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1.
Externally shelled cephalopods were important elements in open marine habitats throughout Earth history. Paleotemperatures calculated on the basis of the oxygen isotope composition of their shells can provide insights into ancient marine systems as well as the ecology of this important group of organisms. In some sedimentary deposits, however, the aragonitic shell of the ammonite or nautilid is poorly or not preserved at all, while the calcitic structures belonging to the jaws are present. This study tests for the first time if the calcitic jaw structures in fossil cephalopods can be used as a proxy for paleotemperature. We first analyzed the calcitic structures on the jaws of Recent Nautilus and compared the calculated temperatures of precipitation with those from the aragonitic shell in the same individuals. Our results indicate that the jaws of Recent Nautilus are secreted in isotopic equilibrium, and the calculated temperatures approximately match those of the shell. We then extended our study to ammonites from the Upper Cretaceous (Campanian) Pierre Shale of the U.S. Western Interior and the age-equivalent Mooreville Chalk of the Gulf Coastal Plain. In the Pierre Shale, jaws occur in situ inside the body chambers of well-preserved Baculites while in the Mooreville Chalk, the jaw elements appear as isolated occurrences in the sediment and the aragonitic shell material is not preserved. For the Pierre Shale specimens, the calculated temperatures of well-preserved jaw material match those of well-preserved shell material in the same individual. Analyses of the jaw elements in the Mooreville Chalk permit a comparison of the paleotemperatures between the two sites, and show that the Western Interior is warmer than the Gulf Coast at that time. In summary, our data indicate that the calcitic jaw elements of cephalopods can provide a reliable geochemical archive of the habitat of fossil forms.  相似文献   

2.
A new strain of Manila clam with orange shell color was produced after selection within a full-sib family for two generations. In the present study, the shell length, height, and width, and the live body weight of the orange strain were measured, and their correlation coefficients were calculated. The shell morphological traits were used as independent variables, and the live body weight was used as the dependent variable for calculating the path coefficients, correlation index, and determination coefficients. The results showed that the correlation coefficients between each shell morphological trait and the live body weight were all highly significant (P < 0.01). The correlation indices (R2) of morphological traits against the live body weight of clams were larger than 0.85, indicating that the morphology traits were the main factors affecting the body weight. Multiple regression equations were obtained to estimate shell length X1 (cm), shell height X2 (cm), and shell width X3 (cm) against live body weight Y (g): Y = ? 2.62 + 0.34 X1 + 0.145 X2, (X1 < 0.05, X2 < 0.05). The results suggest that the shell length could be used as the main trait for selective breeding and could indirectly make a large improvement in the weight trait.  相似文献   

3.
This study is a preliminary quantitative analysis of Paleocene calcareous nannofossil assemblages of the Tenida area (Egypt) in order to establish a detailed biostratigraphic framework as well as to reconstruct the paleoclimatic trends. A total of 48 samples with an average sample spacing of 1.5 m allowed the identification of 63 calcareous nannofossil species belonging to 19 different genera. The preservation of the studied samples varies from poor to moderate and is characterized by the frequent presence of small frangible placoliths, and nannoliths. This study recognizes three calcareous nannofossil biozones in the Danian-Thanetian time interval; Chiasmolithus danicus (NP3) Zone, Ellipsolithus macellus (NP4) Zone, and Heliolithus kleinpellii (NP6) Zone. Moreover, the multivariate statistical analysis of the calcareous nannofossil communities reveals a relationship between the distribution of these nannofossil assemblages and variations in paleoclimatic trends. Accordingly, the relative abundances of Coccolithus pelagicus in addition to nine calcareous nannofossil genera along with the diversity and preservation indices of calcareous nannofossil elements have been used to elucidate changes in paleoclimatic trends. Based on the cyclic change from cold to warm climates, it was possible to subdivide the Paleocene Period recorded in the Tenida section into four paleoclimatic intervals. The oldest is a global cooling trend spanning 2.01 Myr long, starting in the early Paleocene (Danian) during the deposition of the lower part of the Kharga Shale Member. This cooling trend is followed by a ~ 0.56 Myr warming trend during deposition of the middle part of the Kharga Shale Member that was followed by a return to a cooling mode, with an estimated duration of roughly 1.67 Myr. The last interval includes a 0.39 Myr long period at the Selandian/Thanetian boundary interval, which is dominated by a global warming trend during deposition of the upper part of the Upper Kharga Shale Member.  相似文献   

