Lake In Mexico Research Articles

A metagenomic study of antibiotic resistance genes in a hypereutrophic subtropical lake contaminated by anthropogenic sources

Antibiotic resistance genes (ARGs) are a major threat to human and environmental health. This study investigated the occurrence and distribution of ARGs in Lake Cajititlán, a hypereutrophic subtropical lake in Mexico contaminated by anthropogenic sources (urban wastewater and runoff from crop and livestock production). ARGs (a total of 475 genes) were detected in 22 bacterial genera, with Pseudomonas (144 genes), Stenotrophomonas (88 genes), Mycobacterium (54 genes), and Rhodococcus (27 genes) displaying the highest frequencies of ARGs. Among these, Pseudomonas aeruginosa and Stenotrophomonas maltophilia showed the highest number of ARGs. The results revealed a diverse array of ARGs, including resistance to macrolides (11.55 %), aminoglycosides (8.22 %), glycopeptides (6.22 %), tetracyclines (4 %), sulfonamides (4 %), carbapenems (1.11 %), phenicols (0.88 %), fluoroquinolones (0.44 %), and lincosamides (0.22 %). The most frequently observed ARGs were associated with multidrug resistance (63.33 %), with MexF (42 genes), MexW (36 genes), smeD (31 genes), mtrA (25 genes), and KHM-1 (22 genes) being the most common. Lake Cajititlán is a recreational area for swimming, fishing, and boating, while also supporting irrigation for agriculture and potentially acting as a drinking water source for some communities. This raises concerns about the potential for exposure to antibiotic-resistant bacteria through these activities. The presence of ARGs in Lake Cajititlán poses a significant threat to both human and environmental health. Developing strategies to mitigate the risks of antibiotic resistance, including improving wastewater treatment, and promoting strategic antibiotic use and disposal, is crucial. This study represents a significant advancement in the understanding of antibiotic resistance dynamics in a hypereutrophic subtropical lake in a developing country, providing valuable insights for the scientific community and policymakers.

Biomineralization of amorphous Fe-, Mn- and Si-rich mineral phases by cyanobacteria under oxic and alkaline conditions

Abstract. Iron and manganese are poorly soluble elements in oxic and alkaline solutions, whereas they are much more soluble under anoxic conditions. As a result, the formation of authigenic mineral phases rich in Fe and/or Mn has traditionally been viewed as diagnostic of global or local anoxic conditions. Here we reveal that some specific cyanobacteria of very small size (< 2 µm, i.e., picocyanobacteria) can biomineralize abundant, authigenic Fe(III)-, Mn(IV)- and Si-rich amorphous phases under oxic conditions in an alkaline lake in Mexico. The resulting biominerals cluster as small globules arranged as rings around the division septum of cyanobacterial cells. These rings are enveloped within an organic, likely polysaccharidic envelope and are partially preserved, at least morphologically, upon sedimentation. Based on their 16S rDNA sequence, these cyanobacteria were affiliated with the Synechococcales order. The high Fe and Mn enrichment of the biominerals questions the systematic inference of anoxic conditions based on their detection. Moreover, this process scavenges iron from the water column, an overlooked biological contribution to the Fe cycle. Finally, it reveals a new case of controlled biomineralization of Si-rich phases by bacteria.

Seasonal Diversity and Morphometric Variations of Rotifers in Relation to Selected Environmental Variables from a Tropical High-Altitude Lake in Mexico

We studied the species diversity and morphometric variations of rotifers from a high-altitude water body, the Llano reservoir, in central Mexico during September 2021 to August 2022. Samples were collected from four stations in the reservoir every month. During the study period we were able to identify 54 monogonont rotifer species of which Polyarthra vulgaris, Lecane closterocerca, Trichocerca porcellus, Lepadella patella and Keratella cochlearis were numerically the most abundant. Depending on the season, the total rotifer density varied from 250 to 2450 ind L−1. Canonical correspondence analysis (CCA) showed that the species of Trichocerca similis, Keratella cochlearis, Mytilina ventralis and Scaridium longicaudum were directly related to temperature. Using rotifer species richness and abundance data, we derived the Shannon diversity index, saprobic index and rotifer trophic state index. Data on the geometric morphometrics showed that Keratella cochlearis was found in two of the three climatic periods of the year (dry and winter), while Lecane closterocerca and Trichocerca porcellus occurred during the three climatic seasons (dry, winter and rainy) and formed three distinct clusters in relation to body size.

