Search

Comprehensive assessments of functional diversity are needed to understand ecosystem alterations under global changes. The 'Fun-eDNA' approach characterises functional diversity by assigning traits to taxonomic units obtained through environmental DNA (eDNA) sampling. By simultaneously analysing an unprecedented number of taxa over broad spatial scales, the approach provides a whole-ecosystem perspective of functional diversity. Fun-eDNA is increasingly used to tackle multiple questions, but aligning eDNA with traits poses several conceptual and technical challenges. Enhancing trait databases, improving the annotation of eDNA-based taxonomic inventories, interdisciplinary collaboration, and conceptual harmonisation of traits are key steps to achieve a comprehensive assessment of diverse taxa. Overcoming these challenges can unlock the full potential of eDNA in leveraging measures of ecosystem functions from multi-taxa assessments., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Soil hosts key components of terrestrial biodiversity providing essential services to the below- and above-ground ecosystems. The worldwide retreat of glaciers is exposing new deglaciated terrains, offering a unique opportunity to understand the development of soil ecosystems under a changing climate. Many studies have investigated how biotic communities change after deglaciation, but protists have often been overlooked despite their key role in multiple ecosystem functions. Here, we aim to understand how protist communities develop along glacier forelands, describing their successional trajectories. Protist communities were characterized in 1251 soil samples from 46 glacier forelands across four continents. We used environmental DNA metabarcoding to identify the Molecular Operational Taxonomic Units (MOTUs) of protists based on a universal eukaryotic marker. The detected MOTUs were combined with information on multiple traits to assess how the functional diversity and composition of protist communities vary through time. Immediately after glacier retreat, protist communities are like those of polar and high-altitude habitats, with consumers being the dominant trophic group, followed by a relevant presence of phototrophs, while parasites were underrepresented. Over the succession, we detected an increase in taxonomic and functional diversity, but some highly specialized groups (e.g. phototrophic algae) declined. The use of a trait-based approach allowed us to identify distinct successional patterns depending on functional groups. Through the functional characterization of a crucial but understudied component of soil biotic communities, our study added one of the final pieces needed to predict how soil ecosystems will develop in the rapidly changing environment of glacier forelands., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

The global retreat of glaciers is dramatically altering mountain and high-latitude landscapes, with new ecosystems developing from apparently barren substrates 1-4 . The study of these emerging ecosystems is critical to understanding how climate change interacts with microhabitat and biotic communities and determines the future of ice-free terrains 1,5 . Here, using a comprehensive characterization of ecosystems (soil properties, microclimate, productivity and biodiversity by environmental DNA metabarcoding 6 ) across 46 proglacial landscapes worldwide, we found that all the environmental properties change with time since glaciers retreated, and that temperature modulates the accumulation of soil nutrients. The richness of bacteria, fungi, plants and animals increases with time since deglaciation, but their temporal patterns differ. Microorganisms colonized most rapidly in the first decades after glacier retreat, whereas most macroorganisms took longer. Increased habitat suitability, growing complexity of biotic interactions and temporal colonization all contribute to the increase in biodiversity over time. These processes also modify community composition for all the groups of organisms. Plant communities show positive links with all other biodiversity components and have a key role in ecosystem development. These unifying patterns provide new insights into the early dynamics of deglaciated terrains and highlight the need for integrated surveillance of their multiple environmental properties 5 ., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

The development of terrestrial ecosystems depends greatly on plant mutualists such as mycorrhizal fungi. The global retreat of glaciers exposes nutrient-poor substrates in extreme environments and provides a unique opportunity to study early successions of mycorrhizal fungi by assessing their dynamics and drivers. We combined environmental DNA metabarcoding and measurements of local conditions to assess the succession of mycorrhizal communities during soil development in 46 glacier forelands around the globe, testing whether dynamics and drivers differ between mycorrhizal types. Mycorrhizal fungi colonized deglaciated areas very quickly (< 10 yr), with arbuscular mycorrhizal fungi tending to become more diverse through time compared to ectomycorrhizal fungi. Both alpha- and beta-diversity of arbuscular mycorrhizal fungi were significantly related to time since glacier retreat and plant communities, while microclimate and primary productivity were more important for ectomycorrhizal fungi. The richness and composition of mycorrhizal communities were also significantly explained by soil chemistry, highlighting the importance of microhabitat for community dynamics. The acceleration of ice melt and the modifications of microclimate forecasted by climate change scenarios are expected to impact the diversity of mycorrhizal partners. These changes could alter the interactions underlying biotic colonization and belowground-aboveground linkages, with multifaceted impacts on soil development and associated ecological processes., (© 2024 The Authors. New Phytologist © 2024 New Phytologist Foundation.)

