Comparisons of δ13C values in leaves of aquatic macrophytes from different habitats in Britain and Finland; some implications for photosynthetic processes in aquatic plants

Summary The ¹³C values of submerged aquatic plants from contrasting but relatively defined habitats, and the ¹³C values of emergent, floating and submerged leaves of dimorphic aquatic plants, were measured. In many instances the ¹³C values of dissolved inorganic carbon in the water were also measured. Plant ¹³C values in the vicinity of-40 to-50 were found in rapidly flowing spring waters with carbonate ¹³C values of-16 to-21, consistent with the notion that species such as Fontinalis antipyretica almost exclusively assimilate free CO₂ via RuP₂ carboxylase. Plant ¹³C values in the vicinity of-10 to-15 in sluggish water with carbonate ¹³C values of about-5 were observed, consistent with the notion that boundary layer diffusion and/or HCO₃ ^- uptake may determine the ¹³C value of submerged aquatic plants in these circumstances. Comparisons of ¹³C values of the same or related species growing in waters of similar carbonate ¹³C value but different flow rates confirmed this view; more negative ¹³C values were frequently associated with plants in fast moving water. In Britain, but not in Finland, the ¹³C values of submerged leaves of dimorphic plants were almost invariably more negative than in aerial leaves. The ¹³C value of carbonate from chalk streams and in acid springs indicate substantial inputs of respiratory CO₂, as opposed to atmospheric carbon. The contributions of these variations in ¹³C of the carbon source, and of isotope fractionation in diffusion, to the ¹³C value of submerged parts of dimorphic plants is discussed.     

Stable isotope and radiocarbon compositions of methane emitted from tropical rice paddies and swamps in Southern Thailand

Stable isotopes (¹³C, D) and radiocarbon weremeasured in methane bubbles emitted from rice paddies and swamps in southernThailand. Methane emitted from the Thai rice paddies was enriched in¹³C (mean ¹³C; –51.5 ±7.1 and–56.5 ± 4.6 for mineral soil and peat soil paddies,respectively)relative to the reported mean value of methane from temperate rice paddies(– 63 ± 5). Large seasonal variation was observed in¹³C(32) in the rice paddies, whereas variationinD was much more smaller (20), indicating that variation in¹³C is due mainly to changes in methane production pathways.Values of ¹³C were lower in swamps (–66.1 ±5.1)than in rice paddies. The calculated contribution of acetate fermentation from¹³C value was greater in rice paddies (mineral soils:62–81%, peat soils: 57–73%) than in swamps (27–42%). Din methane from Thai rice paddies (–324± 7 (n=46)) isrelativelyhigher than those from 14 stations in Japanese rice paddies ranging from–362 ± 5 (Mito: n=2) to –322 ± 8(Okinawa: n=3), due tohigher D in floodwaters. ¹⁴C content in methane produced fromThai rice paddies (127±1 pMC) show higher ¹⁴Cactivity compared with previous work in paddy fields and those from Thai swamps(110±2 pMC).     

Carbon isotope composition in relation to leaf gas exchange and environmental conditions in Hawaiian Metrosideros polymorpha populations

Summary Carbon isotope composition, photosynthetic gas exchange, and nitrogen content were measured in leaves of three varieties of Metrosideros polymorpha growing in sites presenting a variety of precipitation, temperature and edaphic regimes. The eight populations studied could be divided into two groups on the basis of their mean foliar ¹³C values, one group consisting of three populations with mean ¹³C values ca.-26 and another group with ¹³C values ca.-28. Less negative ¹³C values appeared to be associated with reduced physiological availability of soil moisture resulting from hypoxic conditions at a poorly drained high elevation bog site and from low precipitation at a welldrained, low elevation leeward site. Gas exchange measurements indicated that foliar ¹³C and intrinsic wateruse efficiency were positively correlated. Maximum photosynthetic rates were nearly constant while maximum stomatal conductance varied substantially in individuals with foliar ¹³C ranging from-29 to-24. In contrast with the patterns of ¹³C observed, leaf nitrogen content appeared to be genetically determined and independent of site characteristics. Photosynthetic nitrogenuse efficiency was nearly constant over the range of ¹³C observed, suggesting that a compromise between intrinsic water- and N-use efficiency did not occur. In one population variations in foliar ¹³C and gas exchange with leaf cohort age, caused the ratio of intercellular to atmospheric partial pressure of CO₂ predicted from gas exchange and that calculated from ¹³C to be in close agreement only in the two youngest cohorts of fully expanded leaves. The results indicated that with suitable precautions concerning measurement protocol, foliar ¹³C and gas exchange measurements were reliable indicators of potential resource use efficiency by M. polymorpha along environmental gradients.     

