnitude of the nighttime variation in δ¹³C_R and the respiratory apparent fractionation, we measured the δ¹³C_R, the leaf gas exchange, and the δ¹³C of the respiratory substrates of 22 species present in the agricultural-pastoral zone of the Songnen Plain, northeast China. The species studied were grouped into C₃ and C₄ plants, trees, grasses, and herbs. A significant nocturnal shift in δ¹³C_R was detected in 20 of the studied species, with the magnitude of the shift ranging from 1‰ to 5.8‰. The magnitude of the nighttime variation in δ¹³C_R was strongly correlated with the daytime cumulative carbon assimilation, which suggests that variation in δ¹³C_R were influenced, to some extent, by changes in the contribution of malate decarboxylation to total respiratory CO₂ flux. There were no differences in the magnitude of the nighttime variation in δ¹³C_R between the C₃ and C₄ plants, as well as among the woody plants, herbs and graminoids. Leaf respired CO₂ was enriched in ¹³C compared to biomass, soluble carbohydrates and lipids; however the magnitude of enrichment differed between 8 pm and 4 am, which were mainly caused by the changes in δ¹³C_R. We also detected the plant functional type differences in respiratory apparent fractionation relative to biomass at 4 am, which suggests that caution should be exercised when using the δ¹³C of bulk leaf material as a proxy for the δ¹³C of leaf-respired CO₂.     

Two Carotenoid Oxygenases Contribute to Mammalian Provitamin A Metabolism

Mammalian genomes encode two provitamin A-converting enzymes as follows: the β-carotene-15,15′-oxygenase (BCO1) and the β-carotene-9′,10′-oxygenase (BCO2). Symmetric cleavage by BCO1 yields retinoids (β-15′-apocarotenoids, C₂₀), whereas eccentric cleavage by BCO2 produces long-chain (>C₂₀) apocarotenoids. Here, we used genetic and biochemical approaches to clarify the contribution of these enzymes to provitamin A metabolism. We subjected wild type, Bco1^−/−, Bco2^−/−, and Bco1^−/−Bco2^−/− double knock-out mice to a controlled diet providing β-carotene as the sole source for apocarotenoid production. This study revealed that BCO1 is critical for retinoid homeostasis. Genetic disruption of BCO1 resulted in β-carotene accumulation and vitamin A deficiency accompanied by a BCO2-dependent production of minor amounts of β-apo-10′-carotenol (APO10ol). We found that APO10ol can be esterified and transported by the same proteins as vitamin A but with a lower affinity and slower reaction kinetics. In wild type mice, APO10ol was converted to retinoids by BCO1. We also show that a stepwise cleavage by BCO2 and BCO1 with APO10ol as an intermediate could provide a mechanism to tailor asymmetric carotenoids such as β-cryptoxanthin for vitamin A production. In conclusion, our study provides evidence that mammals employ both carotenoid oxygenases to synthesize retinoids from provitamin A carotenoids.     

C(3) Photosynthesis and Crassulacean Acid Metabolism in a Kansas Rock Outcrop Succulent, Talinum calycinum Engelm. (Portulacaceae)

The potential for Crassulacean acid metabolism (CAM) was investigated in the sandstone outcrop succulent Talinum calycinum in central Kansas. Field studies revealed CAM-like diurnal acid fluctuations in these plants. These fluctuations persisted under all moisture and temperature regimes in the laboratory. Despite this CAM-like acid metabolism, simultaneous gravimetric determinations of day- and nighttime transpiration rates indicated the presence of a C₃ gas exchange pattern. Subsequent analyses of diurnal CO₂ and H₂O exchange patterns under well-watered conditions and after 3, 5, and 7 days of drought confirmed these findings, though low rates of nocturnal CO₂ uptake were observed on the fifth night after continuous drought. Finally, the δ¹³C/¹²C value of this succulent, −27.8‰, emphasizes the insignificance of any nocturnal CO₂ uptake in the lifelong accumulation of carbon in this species. Thus, it is proposed that T. calycinum is a C₃ plant with some CAM characteristics, including the ability to re-fix respiratory CO₂ at night under all moisture regimes, potentially resulting in a conservation of carbon, and occasionally to fix atmospheric CO₂ at night. These findings may prove to be common among rock outcrop succulents.     