4.
Shells of calcifying foraminifera play a major role in marine biogeochemical cycles; fossil shells form important archives for paleoenvironment reconstruction. Despite their importance in many Earth science disciplines, there is still little consensus on foraminiferal shell mineralization. Geochemical, biochemical, and physiological studies showed that foraminiferal shell formation might take place through various and diverse mineralization mechanisms.In this study, we contribute to benthic foraminiferal shell calcification through deciphering crystallite organization within the shells. We base our conclusions on results gained from electron backscattered diffraction (EBSD) measurements and describe microstructure/texture characteristics within the laminated shell walls of the benthic, symbiontic foraminifera: Ammonia tepida, Amphistegina lobifera, Amphistegina lessonii. We highlight crystallite assembly patterns obtained on differently oriented cuts and discuss crystallite sizes, morphologies, interlinkages, orientations, and co-orientation strengths.We show that: (i) crystals within benthic foraminiferal shells are mesocrystals, (ii) have dendritic-fractal morphologies and (iii) interdigitate strongly. Based on crystal size, we (iv) differentiate between the two layers that comprise the shells and demonstrate that (v) crystals in the septa have different assemblies relative to those in the shell walls. We highlight that (vi) at junctions of different shell elements the axis of crystal orientation jumps abruptly such that their assembly in EBSD maps has a bimodal distribution. We prove (vii) extensive twin-formation within foraminiferal calcite; we demonstrate (viii) the presence of two twin modes: 60°/[0 0 1] and 77°/~[6 –6 1] and visualize their distributions within the shells.In a broader perspective, we draw conclusions on processes that lead to the observed microstructure/texture patterns.  相似文献   

5.
A modification of the classical calcium alginate enzyme entrapment technique is described aiming to overcome some of the limitations of the former gel-based biocatalysts. Dried alginate entrapped enzymes (DALGEEs) were obtained dehydrating calcium alginate gel beads containing entrapped enzymes. A fructosyltransferase from Aspergillus aculeatus, present in Pectinex Ultra SP-L, was entrapped using this technique. The resulting DALGEEs were successfully tested both operating batchwise and in a continuous fixed-bed reactor for fructooligosaccharides (FOS) synthesis from sucrose. Interestingly, DALGEEs did not re-swell upon incubation in concentrated (600 g/L) sucrose solutions, probably due to the lowered water activity (aw) of such media. Confocal laser scanning microscopy of DALGEEs revealed that the enzyme molecules accumulated preferably in the shell of the particles. DALGEEs showed an approximately 30-fold higher volumetric activity (300 U/mL) compared with the calcium alginate gel beads. Moreover, a significant enhancement (40-fold) of the space-time-yield of fixed-bed bioreactors was observed when using DALGEEs as biocatalyst compared with gel beads (4030 g/day L of FOS vs. 103 g/day L). The operational stability of fixed-bed reactors packed with DALGEEs was extraordinary, providing a nearly constant FOS composition of the outlet during at least 700 h. It was also noticeable their resistance against microbial attack, even after long periods of storage at room temperature. The DALGEEs immobilisation strategy may also be useful for other biotransformations, in particular when they take place in low aw media.  相似文献   

6.
《Process Biochemistry》2007,42(4):681-685
The potential application of dry biomass of a cyanobacterium Anacystis nidulans as a supplement in SSF for the production of laccase from Pleurotus ostreatus was evaluated. Experiments were carried out in solid culture using groundnut shell as a basic substrate supplemented with four independent nitrogen sources (ammonium sulphate, urea, yeast extract and dry powder of cyanobacteria). All the four supplements enhanced the enzyme yield, and yeast extract showed precedence over inorganic nitrogenous sources. However, when dry biomass of A. nidulans was used as an additive to groundnut shell (agricultural residues), it supported maximum cell growth (56.83 ± 5.56 mg/g dry substrate) and laccase production (49.21 ± 4.89 U/g dry substrate). Addition of 1 mM copper salt in the medium containing groundnut shell supplemented with yeast extract gave laccase activity of 32.64 ± 3.4 U/g dry substrate. When dry powder of cyanobacterial biomass was used as N-supplement, laccase production enhanced to 65.42 ± 6.48 U/g dry substrate. In addition to the enhancement to enzyme production inhibitory effects of high concentrations of copper was also diminished in the medium having dry cyanobacterial biomass. This study, forms the first report on the potential application of cyanobacterial biomass as an additive for production of laccase by Pleurotus ostraetus MTCC 1804 in solid state fermentation and has relevance in scale-up production of this fungal enzyme of commercial significance.  相似文献   