The Use of the [13C]/[12C] Ratio for the Assay of the Microbial Oxidation of Hydrocarbons

The study deals with a comparative analysis of the relative abundances of the carbon isotopes ¹²C and ¹³C in the metabolites and biomass of the Burkholderia sp. BS3702 and Pseudomonas putida BS202-p strains capable of utilizing aliphatic (n-hexadecane) and aromatic (naphthalene) hydrocarbons as sources of carbon and energy. The isotope compositions of the carbon dioxide, biomass, and exometabolites produced during the growth of Burkholderia sp. BS3702 on n-hexadecane (¹³C = –44.6 ± 0.2) were characterized by the values of ¹³C_{CO 2} = –50.2 ± 0.4, ¹³C_biom = –46.6 ± 0.4, and ¹³C_exo = –41.5 ± 0.4, respectively. The isotope compositions of the carbon dioxide, biomass, and exometabolites produced during the growth of the same bacterial strain on naphthalene (¹³C = –21 ± 0.4) were characterized by the isotope effects ¹³C_{CO 2} = –24.1 ± 0.4, ¹³C_biom = –19.2 ± 0.4, and ¹³C_exo = –19.1 ± 0.4, respectively. The possibility of using the isotope composition of metabolic carbon dioxide for the rapid monitoring of the microbial degradation of petroleum hydrocarbons in the environment is discussed.     

Effect of size on the energy acquired in species of the fish from a neotropical reservoir,Brazil

In this paper we analysed autotrophic sources of the carbon ( ¹³C) and the trophic position ( ¹⁵N) of Leporinus friderici in the influence area of Corumbá Reservoir, Brazil. We collected samples of muscles of fish from different sizes riparian vegetation, C₄ grasses, zooplankton, periphyton and particulate organic carbon (POC). There were significant differences for the carbon isotope proportion found in muscles of L.friderici in the different size groups analysed. The highest values of ¹³C recorded for middle sized individuals is attributed to the large contribution of C₄ plants in their diet. Small individuals sampled upstream also receive similar contribution from C₄ plants. In contrast the same size group sampled downstream from the reservoir, has a much smaller of C₄ plants. The ¹³C negative character of small individuals from downstream is due to the larger contribution of C₃ plants (except periphyton). At larger sizes we found intermediate ¹³C values. The ¹⁵N proportions we found for each size group were not significantly different, however we found decreasing mean values with increasing size. The trophic level calculated from the dietary data was higher than that found with the ¹³C concentration in the muscle, except for small individuals, when the values were equal.     

Feeding level and individual metabolic rate affect δ13C and δ15N values in carp: implications for food web studies

Stable isotope analyses are often used to calculate relative contributions of multiple food sources in an animals diet. One prerequisite for a precise calculation is the determination of the diet-tissue fractionation factor. Isotopic ratios in animals are not only affected by the composition of the diet, but also by the amount of food consumed. Previous findings regarding the latter point are controversial. As stable isotope analyses have often been used to investigate aquatic food webs, an experiment with carp (Cyprinus carpio L.) was carried out to test the influence of the feeding level and individual metabolic rate on ¹³C and ¹⁵N values of the whole body. After an initial phase, 49 carp were assigned randomly to four groups and fed the same diet at different levels for 8 weeks. For 15 fish, the energy budget was determined by indirect calorimetry. Feed and individual fish were analysed for their proximate composition, gross energy content and ¹³C and ¹⁵N values. ¹³C and ¹⁵N values differed significantly at different feeding levels. While ¹³C values of the lipids and ¹⁵N values decreased with increasing feeding rate, ¹³C values of the lipid-free matter showed a non-linear pattern. Data obtained from fish held in the respirometric system revealed a relationship between ¹³C values and the percentage retention of metabolizable energy. Our results show that reconstructing the diets of fish from the isotopic ratios when the feeding level and individual metabolic rates are unknown would introduce an error into the data used for back-calculation of up to 1 for both ¹³C and ¹⁵N values and may have substantial effects on the results of calculated diets. As other workers have pointed out, the development and application of stable isotopes to nutritional ecology studies is a field in its infancy and gives rise to erroneous, misleading results without nutritional, physiological and ecological knowledge.     