Nematodes are keystone actors of soil, freshwater and marine ecosystems, but the complexity of morphological identification has limited broad-scale monitoring of nematode biodiversity. DNA metabarcoding is increasingly used to assess nematode diversity but requires universal primers with high taxonomic coverage and high taxonomic resolution. Several primers have been proposed for the metabarcoding of nematode diversity, many of which target the 18S rRNA gene. In silico analyses have a great potential to assess key parameters of primers, including taxonomic coverage, resolution and specificity. Based on a recently-available reference database, we tested in silico the performance of fourteen commonly used and one newly optimized primer for nematode metabarcoding. Most primers showed very good coverage, amplifying most of the sequences in the reference database, while four markers showed limited coverage. All primers showed good taxonomic resolution. Resolution was particularly good if the aim was the identification of higher-level taxa, such as genera or families. Overall, species-level resolution was higher for primers amplifying long fragments. None of the primers was highly specific for nematodes as, despite some variation, they all amplified a large number of other eukaryotes. Differences in performance across primers highlight the complexity of the choice of markers appropriate for the metabarcoding of nematodes, which depends on a trade-off between taxonomic resolution and the length of amplified fragments. Our in silico analyses provide new insights for the identification of the most appropriate primers, depending on the study goals and the origin of DNA samples. This represents an essential step to design and optimize metabarcoding studies assessing nematode diversity., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Ficetola et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

The mechanisms underlying plant succession remain highly debated. Due to the local scope of most studies, we lack a global quantification of the relative importance of species addition 'versus' replacement. We assessed the role of these processes in the variation (β-diversity) of plant communities colonizing the forelands of 46 retreating glaciers worldwide, using both environmental DNA and traditional surveys. Our findings indicate that addition and replacement concur in determining community changes in deglaciated sites, but their relative importance varied over time. Taxa addition dominated immediately after glacier retreat, as expected in harsh environments, while replacement became more important for late-successional communities. These changes were aligned with total β-diversity changes, which were more pronounced between early-successional communities than between late-successional communities (>50 yr since glacier retreat). Despite the complexity of community assembly during plant succession, the observed global pattern suggests a generalized shift from the dominance of facilitation and/or stochastic processes in early-successional communities to a predominance of competition later on., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

The worldwide retreat of glaciers is causing a faster than ever increase in ice-free areas that are leading to the emergence of new ecosystems. Understanding the dynamics of these environments is critical to predicting the consequences of climate change on mountains and at high latitudes. Climatic differences between regions of the world could modulate the emergence of biodiversity and functionality after glacier retreat, yet global tests of this hypothesis are lacking. Nematodes are the most abundant soil animals, with keystone roles in ecosystem functioning, but the lack of global-scale studies limits our understanding of how the taxonomic and functional diversity of nematodes changes during the colonization of proglacial landscapes. We used environmental DNA metabarcoding to characterize nematode communities of 48 glacier forelands from five continents. We assessed how different facets of biodiversity change with the age of deglaciated terrains and tested the hypothesis that colonization patterns are different across forelands with different climatic conditions. Nematodes colonized ice-free areas almost immediately. Both taxonomic and functional richness quickly increased over time, but the increase in nematode diversity was modulated by climate, so that colonization started earlier in forelands with mild summer temperatures. Colder forelands initially hosted poor communities, but the colonization rate then accelerated, eventually leveling biodiversity differences between climatic regimes in the long term. Immediately after glacier retreat, communities were dominated by colonizer taxa with short generation time and r-ecological strategy but community composition shifted through time, with increased frequency of more persister taxa with K-ecological strategy. These changes mostly occurred through the addition of new traits instead of their replacement during succession. The effects of local climate on nematode colonization led to heterogeneous but predictable patterns around the world that likely affect soil communities and overall ecosystem development., (© 2023 The Authors. Global Change Biology published by John Wiley & Sons Ltd.)