7.
《Process Biochemistry》2007,42(7):1069-1074
Crab shell (CS) waste samples (particle size 3–10 and 20–35 mm) were inoculated with the newly isolated Pseudomonas aeruginosa F722 to study the efficiency of microbial demineralization (DM) and deproteinization (DP) in the process of extracting chitin. The inoculated waste was incubated for 7 days at 25, 30 and 35 °C. Various concentrations of glucose were supplemented as carbon source. At the optimal temperature of 30 °C, DM was 92% and DP was 63% DP, whereas the pH dropped from initial pH 8.0 to 4.1. In comparative experiments with different amounts of CS waste, 5% CS waste treatment was shown to be the optimal amount for efficient DM. A positive relationship is correlated between DM and glucose concentration (r2 = 0.821), whereas a negative relationship is correlated between DM and pH (r2 = 0.793). DP and protease activity were little affected by different crab shell sizes.  相似文献   

8.
Cedrela odorata and C. fissilis are two tropical tree species that have been widely harvested for their timber. In response to this heavy exploitation, the species have been listed in Appendix III of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). The aim of this study was to provide important elements necessary for the making of CITES Non-Detriment Findings for Cedrela spp. in Bolivia using a wide variety of sources of information on its distribution, population structure, and management at multiple spatial scales. A national large-scale database of forest inventories was created, including information about trees of certain species with diameter at breast height (dbh) ≥20. These data were used to make non-detriment findings (NDFs) following CITES guidance for timber species. Spatial prediction of Cedrela habitat revealed a consistent pattern of habitat probability across Bolivia. The genus occurs in areas formerly or currently occupied by ten of the twelve forest types described as habitat for Cedrela odorata and C. fissilis, with a density ranging from 0.4 to 159 trees > 60 cm dbh per 100 ha. Based on these data, the annual export quota for Cedrela in Bolivia should be 3513.1 m3 of timber. This country-level case study could provide a roadmap for other studies that may eventually lead to uplisting the genus. Including Cedrela in CITES Appendix II may help to ensure that its harvest to supply international markets is conducted in a sustainable manner, without damaging the target species or their ecosystem.  相似文献   

9.
Background: Genetic predispositions to disease have focused on highly penetrant causative changes in tumor suppressor genes or genes associated with DNA mismatch repair. New investigations are revealing new genetic associations with disease that are more subtle in their association with disease and require characterization. Methods: In this report we have examined the tumor characteristics in a group of patients who have been shown to harbor two polymorphisms in two genes that are associated with the immune system NOD2 and TNFα. Results: Colorectal cancers from patients with NOD2 3020insC and TNFα-1031T/T constitutional changes are mostly right-sided disease (OR = 2.21, p = 0.03) with a tendency to higher stages (OR = 2.41, p = 0.06), increased number of associated polyps (OR = 1.77, p = 0.16) and later age of average age of disease onset (p = 0.039). Conclusion: The results reveal that there appear to be specific characteristics associated with the tumors that may aid in determining management strategies to reduce the risk of disease.  相似文献   

10.
Myostatin (MSTN) is a member of the transforming growth factor-β superfamily which negatively regulates growth of muscle tissue. In this study, 103 cultivated Chlamys farreri individuals were screened for polymorphisms in the MSTN gene using PCR-single strand conformation polymorphism (PCR-SSCP) and DNA sequencing methods. Two mutations were found: A/G at position 327 in exon 2, which caused an amino acid change from Thr to Ala (Thr305Ala), and C/T at position 289 in exon 3, which caused an amino acid change from Cys to Arg (Cys422Arg). One way ANOVA of the SNPs and growth traits showed that genotype GG of primer M5 had significantly higher body mass, soft-tissue mass, adductor muscle mass, shell length, shell height, absolute growth rate of shell height and body mass than those of genotype AG and AA (P < 0.05). Genotype frequencies of genotype AA, AG and GG were 68.94%, 27.18% and 3.88%, respectively. The results present evidence that the C. farreri MSTN gene may be selected as a candidate gene for these growth traits.  相似文献   