Capacity for chlorophyll synthesis in heat-bleached 70S ribosome-deficient rye leaves

The leaves of young rye plants (Secale cereale L.) grown at 32° were deficient in chlorophyll and in chloroplastic rRNA as compared to those grown at 22°, which developed normally. Both chlorophyll accumulation and the formation of plastidic rRNA were largely restored at 32° when the plants were transfered several times for 1 h per day to 22°. In the chlorotic 32°-grown rye leaves the in vivo activity of -aminolevulinate synthetase was very low. Aminolevulinate dehydratase however, exhibited high activity in extracts from 32°-grown leaves and was localized in the plastid fraction isolated from the chlorotic leaf tissue. After application of -aminolevulinic acid to chlorotic parts of leaves growing at 32°, protochlorophyll(ide) was formed and accumulated in the dark. In the light, the protochlorophyll(ide) was photooxidized at 32°. The results suggest a cytoplasmic site of synthesis for the series of enzymes converting -aminolevulinate to protochlorophyll(ide). It is concluded that an inhibition of -aminolevulinate synthetase and the photooxidation of protochlorophyll(ide) or chlorophyll are responsible for the chlorosis of the leaves at 32°.Abbreviations ALA -aminolevulinic acid - ALAD -aminolevulinate dehydratase - ALAS -aminolevulinate synthetase     

Factors influencing the stable carbon and oxygen isotopic composition ofPorites lutea coral skeletons from Phuket,South Thailand

We determined the ¹⁸O and ¹³C composition of the same fixed growth increment in severalPorites lutea coral skeletons from Phuket, South Thailand. Skeletal growth rate and ¹⁸O are inversely related. We explain this in terms of McConnaughey's kinetic isotopic disequilibria model. Annual trends in ¹⁸O cannot be solely explained by observed variations in seawater temperature or salinity and may also reflect seasonal variations in calcification rate. Coral tissue chlorophylla content and ¹³C of the underlying 1 mm of skeleton are positively related, suggesting that algal modification of the dissolved inorganic carbonate pool is the main control on skeletal ¹³C. However, in corals that bleached during a period of exceptionally high seawater temperatures in the summer of 1991, ¹³C of the outer 1 mm of skeleton and skeletal growth rate (over 9 months up to and including the bleaching event) are inversely related. Seasonal variations in °¹³C may reflect variations in calcification rate, zooxanthellae photosynthesis or in seawater ¹³C composition. Bleached corals had reduced calcification over the 9-month period up to and including the bleaching event and over the event they deposited carbonate enriched in¹³C and¹⁸O compared with unaffected corals. However, calcification during the event was limited and insufficient material was deposited to influence significantly the isotopic signature of the larger seasonal profile samples. In profile, overall decreases in ¹⁸O and ¹³C were observed, supporting evidence that positive temperature anomalies caused the bleaching event and reflecting the loss of zooxanthellae photosynthesis.     