Ice-free areas are expanding worldwide due to dramatic glacier shrinkage and are undergoing rapid colonization by multiple lifeforms, thus representing key environments to study ecosystem development. It has been proposed that the colonization dynamics of deglaciated terrains is different between surface and deep soils but that the heterogeneity between communities inhabiting surface and deep soils decreases through time. Nevertheless, tests of this hypothesis remain scarce, and it is unclear whether patterns are consistent among different taxonomic groups. Here, we used environmental DNA metabarcoding to test whether community diversity and composition of six groups (Eukaryota, Bacteria, Mycota, Collembola, Insecta, and Oligochaeta) differ between the surface (0-5 cm) and deeper (7.5-20 cm) soil at different stages of development and across five Alpine glaciers. Taxonomic diversity increased with time since glacier retreat and with soil evolution. The pattern was consistent across groups and soil depths. For Eukaryota and Mycota, alpha-diversity was highest at the surface. Time since glacier retreat explained more variation of community composition than depth. Beta-diversity between surface and deep layers decreased with time since glacier retreat, supporting the hypothesis that the first 20 cm of soil tends to homogenize through time. Several molecular operational taxonomic units of bacteria and fungi were significant indicators of specific depths and/or soil development stages, confirming the strong functional variation of microbial communities through time and depth. The complexity of community patterns highlights the importance of integrating information from multiple taxonomic groups to unravel community variation in response to ongoing global changes., (© 2022 The Authors. Molecular Ecology published by John Wiley & Sons Ltd.)

Understanding how anthropization impacts the assembly of species onto communities is pivotal to go beyond the observation of biodiversity changes and reveal how disturbances affect the environmental and biotic processes shaping biodiversity. Here, we propose a simple framework to measure the assembly processes underpinning functional convergence/divergence patterns. We applied this framework to northern Amazonian fish communities inventoried using environmental DNA in 35 stream sites and 64 river sites. We found that the harsh and unstable environmental conditions characterizing streams conveyed communities towards functional convergence, by filtering traits related to food acquisition and, to a lower extent, dispersal. Such environmental filtering also strengthened competition by excluding species having less competitive food acquisition traits. Instead, random species assembly was more marked in river communities, which may be explained by the downstream position of rivers facilitating the dispersion of species. Although fish assembly rules differed between streams and river fish communities, anthropogenic disturbances reduced functional divergence in both ecosystems, with a reinforcement of both environmental filtering and weaker competitor exclusion. This may explain the substantial biodiversity alterations observed under slight deforestation levels in Neotropical freshwater ecosystems and underlines their vulnerability to anthropic disturbances that not only affect species persistence but also modify community assembly rules.

Freshwater ecosystems are among the most endangered ecosystem in the world. Understanding how human activities affect these ecosystems requires disentangling and quantifying the contribution of the factors driving community assembly. While it has been largely studied in temperate freshwaters, tropical ecosystems remain challenging to study due to the high species richness and the lack of knowledge on species distribution. Here, the use of eDNA-based fish inventories combined to a community-level modelling approach allowed depicting of assembly rules and quantifying the relative contribution of geographic, environmental and anthropic factors to fish assembly. We then used the model predictions to map spatial biodiversity and assess the representativity of sites surveyed in French Guiana within the EU Water Framework Directive (WFD) and highlighted areas that should host unique freshwater fish assemblages. We demonstrated a mismatch between the taxonomic and functional diversity. Taxonomic assemblages between but also within basins were mainly the results of dispersal limitation resulting from basin isolation and natural river barriers. Contrastingly, functional assemblages were ruled by environmental and anthropic factors. The regional mapping of fish diversity indicated that the sites surveyed within the EU WFD had a better representativity of the regional functional diversity than taxonomic diversity. Importantly, we also showed that the assemblages expected to be the most altered by anthropic factors were the most poorly represented in terms of functional diversity in the surveyed sites. The predictions of unique functional and taxonomic assemblages could, therefore, guide the establishment of new survey sites to increase fish diversity representativity and improve this monitoring program., (© 2022 The Authors. Global Change Biology published by John Wiley & Sons Ltd.)