The hidden role of dissolved organic carbon in the biogeochemical cycle of carbon in modern redox-stratified lakes

Abstract. The dissolved organic carbon (DOC) reservoir plays a critical role in the C cycle of marine and freshwater environments because of its size and implication in many biogeochemical reactions. Although it is poorly constrained, its importance in ancient Earth's C cycles is also commonly invoked. Yet DOC is rarely quantified and characterized in modern stratified analogues. In this study, we investigated the DOC reservoirs of four redox-stratified alkaline crater lakes in Mexico. We analyzed the concentrations and isotopic compositions of DOC throughout the four water columns and compared them with existing data on dissolved inorganic and particulate organic C reservoirs (DIC and POC). The four lakes have high DOC concentrations with great variability between and within the lakes (averaging 2 ± 4 mM; 1 SD, n=28; i.e., from ∼ 15 to 160 times the amount of POC). The δ13CDOC signatures also span a broad range of values from −29.3 ‰ to −8.7 ‰ (with as much as 12.5 ‰ variation within a single lake). The prominent DOC peaks (up to 21 mM), together with their associated isotopic variability, are interpreted as reflecting oxygenic and/or anoxygenic primary productivity through the release of excess fixed carbon in three of the lakes (Alberca de los Espinos, La Preciosa, and Atexcac). By contrast, the variability of [DOC] and δ13CDOC in the case of Lake Alchichica is mainly explained by the partial degradation of organic matter and the accumulation of DOC in anoxic waters. The DOC records detailed metabolic functions such as active DIC-uptake and DIC-concentrating mechanisms, which cannot be inferred from DIC and POC analyses alone but which are critical to the understanding of carbon fluxes from the environment to the biomass. Extrapolating our results to the geological record, we suggest that anaerobic oxidation of DOC may have caused the very negative C isotope excursions in the Neoproterozoic. It is, however, unlikely that a large oceanic DOC reservoir could overweigh the entire oceanic DIC reservoir. This study demonstrates how the analysis of DOC in modern systems deepens our understanding of the C cycle in stratified environments and helps to set boundary conditions for the Earth's past oceans.

Diagenetic formation of stevensite by replacement of diatom frustules in the sediments of the alkaline Lake Alchichica (Mexico)

AbstractBetter understanding the conditions of formation of authigenic Mg‐silicates and their reactivity is key to interpret the palaeoenvironmental message carried by the sedimentary record and evaluate the effect of reverse weathering, a process involved in long‐term climate evolution. Microbialites from most alkaline crater lakes in Mexico contain Mg‐silicates except those in Lake Alchichica, where concentration of orthosilicic acid is low (<26 μm). This study investigated the first metre of sediments in Lake Alchichica in order to check how their mineralogy compared with that of shoreline microbialites. The mineralogy and chemistry of the sediment column were determined, together with the pore water chemistry, providing insights on the processes occurring during early diagenesis. Below ca 3 cm in depth, diatom frustules are progressively pseudomorphized into Al‐poor Mg‐silicates with a composition corresponding to stevensite. This diagenetic process is massive and the resulting silicate represents between 30 and 53 wt.% of the sediment content at all depths. This observation questions the possibility to infer lake palaeochemistry from the presence/absence of Mg‐silicates in the sedimentary record. Moreover, it allowed refinement of the conditions under which Mg‐silicates authigenesis occurs: the saturation of the solution should be higher or equal to the solubility of a Mg‐silicate phase close to that of ‘amorphous sepiolite’. Although the solubility of authigenic silicates is a key parameter of reverse weathering modelling during geological times, it is still debated. In this study, a solubility constant deduced from a natural system is proposed that should be considered when modelling the formation of Mg‐silicates in a natural environment. The proportion of reverse weathering associated with this solubility constant could be higher than previously predicted based on experiments and thus have a greater impact on climate stability over geological timescales.