Effect of organic and inorganic fertigation on yields, δ15N values, and δ13C values of tomato (Lycopersicon esculentum Mill. cv. Saturn)

We examined the effects of fertilizer application, especially the effects of fertigation and types of fertilizer (inorganic and organic) on yields and ¹⁵N and ¹³C values of tomato (Lycopersicon esculentum Mill. cv. Saturn). Fertigation is a method in which an appropriate diluted liquid fertilizer is applied to the plants each time they are drip-irrigated. We developed a method of organic fertigation using corn steep liquor (CSL) as the liquid fertilizer, because it is an industrial byproduct of cornstarch manufacture and can be used very effectively. We compared fruit yield, mineral content, ¹⁵N value, and ¹³C value of tomatoes grown under three different fertilizer treatments, basal dressing: basal dressing with granular chemical fertilizer; inorganic fertigation: fertigation with liquid chemical fertilizer; and organic fertigation: fertigaion with CSL. Mineral contents of tomatoes grown with basal dressing were generally lower than those grown under either fertigation treatment. These results indicated that yields and mineral contents were influenced more by the method of fertilizer application than by whether the fertilizers were inorganic or organic. There were, however, significant differences in the ¹⁵N values of tomato fruits grown under different types of fertilizer applications, especially between inorganic and organic fertilizers. The ¹⁵N value of the chemical fertilizer used for basal dressing was 0.81 ± 0.45{}, that of the chemical fertilizer for fertigation was 0.00 ± 0.04{}, and that of CSL was 8.50 ± 0.71{}. The ¹⁵N values of the soils reflected the ¹⁵N values of the fertilizers. Moreover, the ¹⁵N values of the fruits corresponded to the ¹⁵N values of the applied fertilizers. The ¹⁵N values were 3.18 ± 1.34{} in the fruits grown with a basal dressing of chemical fertilizer, 0.30 ± 0.61 in those grown under inorganic fertigation, and 7.09 ± 0.68 in those grown under organic fertigation. On the other hand, although the ¹³C values in the soil also reflected the ¹³C values of the applied fertilizers, there was no significant difference in the ¹³C values of fruits among the different treatments. In conclusion, because the ¹⁵N values of fertilizers correlated well with those of the fruits, it may be possible to use ¹⁵N values as an indicator of organic products.     

Oxygen and carbon isotope composition of parasitic plants and their hosts in southwestern Australia

We measured leaf dry matter ¹⁸O and ¹³C in parasitic plants and their hosts growing in southwestern Australia. Parasite/host pairs included two mistletoe species, three species of holoparasites, and five species of root hemiparasites. Among these parasite functional types, significant variation was observed in parasite/host isotopic differences for both ¹⁸O (P<0.0001, n=65) and ¹³C (P<0.0001, n=64). Mistletoes were depleted in both ¹⁸O and ¹³C compared to their hosts; parasite/host differences were –4.0 for ¹⁸O (P<0.0001) and –1.9 for ¹³C (P<0.0001). The lower ¹⁸O in mistletoe leaf dry matter compared to their hosts is consistent with the frequently observed high transpiration rates of these parasites. Root hemiparasites were also depleted in ¹⁸O and ¹³C compared to their hosts, but not to the same extent as mistletoes; parasite/host differences were –1.0 for ¹⁸O (P=0.04) and –1.2 for ¹³C (P=0.0006). In contrast to mistletoes and root hemiparasites, holoparasites were enriched in both ¹⁸O and ¹³C compared to their hosts; parasite/host differences were +3.0 for ¹⁸O (P<0.0001) and +1.5 for ¹³C (P=0.02). The enrichment in ¹⁸O for holoparasite dry matter did not result from more enriched tissue water; holoparasite tissue water ¹⁸O was less than host leaf water ¹⁸O by a difference of –3.8 when sampled at midday (P=0.0003). Enrichment of holoparasites in ¹³C compared to their hosts is consistent with a generally observed pattern of enrichment in heterotrophic plant tissues. Results provide insights into the ecology of parasitic plants in southwestern Australia; additionally, they provide a context for the formulation of specific hypotheses aimed at elucidating mechanisms underlying isotopic variations among plants.     