Assessing the impact of human activity on ecosystems often links local biodiversity to disturbances measured within the same locality. However, remote disturbances may also affect local biodiversity. Here, we used environmental DNA metabarcoding to evaluate the relationships between vertebrate biodiversity (fish and mammals) and disturbance intensity in two Amazonian rivers. Measurements of anthropic disturbance -here forest cover losses- were made from the immediate vicinity of the biodiversity sampling sites to up to 90 km upstream. The findings suggest that anthropization had a spatially extended impact on biodiversity. Forest cover losses of <11% in areas up to 30 km upstream from the biodiversity sampling sites were linked to reductions of >22% in taxonomic and functional richness of both terrestrial and aquatic fauna. This underscores the vulnerability of Amazonian biodiversity even to low anthropization levels. The similar responses of aquatic and terrestrial fauna to remote disturbances indicate the need for cross-ecosystem conservation plans that consider the spatially extended effects of anthropization., (© 2022. The Author(s).)

Environmental DNA (eDNA) is gaining a growing popularity among scientists but its applicability to biodiversity research and management remains limited in river systems by the lack of knowledge about the spatial extent of the downstream transport of eDNA. Here, we assessed the ability of eDNA inventories to retrieve spatial patterns of fish assemblages along two large and species-rich Neotropical rivers. We first examined overall community variation with distance through the distance decay of similarity and compared this pattern to capture-based samples. We then considered previous knowledge on individual species distributions, and compared it to the eDNA inventories for a set of 53 species. eDNA collected from 28 sites in the Maroni and 25 sites in the Oyapock rivers permitted to retrieve a decline of species similarity with increasing distance between sites. The distance decay of similarity derived from eDNA was similar and even more pronounced than that obtained with capture-based methods (gill-nets). In addition, the species upstream-downstream distribution range derived from eDNA matched to the known distribution of most species. Our results demonstrate that environmental DNA does not represent an integrative measure of biodiversity across the whole upstream river basin but provides a relevant picture of local fish assemblages. Importantly, the spatial signal gathered from eDNA was therefore comparable to that gathered with local capture-based methods, which describes fish fauna over a few hundred metres., (© 2021 John Wiley & Sons Ltd.)

Background: Environmental DNA [eDNA] metabarcoding has recently emerged as a non-destructive alternative to traditional sampling for characterising species assemblages., New Information: We here provide a consistent dataset synthetising all eDNA sampling sites in French Guiana to date. Field collections have been initiated in 2014 and have continued until 2019. This dataset is however a work in progress and will be updated after each collecting campaign. We also provide a taxon by site matrix for fishes presence / absence as inferred from eDNA. Our aim is to allow a transparent communication to the stakeholders and provide the foundation for a monitoring programme based on eDNA. The lastest version of the dataset is publicly and freely accessible through the CEBA geoportal (http://vmcebagn-dev.ird.fr) or through the French Guiana geographic portal (https://www.geoguyane.fr)., (Jérôme Murienne, Isabel Cantera, Axel Cerdan, Kévin Cilleros, Jean-Baptiste Decotte, Tony Dejean, Régis Vigouroux, Sébastien Brosse.)

Species trees have traditionally been inferred from a few selected markers, and genome-wide investigations remain largely restricted to model organisms or small groups of species for which sampling of fresh material is available, leaving out most of the existing and historical species diversity. The genomes of an increasing number of species, including specimens extracted from natural history collections, are being sequenced at low depth. While these data sets are widely used to analyse organelle genomes, the nuclear fraction is generally ignored. Here we evaluate different reference-based methods to infer phylogenies of large taxonomic groups from such data sets. Using the example of the Oleeae tribe, a worldwide-distributed group, we build phylogenies based on single nucleotide polymorphisms (SNPs) obtained using two reference genomes (the olive and ash trees). The inferred phylogenies are overall congruent, yet present differences that might reflect the effect of distance to the reference on the amount of missing data. To limit this issue, genome complexity was reduced by using pairs of orthologous coding sequences as the reference, thus allowing us to combine SNPs obtained using two distinct references. Concatenated and coalescence trees based on these combined SNPs suggest events of incomplete lineage sorting and/or hybridization during the diversification of this large phylogenetic group. Our results show that genome-wide phylogenetic trees can be inferred from low-depth sequence data sets for eukaryote groups with complex genomes, and histories of reticulate evolution. This opens new avenues for large-scale phylogenomics and biogeographical analyses covering both the extant and the historical diversity stored in museum collections., (© 2019 John Wiley & Sons Ltd.)