11.
The influence of global climate change will potentially alter the salinity of aquatic ecosystems. This represents a tremendous challenge for societies worldwide. Different sources of salinization (natural or anthropogenic) amplify the introduction of salt in rivers and streams, causing an increase of salt flowing down to estuarine and coastal areas. In this study, Corbicula fluminea and Scrobicularia plana have been selected because of their large tolerance for salinity variation (euryhaline organisms). They will allow the study of effect on the whole spectrum of salinity from fresh to marine waters respectively. The aim was to study the impact of experimental salinity stress at physiological, biochemical and behavioral levels by exposing both species to a salinity close to their limit range of tolerance, 15 practical salinity unit (psu), and at their field salinity (1.5 psu and 30 psu for C. fluminea and S. plana respectively) in the presence or absence of food during 2 and 7 days of exposure. Negative impacts of hyper saline condition for C. fluminea (15 psu) and hypo saline condition for S. plana (15 psu) have been measured at biochemical, physiological and behavioral levels. At sub-individual and individual levels, structural and energetic parameters and behavioral impairments seemed to be suitable biomarkers to assess salinity stress on C. fluminea and S.plana. After exposure to the limit of salinity tolerance (15psu) for both organisms, fitness modifications could appear, and may participate in endangering populations.  相似文献   

12.
Medicinal leeches (Hirudo verbana) thermoregulate with respect to their sanguivorous feeding behavior. Immediate postprandial preferences are for warmer than their initial acclimation temperature (Ta, 21 °C, Petersen et al. 2011), while unfed leeches have a lower preferred temperature (Tpref, 12.5 °C). This may reduce energy expenditure and defer starvation if feeding opportunities are limited. Energetic benefits may have an associated cost if low temperatures reduce mobility and the ability to locate further hosts. These costs could be limited if mobility is unimpaired at low temperatures, or if acclimation can restore locomotor performance to the levels at Ta. The transition from Ta to the unfed Tpref significantly reduced speed and propulsive cycle frequency during swimming, and extension and retraction rates during crawling. Aerobic metabolic rate was also reduced from 0.20±0.03 W kg−1 at Ta to 0.10±0.03 W kg−1 at Tpref. The Q10 values of 1.7–2.9 for energetic and swimming parameters indicate a substantial temperature effect, although part of the decline in swimming performance can be attributed to temperature-related changes in water viscosity. 6 weeks at Ta resulted in no detectable acclimation in locomotor performance or aerobic metabolism. The energetic savings associated with a lower Tpref in unfed leeches effectively doubled the estimated time until depletion of energy reserves. Given that some mobility is still retained at Tpref, and that acclimation is in itself costly, the energetic benefits of selecting cooler temperatures between feedings may outweigh the costs associated with reduced locomotor performance.  相似文献   

13.
《Process Biochemistry》2014,49(5):745-750
The present study was designed to investigate the effect of bromopropylate on its own transport rate, glycolysis and tricarboxylic acid cycle metabolite levels, adenine nucleotides, and membrane lipid peroxidation (LPO) as well as the activities of mitochondrial electron transport chain (ETC) enzymes in eukaryotic Trichoderma harzianum. The transport rate of bromopropylate reached a maximum level within the first 24 h of incubation for all studied concentrations. The succinate dehydrogenase (SDH) and cytochrome c oxidase (CCO) activities reached their maxima at 72 h for 2.5 and 10 mg/L of bromopropylate, respectively. In addition, the intracellular pyruvate levels increased for bromopropylate concentrations up to 2.5 mg/L. The maximum intracellular α-ketoglutarate level was determined at 5 mg/L, while the intracellular fumarate and citrate levels reached their maximums at 7.5 mg/L of bromopropylate. The variations in the adenine nucleotide levels showed a positive correlation with both α-ketoglutarate and fumarate levels. Nevertheless, the LPO levels increased with increasing bromopropylate concentrations. These results may indicate that the membrane becomes more damaged from an impaired respiratory chain, which may then cause an increase in electron leakage.  相似文献   