Quantifying the ecosystem service value dynamics in response to land use and land cover changes in Michoacán, Mexico

To quantify the variation in the ecosystem service values (ESV) in response to land use and land cover changes in the Lake Pátzcuaro watershed, the third largest lake in Mexico, Landsat images from the period 1986–2022 were analyzed and a benefits transfer method was used. Forest class and agricultural soils were the dominant covers occupying an average of 42% and 25% of the study area, respectively. The most evident change was a reduction of 2550 ha of the Lake Pátzcuaro surface. Further, with the natural vegetation increasing gradually due to the abandonment of agricultural lands, the ESV also increased from 279.1 to 302.0 million dollars from 1986–2022. These results suggest a positive influence on the protection of natural environments, and although it was not evaluated here but given the lake area reduction, they highlight the need to avoid detrimental actions that affect the biota inhabiting it.

Population analysis of smallmouth silverside, Chirostoma chapalae Jordan & Snyder, 1899 (Atherinopsidae), an endemic fish in Lake Chapala, Mexico

Abstract A sample was analyzed of 1,129 specimens of Chirostoma chapalae from Lake Chapala, Mexico obtained from catches of local fishers between October 2016 and September 2017. Size frequency, length-weight relationships (LWRs), growth, mortality, exploitation rate, survival rate, and relative condition index were estimated. The LWRs showed negative allometric growth in females (b = 2.735) and isometric growth in males (b = 3.000). The SL∞ was 9.20 cm in females and 8.94 cm in males, and the growth coefficient k ranged from 1.80 year−1 to 1.90 year−1, and the t0 from 0.119 to 0.113. Estimated total mortality (Z = 3.20 year−1), natural mortality (M = 1.56 year−1), and fishing mortality (F = 1.64 year−1) values were higher in females. C. chapalae is at optimum, underexploited levels according to the exploitation rate (E = 0.42 year−1). The SL at first capture (SLpc) was 65.8 mm and the SL at retention (SLpr) of the fishing gear was 62.4 mm. This indicates that more than 50% of the fish caught enter the fishery at a larger size. The condition factor (Kn) showed body robustness in fall and subsequent spawning from spring onwards. Our findings contribute to the future development of fisheries management strategies in the largest lake in Mexico.

Organochlorine pesticides and polychlorinated biphenyls in high mountain lakes, Mexico.

Pollution levels of organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) were investigated in the El Sol and the La Luna alpine lakes. The lakes are located in central Mexico, in the crater of the Nevado de Toluca volcano. The El Sol and the La Luna lakes are extremely relevant in Mexico and in the world because they are recognized as pristine regions and environmental reservoirs. Samples of atmospheric aerosol, sediment, plankton, and Tubifex tubifex (sludge worm) were collected at three different sample locations for three years (2017, 2018, and 2019) at three different times of year, meaning that the weather conditions at the time of sampling were different. Pollutants were analysed by gas chromatography-mass spectrometry with negative chemical ionisation (GC-MS/NCI). Endosulfan was the most frequent and abundant pollutant, showing the highest peaks of all. Atmospheric aerosol revealed Σ2 = 45pg/m3, including α and β, while sediment lakes displayed α, β and endosulfan sulfate as Σ3 = 1963pg/g, whereas plankton and Tubifex tubifex showed Σ2 = 576pg/g and 540pg/g for α and β respectively. Results of endosulfan ratios (α/β) and (α-β/endosulfan sulfate) suggest that both fresh and old discharges continue to arrive at the lakes. This study shows for the first time the pollution levels of OCP and PCB in high mountain lakes in Mexico. These results that must be considered by policy makers to mitigate their use in the various productive activities of the region.

Cyclotella (Bacillariophyceae) species present in sediments dating to Marine Isotope Stage 5 from Lake Chalco, central Mexico, with special reference to two new species: Cyclotella poyeka and Cyclotella tlalocii