Environmental DNA (eDNA) metabarcoding is a promising tool to estimate aquatic biodiversity. It is based on the capture of DNA from a water sample. The sampled water volume, a crucial aspect for efficient species detection, has been empirically variable (ranging from few centiliters to tens of liters). This results in a high variability of sampling effort across studies, making comparisons difficult and raising uncertainties about the completeness of eDNA inventories. Our aim was to determine the sampling effort (filtered water volume) needed to get optimal inventories of fish assemblages in species-rich tropical streams and rivers using eDNA. Ten DNA replicates were collected in six Guianese sites (3 streams and 3 rivers), resulting in sampling efforts ranging from 17 to 340 liters of water. We show that sampling 34 liters of water detected more than 64% of the expected fish fauna and permitted to distinguish the fauna between sites and between ecosystem types (stream versus rivers). Above 68 liters, the number of detected species per site increased slightly, with a detection rate higher than 71%. Increasing sampling effort up to 340 liters provided little additional information, testifying that filtering 34 to 68 liters is sufficient to inventory most of the fauna in highly diverse tropical aquatic ecosystems.

Objective: To analyse the relationship between the primary diagnosis on admission to an Acute Geriatric Unit (AGU) and the risk of hospital mortality and one year after discharge, Material and Methods: A longitudinal study was conducted on patients admitted to the Central Hospital AGU Red Cross in Madrid in 2009. The admission diagnosis was grouped by Diagnosis Related Groups (DRGs). The date of death was collected from the medical charts and the National Death Index Ministry of Health report. The main outcome of study was the association between diagnoses on admission and functional impairment at discharge (measured as a loss of 10 or more points between the Barthel Index at discharge and that on admission), mortality during hospitalization, at 3 months and one year after discharge. The multivariate analysis was adjusted for age, sex, comorbidity, functional and cognitive status, and serum albumin., Results: The study included1147 patients, with a mean age of 86.7 years (SD±6.7), and 66% were women. During admission, 10.1% of patients died and 36.6% had functional impairment at discharge. After discharge, 25.5% died at 3 months, and 42.2% at one year. The distribution of the primary diagnoses at admission (between parentheses hospital mortality and at year) were heart failure, 21.4% (8.1% and 37.4%), pneumonia,13.3% (12.3% and 46.4%), and aspiration pneumonia, 4.7% (27.5%, y 71%), respiratory diseases,13.3% (6.6% and 38.2%), urinary infection,10.2% (5.1% and 42.7%), and stroke (excluding AIT), 9.9% (13.3% and 46.9%). In the multivariate analysis, only admissions due to aspiration pneumonia were independently associated with increased risk of hospital mortality (odds ratio, 2.23; 95% CI=1.13 to 44.42), and stroke with increased risk of functional impairment at discharge (odds ratio, 6.01; 95% CI=3.42-10.57). No diagnosis was independently associated with increased risk of death at 3 months and at year, Conclusions: Admission from aspiration pneumonia carries an increased risk of death in elderly patients hospitalised for acute medical conditions. After discharge, the risk of death must be attributed to factors other than the admission diagnosis., (Copyright © 2015 SEGG. Published by Elsevier Espana. All rights reserved.)