14.
The degradation of wood by brown rot fungi has been studied intensely for many years in order to facilitate the preservation of in-service wood. In this work we used X-ray diffraction to examine changes in wood cellulose crystallinity caused by the brown rot fungi Gloeophyllum trabeum, Coniophora puteana, and two isolates of Serpula lacrymans. All fungi increased apparent percent crystallinity early in the decay process while decreasing total amounts of both crystalline and amorphous material. Data also showed an apparent decrease of approximately 0.05 Å in the average spacing of the crystal planes in all degraded samples after roughly 20% weight loss, as well as a decrease in the average observed relative peak width at 2θ = 22.2°. These results may indicate a disruption of the outer most semi-crystalline cellulose chains comprising the wood microfibril. X-ray diffraction analysis of wood subjected to biological attack by fungi may provide insight into degradative processes and wood cellulose structure.  相似文献   

15.
At lambing, ewes (Ovis aries) tend to isolate themselves from the flock and this is believed to be beneficial to the early relationship with their young. Later, they return to the flock and lambs progressively interact with other conspecifics. This study focused on the importance of isolation and socio-spatial factors on maternal recognition in lambs. All the ewes gave birth in individual 2 m2 pens and lambs were able to interact freely with their mothers. A first experiment investigated the role of isolation: INT1 lambs could progressively interact with other mothers and young in a pen which was increased in surface area from birth to 24 h (0–6 h: 1 ewe, 1 litter, 2 m2; 6–12 h: 2 ewes, 2 litters, 4 m2; 12–24 h: 4 ewes, 4 litters, 8 m2); ISO lambs remained in small pens (2 m2) and only with their mothers for 24 h. The lambs were submitted to a 5 min choice test between their own mothers and an alien ewe. At 12 h INT1 lambs, unlike ISO lambs, showed a clear preference for their mothers. ISO lambs still did not show any maternal preference at 24 h. The second experiment investigated the relative importance of social interactions while keeping the same space allowance for the two groups: INT2 lambs as described for INT1 in Experiment 1, and SPA lambs that remained isolated from the flock with their mothers but had the same increase in pen surface area as INT2 lambs. Both INT2 and SPA lambs displayed a preference for their own mothers at 12 h but INT2 lambs did so more rapidly. Our data also suggest that INT1 and INT2 lambs chose their own mothers at a distance. These results demonstrate that progressively increasing social interactions with flock members other than the mother facilitates the establishment of a preference for the mother in lambs. Increasing space allowance only also seems to have facilitating effects. This suggests that a rich socio-spatial environment from birth has beneficial effects on the development of filial bonding.  相似文献   

16.
Mitogen-activated protein kinases (MAPKs) are a group of cytoplasmic phosphoproteins that constitute the central core of the signalling network to respond to stress in most organisms. Their role in stress responses has been extensively studied in organisms from yeast to humans, and recently, their presence has also been described in higher plants as well as in micro- and macroalgae. In this study, we demonstrate via short experiments (1 h in duration), the rapid activation of two MAPKs similar to p38 and JNK of mammalian cells, in the Arctic kelps Laminaria solidungula and Saccharina latissima exposed to temperature and UV stress. The molecular mass of p38 is 40 kDa in L. solidungula and 42 kDa in S. latissima, while two JNKs were detected in both species, of 36 and 42 kDa in L. solidungula, and 36 and 40 kDa in S. latissima. These MAPKs are highly phosphorylated in response to temperature and UV light. In S. latissima, both p38 and the JNK showed higher phosphorylation at 2 °C than at 7 °C, while the reverse response was shown for L. solidungula. In addition, a significant increase in phosphorylation of both kinases was found following exposure to UV radiation (UVR). Exposure to PAR + UVA + UVB induced higher phosphorylation than PAR + UVA in L. solidungula, especially at 7 °C. In S. latissima, this response occurred only with JNK, and no differences in p38 phosphorylation between PAR + UVA and PAR + UVA + UVB at any temperature were observed. These results indicate the possible participation of MAPK-like proteins in response to stress in Arctic kelps, and that their activation is species-specific.  相似文献   

17.
Pereskia aculeata and Pereskia grandifolia have been studied widely due to their high nutritional and therapeutic values. However, little is known about the biological requirements of their seeds for the various germination factors. Thus, this experiment aimed to evaluate the thermal effects on the germination of these species at the temperatures of 24 °C, 27 °C, 30 °C, 33 °C and 36 °C. After verification of the existence of differences in the performance of germination, a non-linear regression was carried out, relating the germination to temperature and identifying its point of maximum efficiency. We found that the lowest synchronization indexes of germination were observed close to 30 °C. The best germination response of the P. aculeata and P. grandifolia was observed at 30 °C and 33 °C, respectively, with greater germination strength and fewer days to attain 63.21% of germinations. The results obtained from the germination of P. aculeata and P. grandifolia can be described by the Weindull distribution model with three parameters, as proposed by Carneiro and Guedes (1992).  相似文献   