Lake Chalco is one of the few lakes in Mexico that has a long, continuous sedimentary sequence covering the Upper Pleistocene (> 150 000 yrs). The diatom content of its lacustrine sediments includes a variety of centric species. In particular, the sediments dating to Marine Isotope Stage 5 (MIS5) were alternately dominated by four Cyclotella spp: C. meneghiniana Kützing, C. quillensis Bailey, and two new species which are described here: C. poyeka and C. tlalocii. These two new Cyclotella spp. resemble C. stylorum Brightwell, because of the presence of marginal chambers. Nevertheless, the new species from Lake Chalco have a different structure of the marginal chambers and marginal fultoportulae with three satellite pores. Cyclotella poyeka and C. tlalocii differ from each other by the relative proportion of the central area to valve face diameter, the central fultoportula arrangement, and the striation pattern. Ecological affinities of the four Cyclotella species in the MIS5 sediments from Lake Chalco are discussed based on their modern distribution (for C. meneghiniana and C. quillensis) and their fossil assemblages (for C. quillensis, C. tlalocii and C. poyeka), leading to the conclusion that C. tlalocii was a freshwater species while C. meneghiniana, C. quillensis, and C. poyeka thrived in saline conditions.

Ambystoma mexicanum, a model organism in developmental biology and regeneration: a colombian experience

Ambystoma mexicanum is a urodele amphibian endemic to Xochimilco Lake in Mexico, it belongs to the salamander family Ambystomatidae. This species has frequently been used as model organism in developmental biology and regeneration laboratories around the world due to its broad regenerative capacities and adaptability to laboratory conditions. In this review we describe the establishment of the first colony of axolotls in Colombia to study tissue regeneration and our perspectives on the use A. mexicanum as a model organism in Colombia are discussed emphasizing its possible uses in regeneration and developmental biology

Feeding habits of Lithobates megapoda (Anura: Ranidae), a threatened leopard frog used for human consumption, in Lake Chapala, Mexico

The Big-footed Leopard Frog (Lithobates megapoda) is a threatened and endemic species from western Mexico. This species has aquatic habits and it is distinguished by its large size, particularly by the length of its legs, reason for which it is captured for human consumption. Also, knowledge about its natural history is scarce and incomplete. In this study, we analyzed the composition of the diet of L. megapoda on the shore of Lake Chapala, the largest lake in Mexico. A total of 69 adult individuals were collected lifeless in fishing nets, during the rainy season (June-October), of which 48 had stomach contents. A total of 96 prey items were identified, which correspond to 13 prey categories. Fish constituted the most dominant prey category in the diet in terms of number, volume, frequency of occurrence, and relative importance. No significant differences were found in the consumption by prey type (aquatic or terrestrial). However, the aquatic preys had a greater relative importance and were more voluminous than the terrestrial ones. The diversity of prey categories, in terms of prey volume, indicates males may have a higher dietary diversity than females, but we not evaluated possible bias. In addition, a significant effect was found in the interaction of size (SVL) of frogs with the average of prey volume. Females (that are larger than males) consumed prey within a wide volume range and, the larger they are, more voluminous are prey. In this way it is possible that intraspecific competition for trophic resources in the environment is reduced. This study helps us understand the trophic ecology of L. megapoda, a frog species that plays an important role in the food web where it lives, as a predator feeding on aquatic and terrestrial organisms.

Seasonal variation of <em>Microcystis aeruginosa</em> and factors related to blooms in a deep warm monomictic lake in Mexico

The occurrence of cyanobacterial blooms has increased globally over the last decades, with the combined effect of climate change and eutrophication as its main drivers. The seasonal dynamic of cyanobacterial blooms is a well-known phenomenon in lakes and reservoirs in temperate zones. Nevertheless, in the tropics, most studies have been performed in shallow and artificial lakes; therefore, the seasonal dynamic of cyanobacterial blooms in deep and eutrophic tropical lakes is still under research. We studied the seasonal variation of the phytoplankton community and the factors associated with Microcystis aeruginosa blooms along the water column of Lake Alberca de Tacámbaro, a warm monomictic crater lake located in Mexico, during 2018 and 2019. According to previous studies performed in 2006 and 2010, this lake was mesotrophic-eutrophic, with Chlorophyta and Bacillariophyta as the dominant groups of the phytoplankton community. During 2018 and 2019, the lake was eutrophic and occasionally, hypertrophic, a phenomenon likely associated with the increase of farmland area around the lake. The dominant species was M. aeruginosa, forming blooms from the surface to 10 m depth in winter, in the hypolimnion in spring and summer, and along the full water column in autumn. These findings suggest that M. aeruginosa in Lake Alberca de Tacámbaro displays seasonal and spatial population dynamics. Total phosphorus, dissolved inorganic nitrogen, water temperature and photosynthetically active radiation were the environmental factors related to M. aeruginosa blooms. Our results suggest that the changes in the structure of the phytoplankton community through time, and M. aeruginosa blooms in Lake Alberca de Tacámbaro, are mainly related to changes in land use from forest to farmland in areas adjacent to the lake, which promoted its eutrophication in the last years through runoffs. Comparative studies with other deep and eutrophic lakes will allow us to gain a deeper understanding of the dynamic of cyanobacterial blooms in natural and artificial water reservoirs strongly stressed by human activities.