C/C ratio changes in crassulacean Acid metabolism plants

¹³C/¹²C ratios have been found in totally combusted leaves of Crassulacean acid metabolism plants to range from −14 to −33 δ ¹³C‰ compared with a limestone standard. Crassulacean acid metabolism plants apparently utilize both ribulose-1, 5-diphosphate carboxylase and phosphoenolpyruvate carboxylase to assimilate atmospheric CO₂ and, depending on environmental conditions, have ¹³C/¹²C ratios indicative of either carboxylase or to any intermediate value. The degree of discrimination against ¹³C and the resultant ¹³C/¹²C ratio from the photosynthetically fixed CO₂ is influenced by environmental conditions and is not a specific and fixed characteristic of a Crassulacean acid metabolism plant. Certain Crassulacean acid metabolism plants may shift their ratios as much as 17 δ ¹³C‰ in specific environments.     

Identification of the leaf vacuole as a major nitrate storage pool

Highly purified vacuoles were isolated from protoplasts derived from green barley (Hordeum vulgare var. Numar) leaves, in order to determine their role as a NO₃⁻ storage sink. α-Mannosidase and acid phosphatase activities were used as markers to identify vacuoles, α-mannosidase being the more suitable. Nitrate and α-mannosidase, which were released from vacuoles destroyed during lysis of protoplasts, moved at unequal rates in the density gradient used for vacuole isolation. Purified vacuoles retained less NO₃⁻ than α-mannosidase during a single washing. Empirically determined corrections were used to account for NO₃⁻ movement in estimating the percentage of total cellular nitrate found in the vacuole. Vacuoles from plants grown in the presence of NO₃⁻ contained 58% of the total cellular NO₃⁻ and therefore represent a major NO₃⁻ storage pool.     

Development of Functional Human NK Cells in an Immunodeficient Mouse Model with the Ability to Provide Protection against Tumor Challenge

Studies of human NK cells and their role in tumor suppression have largely been restricted to in vitro experiments which lack the complexity of whole organisms, or mouse models which differ significantly from humans. In this study we showed that, in contrast to C57BL/6 Rag2^−/−/γ_c ^−/− and NOD/Scid mice, newborn BALB/c Rag2^−/−/γ_c ^−/− mice can support the development of human NK cells and CD56+ T cells after intrahepatic injection with hematopoietic stem cells. The human CD56⁺ cells in BALB/c Rag2^−/−/γ_c ^−/− mice were able to produce IFN-γ in response to human IL-15 and polyI:C. NK cells from reconstituted Rag2^−/−/γ_c ^−/− mice were also able to kill and inhibit the growth of K562 cells in vitro and were able to produce IFN-γ in response to stimulation with K562 cells. In vivo, reconstituted Rag2^−/−/γ_c ^−/− mice had higher survival rates after K562 challenge compared to non-reconstituted Rag2^−/−/γ_c ^−/− mice and were able to control tumor burden in various organs. Reconstituted Rag2^−/−/γ_c ^−/− mice represent a model in which functional human NK and CD56+ T cells can develop from stem cells and can thus be used to study human disease in a more clinically relevant environment.     

Stable isotope canopy effects for sympatric monkeys at Ta? Forest,C?te d'Ivoire

This study tests the hypothesis that vertical habitat preferences of different monkey species inhabiting closed canopy rainforest are reflected in oxygen isotopes. We sampled bone from seven sympatric cercopithecid species in the Taï forest, Côte d''Ivoire, where long-term study has established taxon-specific patterns of habitat use and diet. Modern rib samples (n = 34) were examined for oxygen (δ¹⁸O_ap) and carbon (δ¹³C_ap) from bone apatite (‘bioapatite’), and carbon (δ¹³C_co) and nitrogen (δ¹⁵N_co) from bone collagen. Results are consistent for C₃ feeders in a closed canopy habitat. Low irradiance and evapotranspiration, coupled with high relative humidity and recycled CO₂ in forest understory, contribute to observed isotopic variability. Both δ¹³C_co and δ¹³C_ap results reflect diet; however, δ¹³C values are not correlated with species preference for canopy height. By contrast, δ¹⁸O_ap results are correlated with mean observed height and show significant vertical partitioning between taxa feeding at ground, lower and upper canopy levels. This oxygen isotope canopy effect has important palaeobiological implications for reconstructing vertical partitioning among sympatric primates and other species in tropical forests.     