Background and Objective: To compare the glomerular filtration rate (GFR) of patients aged 75 and older without known renal disease estimated by the MDRD-4-IDMS (MDRD) formula, urine 24h creatinine clearance (24h-CC) and Cockroft-Gault (CG) formula., Patients and Methods: There were included 70 persons aged 75 and older from the geriatric external consultation. We compared the MDRD against the CG and the 24h-CC., Results: Means of GFR were: 24h-CC: 56,60 + or - 22,79 ml/min/1.73 m(2) and CG: 54,27 + or - 15,25 ml/min/1.73 m(2), significantly lower than with MDRD: 69,78 + or - 18,53 ml/min/1.73 m(2) (p<0.001). Age was correlated with the difference between CG and MDRD (coefficient of correlation: 0,59; r(2):0,34; p<0,001). Moderate agreement was obtained between MDRD with CG (k coefficient = 0.37; CI95%=0.19-0.55) and MDRD with 24-CC (k coefficient = 0.39; CI95% = 0.22-0.57)., Conclusions: The MDRD and CG formulas to estimate the FGR in people aged 75 and older are not interchangeable. There are needed specific studies in old people to establish the most precise formula., (Copyright 2009 Elsevier España, S.L. All rights reserved.)

Objective: The aim of this study was to determine the risk of mortality, functional decline and institutionalization, as well as the hospital cost associated with patients with methicillin-resistant Staphylococcus aureus (MRSA) infection in an Acute Geriatric Unit (AGU)., Material and Methods: We retrospectively gathered data on patients admitted to the AGU over a 5-year period (from 1/1/2001 to 1/1/06). Mortality, institutionalization, functional impairment at discharge, length of hospital stay, and hospital costs were compared between patients with and without MRSA. MRSA infection was documented by the microbiology department using culture and antibiogram, as well as by clinical diagnosis of hospital infection registered in the medical record., Results: Data were obtained from 47 patients with MRSA (mean age 86.15+/-5.5 years) and from 4281 patients without MRSA (mean age 85.25+/-6 years). MRSA-infected patients had higher mortality (25.5% vs. 7.7%, p<0.001), worse functional status at discharge (Barthel index 39.43+/-33.05 vs. 55.24+/-34.99, p<0.01) and more frequent institutionalization (29% vs. 9%, p<0.001). Longer length of hospital stay (22.15+/-13.67 vs. 10.64+/-7.69 days, p<0.001) and higher hospital cost per patient (7517.71+/-4639.59 vs 3611.21+/-2609.98 euro, p<0.001) were also observed. In the multivariate analysis adjusted by age, sex, and baseline functional and cognitive status, MRSA infection was independently associated with higher mortality (OR=3.92; 95% CI=1.95-7.86), worse functional status at discharge (OR=2.48; 95% CI=1.22-5.01), institutionalization at discharge (OR=6.50; 95% CI=2.60-12.22), and substantial increase in length of hospital stay (Beta coefficient=11.55 days; 95% CI=9.32-13.75)., Conclusion: MRSA infection in the AGU is associated with higher mortality, worse functional status at discharge and a higher incidence of institutionalization, as well as significantly longer length of stay and higher hospital costs.

We present four cases of patients admitted to an acute geriatric unit with a presumptive diagnosis of bronchiolitis obliterans organizing pneumonia between 2001 and 2005. Because of the difficulty of performing transbronchial biopsy in these patients, who showed a variety of clinical presentations, the suspected diagnosis was based on clinical and radiological findings and a favorable response to empirical corticosteroid treatment. The clinical and radiological features of this entity in the elderly, as well as the therapeutic options, are reviewed.

Videokeratography is a useful method for postoperative evaluation of photorefractive keratectomy (PRK). The aim of this study was to evaluate the percentage of decentration, the relationship between decentration relative to the corneal apex, postoperative astigmatism, and the relationship between decentration and best corrected Snellen visual acuity. Using the TMS-1 videokeratoscope we analyzed the corneal topography of 175 myopic eyes treated with PRKs ranging between -1.30 D and -15.90 D, performed using the Summit UV200 193-nanometer excimer laser, before and at least 6 months after the procedure. Concerning centration, 122 eyes (75%) were centered within 0.5 mm and 53 eyes (30.3%) were decentered more than 0.5 mm, with nine eyes (5.1%) being decentered more than 1 mm. There was a correlation between best spectacle corrected visual acuity and the amount of decentration. The higher attempted corrections showed greater amounts of decentration. Postoperative induced astigmatism was greater in the group with the highest decentration (1.81 +/- 2.64 D) than in the one with lower decentration (0.09 +/- 0.50 D). The most severe complication induced by a decentered PRK is an increase of postoperative astigmatism.