18.
Adaptation to temperature changes is vital to reduce adverse effects on individuals, and some may present phenotypic changes, which might be accompanied with physiological costs in fitness traits. The objective of this study was to determine whether the two strains of the herbivore Eccritotarsus catarinensis, a biological control agent against water hyacinth in South Africa, differ in their responses to temperature according to their geographical origin.We experimentally quantified the responses of the two strains, at three constant temperatures: 20 °C, 25 °C and 30 °C, using laboratory cultures that originated from Brazil and Peru, where climates differ. Reproductive output, egg hatching rate, sex ratio and longevity were recorded at each temperature. Fitness traits for both strains were significantly reduced at 30 °C compared with 25 °C and 20 °C in two successive generations. Nonetheless, Peruvian individuals continued their development at 30 °C, whereas Brazilian individuals that succeeded in emerging did not continue their development. In contrast, sex ratio was unaffected by temperature. The Peruvian strain of E. catarinensis presented different phenotypes depending on temperature and was more adapted to extreme high temperature than the Brazilian strain. The tropical origin of the population induces the insect to tolerate the extreme high temperature. We suggest that the Peruvian strain could be better suited for release to control water hyacinth in nature, particularly in regions where temperature is high.  相似文献   

19.
Maternal nutrient intake during gestation can alter fetal growth. Whilst this has been studied extensively in the sheep, less is known about effects in the bovine. Composite-breed beef heifers were allocated to either a high (H/? = 76 MJ metabolisable energy (ME) and 1.4 kg crude protein (CP)) or low (L/? = 62 MJ ME and 0.4 kg CP daily) nutritional treatment at artificial insemination. Half of each nutritional group changed to an opposite nutritional group at the end of the first trimester (?/H = 82 MJ ME and 1.4 kg CP; ?/L = 62 MJ ME and 0.4 kg CP daily), resulting in 4 treatment groups: HH (n = 16); HL (n = 19); LH (n = 17); LL (n = 19). During the third trimester all heifers were fed the same diets. Fetuses were measured at 4-weekly intervals beginning at day 39 of gestation. Calves were also measured at birth for physical body variables. Low maternal nutrient intake was associated with decreased crown-rump length at day 39 (P < 0.01) and increased thoracic diameter at day 95 (P < 0.01). Umbilical cord diameter was reduced in L/? fetuses in the first trimester (P < 0.05) but was greater in ?/L fetuses in the second trimester compared to their respective H counterparts (P < 0.05). Calf birth weight was decreased in association with ?/L maternal diets (P < 0.05). In conclusion, fetal development of cattle may be affected by maternal nutrition as early as day 39 of gestation. This may be followed by either compensatory fetal growth, or alternatively, preferential fetal tissue growth that is dependant upon maternal nutrition. Clearly, calf birth weight may be altered by maternal nutrition during mid-gestation.  相似文献   

20.
A high-resolution study of the Tethyan Contessa Highway reference section (Gubbio, Italy) has produced a detailed, well-constrained and more complete pattern of the gradual evolutionary origin of the foraminiferal genus Hantkenina from Clavigerinella than hitherto shown. A nine-stage pattern is presented for this evolutionary lineage. The related time frame and duration are assessed using the astronomical calibration of the Contessa Highway record. The evolution of Clavigerinella caucasica to Hantkenina gohrbandti, the real ancestor of Hantkenina, leads in 333 kyr. The H. gohrbandti first appearance is dated at 45.564 Ma. Our findings show that during the middle Eocene, from 46 to 45.5 Ma and with a major pulse centered at 45.521 Ma deep-mesopelagic planktonic foraminifera may have developed different morphological modifications to tolerate temporary exceptionally stressful environmental conditions. These conditions are principally related to poorly oxygenated and usually food-poor waters. This suggests that sustained and expanded oxygen minimum zone conditions and associated enhanced eutrophication might have occurred during that time.  相似文献   

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