Ecological Consequences of Long-Term Exposure of Anabaena variabilis (Cyanophyceae) to Shifts in Environmental Factors

Cultures of Anabaena variabilis were exposed to different light intensities, and the time course of photoadaptation was measured by photosynthetic rate and changes in pigmentation. A shift down in intensity of 284 μEin · m⁻² · sec⁻¹ caused a temporary decrease in the photosynthetic response followed by gradual adaptation to the new conditions. Final chlorophyll a and carotenoid concentrations were reached after 1 day, although other physiological indicators showed that adaptation required 4 days. The parameter I_k was shown to be the best indicator of photoadaptation. A shift up in light intensity of the same magnitude also required 4 days for complete photoadaptation by the culture, although chlorophyll and carotenoid concentrations stabilized within 1 day. A shift down in light intensity of 392 μEin · m⁻² · sec⁻¹ resulted in a temporary attempt to adapt followed by collapse of the population. This demonstrates an apparent threshold in the magnitude of the shift in light intensity which will permit successful adaptation. Simultaneous changes in light intensity and temperature also adversely affected culture populations. Our observations present a possible cause for the decline or prevention of an algal bloom under a fluctuating light regime and suggest that it may be possible to predict this decline as a result of synoptic weather patterns or hydrodynamic influences.     

Decay-Accelerating Factor 1 Deficiency Exacerbates Leptospiral-Induced Murine Chronic Nephritis and Renal Fibrosis

Leptospirosis is a global zoonosis caused by pathogenic Leptospira, which can colonize the proximal renal tubules and persist for long periods in the kidneys of infected hosts. Here, we characterized the infection of C57BL/6J wild-type and Daf1^−/− mice, which have an enhanced host response, with a virulent Leptospira interrogans strain at 14 days post-infection, its persistence in the kidney, and its link to kidney fibrosis at 90 days post-infection. We found that Leptospira interrogans can induce acute moderate nephritis in wild-type mice and is able to persist in some animals, inducing fibrosis in the absence of mortality. In contrast, Daf1^−/− mice showed acute mortality, with a higher bacterial burden. At the chronic stage, Daf1^−/− mice showed greater inflammation and fibrosis than at 14 days post-infection and higher levels at all times than the wild-type counterpart. Compared with uninfected mice, infected wild-type mice showed higher levels of IL-4, IL-10 and IL-13, with similar levels of α-smooth muscle actin, galectin-3, TGF-β1, IL-17, IFN-γ, and lower IL-12 levels at 90 days post-infection. In contrast, fibrosis in Daf1^−/− mice was accompanied by high expression of α-smooth muscle actin, galectin-3, IL-10, IL-13, and IFN-γ, similar levels of TGF-β1, IL-12, and IL-17 and lower IL-4 levels. This study demonstrates the link between Leptospira-induced murine chronic nephritis with renal fibrosis and shows a protective role of Daf1.     

Nitrogen Fixation Associated with the New Zealand Mangrove (Avicennia marina (Forsk.) Vierh. var. resinifera (Forst. f.) Bakh.)

Nitrogenase activity in mangrove forests at two locations in the North Island, New Zealand, was measured by acetylene reduction and ¹⁵N₂ uptake. Nitrogenase activity (C₂H₂ reduction) in surface sediments 0 to 10 mm deep was highly correlated (r = 0.91, n = 17) with the dry weight of decomposing particulate organic matter in the sediment and was independent of light. The activity was not correlated with the dry weight of roots in the top 10 mm of sediment (r = −0.01, n = 13). Seasonal and sample variation in acetylene reduction rates ranged from 0.4 to 50.0 μmol of C₂H₄ m⁻² h⁻¹ under air, and acetylene reduction was depressed in anaerobic atmospheres. Nitrogen fixation rates of decomposing leaves from the surface measured by ¹⁵N₂ uptake ranged from 5.1 to 7.8 nmol of N₂ g (dry weight)⁻¹ h⁻¹, and the mean molar ratio of acetylene reduced to nitrogen fixed was 4.5:1. Anaerobic conditions depressed the nitrogenase activity in decomposing leaves, which was independent of light. Nitrogenase activity was also found to be associated with pneumatophores. This activity was light dependent and was probably attributable to one or more species of Calothrix present as an epiphyte. Rates of activity were generally between 100 and 500 nmol of C₂H₄ pneumatophore⁻¹ h⁻¹ in summer, but values up to 1,500 nmol of C₂H₄ pneumatophore⁻¹ h⁻¹ were obtained.