Reduction in concentration of chromium (VI) by Lysinibacillus macroides isolated from sediments of the Chapala Lake, Mexico.

The Chapala Lake is one of the most polluted lakes in Mexico, due to the in flow of effluents from several industrial plants, the lake accumulates pollutants such as chromium(VI) which is considered important for aquatic ecosystem. This study aimed was to evaluate the ability to decrease the concentration of chromium (VI) by Lysinibacillus macroides 2(1B)104A, isolated from sediments of the Chapala Lake. The strain was identified through 16S rRNA sequencing and phylogenetic analysis. Results showed that this strain grows in concentrations of 50, 100, 200 and 300 mgL-1 Cr(VI), in pH ranging 6 to 7, showing 79.508% reduction in concentration 50 mgL-1, determining that the reduction occurs extracellularly. Likewise, it was observed that Lysinibacillus macroides reduced the concentration of Cr(IV) in the broth, it was not observed that the bacteria could sequester Cr(VI) in the membrane or intracellularly. However, it reduced the concentration of Cr(VI) in the broth. Lysinibacillus macroides 2(1B)104A isolate showed having the ability that decrease the concentration of Cr(VI), which makes it a viable options for bioremediation of water polluted with this metal.

Cultivo de Pomacea patula catemacensis como estrategia de producción piscícola para los productores acuícolas de Catemaco, Veracruz

En la estructura de los ecosistemas dulceacuícolas, los moluscos forman una parte esencial y están estrechamente relacionados a la forma de vida de los pobladores que se encuentran en las riberas de los cuerpos de aguas. El lago de Catemaco se encuentra dentro de los cinco lagos más grandes de México, con una cuenca hidrológica propia de 72.54 kilómetros2 (Pérez y Torres 1992), presenta una producción pesquera importante para la región, y como es de esperarse cuenta con especies endémicas con potencial para el desarrollo de la acuacultura, es un cuerpo de agua del que se extraen diferentes recursos pesqueros que a su vez son aprovechados por los pobladores locales (Lorán et al., 2013). Pomacea patula catemacensis (tegogolo), ocupa el cuarto lugar de importancia en la producción pesquera en Catemaco Veracruz, siendo referente de la gastronomía en la región de Los Tuxtlas, Veracruz, sin embargo, no existen estudios sobre el tamaño de sus poblaciones, aunque se sabe que en la última década la producción ha ido en disminución, se considera que poco más del 60%, debido a la pesca extractiva. Por lo anterior se deben de considerar los diseños de producción a partir del manejo en cautiverio del tegogolo como estrategia para equilibrar sus poblaciones. A la fecha son pocas las técnicas para lograr la domesticación de P. patula catemacensis mediante el diseño de sistemas de producción, impidiendo la constante baja densidad poblacional y el peligro de extinción (INEGI 2010). Los resultados obtenidos nos permitieron identificar que de acuerdo a los usos y costumbres, en la región de Catemaco, se está capturando a P. patula catemacensis sin un control estricto al respecto, por lo que es necesario desarrollar y estudiar bio-tecnias, y presentar estrategias para la producción del molusco, para recuperar las poblaciones en el cuerpo de agua. Se concluyó que la especie P. patula catemacensis puede ser cultivada en encierros que permitan su control, con el apoyo a productores a través de programas de capacitación para el diseño y establecimiento de los cultivos y de esta manera lograr un aprovechamiento sustentable.