The skeletal isotopic composition as an indicator of ecological and physiological plasticity in the coral genus<Emphasis Type="Italic"> Madracis</Emphasis>

Three species of the reef coral genus Madracis display skeletal isotopic characteristics that relate to depth, colony topography, and consequently to coral physiology. The joint interpretation of skeletal ¹³C and ¹⁸O provides information on the ecological plasticity and adaptation to depth of a coral species. Isotopic results are most easily understood in terms of kinetic effects, which reduce both ¹⁸O and ¹³C below isotopic equilibrium values, and metabolic effects, which only influence the skeletal ¹³C. Madracis mirabilis is adapted to depths shallower than 20 m, and shows the greatest range in kinetic effects and the strongest metabolic ¹³C enrichments caused by symbiont photosynthesis. Madracis formosa lives deeper than 40 m, and shows a reduced range of kinetic effects and relatively weak metabolic ¹³C enrichments. Madracis pharensis inhabits depths from 5 to >60 m, and does not attain the strength of kinetic effects of either of the other two species, apparently because it is not quite as well adapted to rapid growth at either extreme.     

Leaf thickness and carbon isotope composition in the Crassulaceae

Summary Measurements of leaf thickness and ¹³C value were obtained for twenty species and three intergeneric hybrids of the Crassulaceae. The data include plants growing in their native habitats and also in greenhouse cultivation. There is a strong relationship between leaf thickness and leaf ¹³C values. The plants with the thickest leaves of ca. 7 to 11 mm had ¹³C values ranging from -11.5 to -13.8. Plants with leaves that were thinner than 2.0 mm all had ¹³C values that were more negative than -23. Plants having intermediate leaf thickness possessed intermediate ¹³C values. The leaf tissue of four genotypes spanning the range of leaf thicknesses all exhibited a two-fold or greater nocturnal increase in titratable acidity. It appears that the differences in leaf thickness and ¹³C values among the tested species are genetically determined.     

Carbon and nitrogen stable isotope ratios of deposit-feeding polychaetes in the Nanakita River Estuary, Japan

Two types of deposit-feeding polychaetes, Neanthes japonica and Notomastus sp., and their surrounding sediments were collected from the Nanakita River Estuary and a small brackish lagoon (Gamõ Lagoon) in northeastern Japan. The samples were examined using stable isotope analysis to assess the site specific feeding mode of the animals and their trophic status. N. japonica is a surface deposit-feeder and Notomastus sp. is a subsurface deposit-feeder. In the estuary, the sedimentary ⁵N tended to become isotopically heavier from the upper estuary (2.0 3.9) to the river mouth (4.3 6.2), while sedimentary organic ¹³C constant value (–26.8 –24.4, average –25.6) throughout the river estuary. The ¹³C values of N. japonica were similar to those of the surrounding sediment in the upper estuary, whereas in the lower estuary, N. japonica had a heavier ¹³C value than the surrounding sediment. The ¹³C and dg ¹⁵N values indicated that the carbon, but not the nitrogen, of N. japonica was derived from upland plants in the upper estuary. In the lower estuary, a significant fraction of carbon of N. japonica was derived from phytoplankton. Notomastus sp. exhibited heavier ¹³C values than the surrounding surface sediment throughout the estuary and had heavier ¹³C values than N. japonica in the same location. These results suggest selective utilization of sedimentary carbon by those animals following bacterial processing and subsequent fractionation. The difference in ¹⁵N between sedimentary organics and corresponding polychaetes was 5 ± 1 and rather higher than 3.4 ± 1.1 expected for normal trophic effects in other animals.     

Origin and trophic importance of detritus-evidence from stable isotopes in the benthos of a small,temperate estuary

Ratios of ¹³C/¹²C and ¹⁵N/¹⁴N were measured in dissolved inorganic carbon (DIC), marginal vegetation, benthic macrodetritus (diameter > 1 mm) and selected invertebrate consumers in the Gamtoos estuary, South Africa to: (1) trace the provenance of benthic detrital deposits, and (2) determine the extent to which three abundant species of macroinvertebrates utilise this resource. DIC was strongly depleted in ¹³C with average ¹³C values (–9.5±0.5) being typical of limnetic waters. Benthic detrital particles (¹³C–24.1±0.3) originated mainly from marginal vegetation (¹³C–25.7±0.3), but their slightly elevated carbon ratio suggests additional input from ¹³C-rich sources-possibly C₄ plants cultivated on the floodplain. Populations of the fossorial ghost shrimp Callianassa kraussi, the bentho-pelagic amphipod Grandidierella lignorum and the epifaunal crab Hymenosoma robiculare together account for 96% of total benthic biomass in the upper regions of this estuary. Marked differences in trophic niches were evident among these three consumer species. Ghost shrimp (¹³C –32.5±0.3) foraged by filter-feeding on fine suspended particulate organic matter (¹³C–31.2±0.5). Amphipods (¹³C–28.0±0.6) utilised some benthic detritus but fed mainly on suspended material. Only the relatively rare crabs (¹³C–23.8±1.5) appeared to utilise benthic detrital particles to any significant extent. In the benthic consumer community of the upper Gamtoos estuary, suspension feeders make up 98% of biomass and thus clearly dominate over deposit feeders. This can be traced to the low contribution of higher plants (c. 13%) to overall carbon production, and detritus originating from macrophytes is consequently relatively unimportant in supporting invertebrate secondary production in this particular system.     