Urinary concentrations of phthalate metabolites in pregnant women living near Chapala Lake, Jalisco, Mexico

Phthalates are esters of phthalic acid used in a broad array of consumer products and food contact surfaces. Phthalates are known endocrine disruptors and oxidant stressors, and exposure has been associated with premature birth, asthma, obesity, insulin resistance and endometriosis. Though many industrializing countries are known to manufacture phthalates, few studies have examined exposure to phthalates in this context, let alone in rural communities where phthalate-containing products are widely used. We evaluated the presence of 16 phthalate metabolites in third trimester pregnant women in three rural communities near the largest lake in Mexico, Lake Chapala, by liquid chromatography coupled to tandem mass spectrometry in 90 urine samples. Phthalate metabolites were found in all samples, where the highest concentration was 1830 ng/mL in mono-ethyl phthalate (mEP), and it was present in 98.9% of all samples. These findings suggest the need for further research on the effect of endocrine disrupting chemicals in developing countries, and public health guidance on opportunities for prevention.

Isolation and Identification of Two Brucella Species from a Volcanic Lake in Mexico.

Brucellosis is a zoonosis caused by bacteria of the Brucella genus. Any source of contamination that could be infectious must be monitored to reduce the risk of exposure to brucellosis, so the purpose of this work was to determine the presence of Brucella spp. on surface water and tilapia (Oreochromis niloticus) skin from a volcanic lake in Mexico. A seasonal sampling during 2016-2017 was carried out at fifteen specific sites for water sampling and five sites for the collection of tilapia fish. From all water and fish samples tested, we found only three isolates of Brucella species. We isolated and identified B. abortus from surface water through bacteriological and molecular techniques, and B. abortus and B. suis from the same tilapia skin sample. The isolated strains likely came from breeding animals that are common to the region, such as infected pigs or cattle with Brucella abortus or B. suis, respectively. A similar finding has not been reported in a water from volcanic lake or tilapia fish in Mexico. We concluded that B. abortus and B. suis are present on the surface water of the volcanic lake and tilapia skin as possible contaminants derived from biological material from cows and pigs carrying this bacterium.

Numerical and Observational Analysis of the Hydro-Dynamical Variability in a Small Lake: The Case of Lake Zirahuén, México

Lake Zirahuén is one of the ecologically better preserved and small-sized lakes in Mexico. Observations revealed that Lake Zirahuén is subjected to a consistent diurnal wind because of the presence of a valley–mountain breeze that triggers semidiurnal and 3–8 h variability oscillations, with the latter among the natural frequencies of the lake, whereas the former and the principal forcing are in near resonance with the second vertical mode. The thermodynamic variability was greater in the metalimnion where the analysis of eigenfunctions shows that higher modes (>2) are important at depths below the thermocline. The numerical modeling adequately described the water temperature evolution and surface layer currents in an averaged manner. The daily observations showed drifts in surface currents, acquiring velocities of up to 0.1 m/s, owing to wind stress, which caused an increase in temperature at the northern section of the lake. Data averaged over three months revealed that the surface currents flow northward with an anticyclonic return to the east part and a pair of cyclonic returns to the northwestern and southwestern parts of the lake; whereas at the vertical, the structure showed two circular regions divided by the thermocline located at a depth of 15–20 m.

Rhythmic episodes of heating and cooling control thermal stratification of two tropical high mountain lakes

Continuous temperature monitoring for two adjacent tropical crater lakes in Mexico at 4200 m amsl shows that the lakes have rhythmic episodes of heating and cooling with a duration of ~ 30 days during the warmest months. The episodes were caused by rise and decline of solar irradiance reaching the lake surface. One lake, El Sol, showed over each heating and cooling episode a stable mixed layer (~ 20 days) and a deeper layer with a weak thermal gradient. Temperatures below the mixed layer warmed progressively by eddy diffusion after the mixed layer formed. Stratification was followed by full mixing of the water column. Within the same crater, an adjacent second lake, La Luna, showed the same cycles of heating and cooling; it stratified daily but not over multiple days. The difference between the lakes (discontinuous polymictic, continuous polymictic) is explained by the lower transparency of El Sol, which led to greater heat uptake near the surface than the more transparent La Luna. Lower transparency of El Sol was caused by modest anthropogenic effects on total suspended solids and nutrient loading, i.e., small deviations from the natural condition of El Sol caused it to differ qualitatively from La Luna. Events observed in these lakes would not have been evident from weekly temperature records.

Assessment of metal contamination and their ecological risks in wetland sediments of the former Texcoco saline lake, Mexico

Texcoco saline lake in Mexico is an important habitat for over 300 species and recharges the Mexico City aquifer. Presently, it is on the verge of disappearance due to increasing anthropogenic pressure and comprises only a few wetland remnants. The impacts of human activities on these wetlands are unknown and uninvestigated. In this regard, this study investigates the degree of metal contamination in this ecosystem during the period of 2015–2018. Twenty-six sediment samples were collected from nine different wetlands of Texcoco Lake and examined for 17 metals (Al, Fe, Ca, Mg, Na, K, As, Ba, Cd, Co, Cr, Cu, Mn, Ni, Pb, Zn, and Hg) using atomic absorption spectroscopy. The degree of metal accumulation and its ecological risks were evaluated on the basis of several indicators (geoaccumulation index, enrichment factor, sediment quality guidelines, ecotoxicological values, and potential ecological risk index). Furthermore, co-occurrence via multivariate statistical techniques was employed using the measured metal concentrations to identify their possible sources. The Cd, Cu, Pb, Zn, and Hg sediment concentrations were higher in magnitude than the background North American shale composite values, suggesting their external origin in all studied sediments. According to the calculated enrichment factors and geoaccumulation index, the sediments were mostly contaminated with Cd and Hg. The sediment quality guidelines and ecotoxicological values together with the ecological risk index suggested probable adverse biological effects due to the presence of Cr, Cu, Ni, and Hg. Cadmium and Hg accounted for nearly 51 and 45% of the ecological threat in the sediments. Based on the knowledge about metal levels and co-occurrence in different matrices and the multivariate data analyses of the detected metal concentrations, we infer that the main sources of metals are as follows: (1) Solonchaks and Vertisol soil types (Ca, Mg, Na, and K); (2) urban-industrial effluents from surrounding municipalities and Mexico City (Cu, Pb, and Zn); and (3) industrial discharges of Xalostoc zone (Cd and Hg) near Texcoco. Our results emphasize the impacts of unregulated discharges from municipal, industrial, and metropolitan zones of Mexico City and construction activities for the elevated metal concentrations in the wetland sediments of Texcoco Lake.

Desulfobotulus mexicanus sp. nov., a novel sulfate-reducing bacterium isolated from the sediment of an alkaline crater lake in Mexico.

A novel Gram-negative, non-spore-forming, vibrio-shaped, anaerobic, alkaliphilic, sulfate-reducing bacterium, designated strain PAR22NT, was isolated from sediment samples collected at an alkaline crater lake in Guanajuato (Mexico). Strain PAR22NT grew at temperatures between 15 and 37 °C (optimum, 32 °C), at pH between pH 8.3 and 10.1 (optimum, pH 9.0-9.6), and in the presence of NaCl up to 10 %. Pyruvate, 2-methylbutyrate and fatty acids (4-18 carbon atoms) were used as electron donors in the presence of sulfate as a terminal electron acceptor and were incompletely oxidized to acetate and CO2. Besides sulfate, both sulfite and elemental sulfur were also used as terminal electron acceptors and were reduced to sulfide. The predominant fatty acids were summed feature 10 (C18 : 1 ω7c and/or C18 : 1 ω9t and/or C18 : 1 ω12t), C18 : 1 ω9c and C16 : 0. The genome size of strain PAR22NT was 3.8 Mb including 3391 predicted genes. The genomic DNA G+C content was 49.0 mol%. Phylogenetic analysis based on 16S rRNA gene sequences showed that it belongs to the genus Desulfobotulus within the class Deltaproteobacteria. Its closest phylogenetic relatives are Desulfobotulus alkaliphilus (98.4 % similarity) and Desulfobotulus sapovorans (97.9 % similarity). Based on phylogenetic, phenotypic and chemotaxonomic characteristics, we propose that the isolate represents a novel species of the genus Desulfobotulus with the name Desulfobotulus mexicanus sp. nov. The type strain is PAR22NT (=DSM 105758T=JCM 32146T).