Foliar δ13C within a temperate deciduous forest: spatial,temporal, and species sources of variation

Summary Foliar ¹³C-abundance (¹³C) was analyzed in the dominant trees of a temperate deciduous forest in east Tennessee (Walker Branch Watershed) to investigate the variation in foliar ¹³C as a function of time (within-year and between years), space (canopy height, watershed topography and habitat) and species (deciduous and coniferous taxa). Various hypotheses were tested by analyzing (i) samples collected from the field during the growing season and (ii) foliar tissues maintained in an archived collection. The ¹³C-value for leaves from the tops of trees was 2 to 3%. more positive than for leaves sampled at lower heights in the canopy. Quercus prinus leaves sampled just prior to autumn leaf fall had significantly more negative ¹³C-values than those sampled during midsummer. On the more xeric ridges, needles of Pinus spp. had more positive ¹³C-values than leaves from deciduous species. Foliar ¹³C-values differed significantly as a function of topography. Deciduous leaves from xeric sites (ridges and slopes) had more positive ¹³C-values than those from mesic (riparian and cove) environments. On the more xeric sites, foliar ¹³C was significantly more positive in 1988 (a dry year) relative to that in 1989 (a year with above-normal precipitation). In contrast, leaf ¹³C in trees from mesic valley bottoms did not differ significantly among years with disparate precipitation. Patterns in foliar ¹³C indicated a higher ratio of net CO₂ assimilation to transpiration (A/E) for trees in more xeric versus mesic habitats, and for trees in xeric habitats during years of drought versus years of normal precipitation. However, A/E (units of mmol CO₂ fixed/mol H₂O transpired) calculated on the basis of ¹³C-values for leaves from the more xeric sites was higher in a wet year (6.6±1.2) versus a dry year (3.4±0.4). This difference was attributed to higher transpiration (and therefore lower A/E) in the year with lower relative humidity and higher average daily temperature. The calculated A/E values for the forest in 1988–89, based on ¹³C, were within ±55% of estimates made over a 17 day period at this site in 1984 using micrometeorological methods.     

Stable isotope ecology in the Ituri Forest

The Ituri Forest, Democratic Republic of Congo (formerly Zaire) is an example of a closed canopy forest showing extreme depletion in ¹³C. ¹³C values for plants from the canopy top, from gaps in the canopy, and from the subcanopy average –29.0±1.7, –30.4±0.9, and –34.0±1.5, respectively. The ¹³C of forest mammals show these differences, with the subcanopy browsers (okapi, dwarf antelope) having ¹³C values for tooth enamel much more negative than subcanopy frugivores who derive their food from the canopy top, and from folivores and omnivores living in gap or clearing areas. Nitrogen isotopes in plants from this ecosystem have an average ¹⁵N value of 5.4±1.8 and do not show significant differences at the 95% confidence interval between plants from the canopy top, from gaps in the canopy, and from the subcanopy. The ¹⁸O_SMOW values of surface waters in the study area are between –2.0 and –2.7. The ¹⁸O_PDB for tooth enamel ranged from –3 to +7.     

Keeling plots for hummingbirds: a method to estimate carbon isotope ratios of respired CO<Subscript>2</Subscript> in small vertebrates

The carbon isotope composition of an animals breath reveals the composition of the nutrients that it catabolizes for energy. Here we describe the use of Keeling plots, a method widely applied in ecosystem ecology, to measure the ¹³C of respired CO₂ of small vertebrates. We measured the ¹³C of Rufous Hummingbirds (Selasphorus rufus) in the laboratory and of Mourning (Zenaida macroura) and White-winged (Z. asiatica) Doves in the field. In the laboratory, when hummingbirds were fed a sucrose based C3 diet, the ¹³C of respired CO₂ was not significantly different from that of their diet (¹³C_{C3 diet}). The ¹³C of respired CO₂ for C3 fasted birds was slightly, albeit significantly, depleted in ¹³C relative to ¹³C_{C3 diet}. Six hours after birds were shifted to a sucrose based C4 diet, the isotopic composition of their breath revealed that birds were catabolizing a mixture of nutrients derived from both the C3 and the C4 diet. In the field, the ¹³C of respired CO₂ from Mourning and White-winged Doves reflected that of their diets: the CAM saguaro cactus (Carnegeia gigantea) and C3 seeds, respectively. Keeling plots are an easy, effective and inexpensive method to measure ¹³C of respired CO₂ in the lab and the field.     

Crassulacean acid metabolism in Kalanchoë species collected in various climatic zones of Madagascar: a survey by δ13C analysis

Summary The carbon isotope compositions of samples of Kalanchoë species collected at the natural stands in Madagascar were determined. The results suggest that all species of the genus Kalanchoë are capable of crassulacean acid metabolism. The observed ¹³C values cover the whole range from –10 to –30. This high diversity of the ¹³C values was found among the species of the genus as well as, in certain cases, within a single species. This suggest that the CAM patterns in Kalanchoë are generally very flexible. The ¹³C values show a clear correlation with the climate of the habitats from where the samples derived. Values indicative of CO₂ fixation taking place exclusively during the night were found in the dry regions of Madagascar, whereas ¹³C values indicative of mixed CO₂ fixation during night and day or of CO₂ fixation entirely during the day are distributed in the humid zones.     

Autotrophic carbon sources for heterotrophic bacterioplankton in a floodplain lake of central Amazon

The relative contribution of autotrophic carbon sources (aquatic macrophytes, flooded forest, phytoplankton) for heterotrophic bacterioplankton was evaluated in a floodplain lake of the Central Amazon. Stable carbon isotopes (¹³C) were used as tracers. Values of ¹³C of different autotrophic sources were compared to those of dissolved organic carbon (DOC) and those of bacterially produced CO₂.The percentage of carbon derived from C₄ macrophytes for bacterially produced CO₂ was the highest, on average 89%. The average ¹³C value of CO₂ from bacterial respiration was –18.5 ± 3.3. Considering a fractionation of CO₂ of 3 by bacterial respiration, ¹³C value was –15.5, near C₄ macrophyte ¹³C value (–13.1).The average value of total DOC ¹³C was –26.8 ± 2.4. The percentage of C₄ macrophytes carbon for total DOC was on average 17%. Considering that bacteria consume mainly carbon from macrophytes, the dominance of C₃ plants for total DOC probably reflects a faster consumption of the former source, rather than a major contribution of the latter source.Heterotrophic bacterioplankton in the floodplain may be an important link in the aquatic food web, transferring the carbon from C₄ macrophytes to the consumers.     

Polar bears make little use of terrestrial food webs: evidence from stable-carbon isotope analysis

Summary The mean stable-carbon isotope ratios (¹³C) for polar bear (Ursus maritimus) tissues (bone collagen –15.7, muscle –17.7, fat –24.7) were close to those of the same tissues from ringed seals (Phoca hispida) (–16.2, –18.1, and –26.1, respectively), which feed exclusively from the marine food chain. The ¹³C values for 4 species of fruits to which polar bears have access when on land in summer ranged from –27.8 to –26.2, typical of terrestrial plants in the Arctic. An animal's ¹³C signature reflects closely the ¹³C signature of it's food. Accordingly, the amount of food that polar bears consume from terrestrial food webs appears negligible, even though some bears spend 1/3 or more of each year on land during the seasons of greatest primary productivity.