Nitrogen isotopes reveal independent origins of N2-fixing symbiosis in extant cycad lineages.

Cycads are ancient seed plants (gymnosperms) that emerged by the early Permian. Although they were common understory flora and food for dinosaurs in the Mesozoic, their abundance declined markedly in the Cenozoic. Extant cycads persist in restricted populations in tropical and subtropical habitats and, with their conserved morphology, are often called 'living fossils.' All surviving taxa receive nitrogen from symbiotic N2-fixing cyanobacteria living in modified roots, suggesting an ancestral origin of this symbiosis. However, such an ancient acquisition is discordant with the abundance of cycads in Mesozoic fossil assemblages, as modern N2-fixing symbioses typically occur only in nutrient-poor habitats where advantageous for survival. Here, we use foliar nitrogen isotope ratios-a proxy for N2 fixation in modern plants-to probe the antiquity of the cycad-cyanobacterial symbiosis. We find that fossilized cycad leaves from two Cenozoic representatives of extant genera have nitrogen isotopic compositions consistent with microbial N2 fixation. In contrast, all extinct cycad genera have nitrogen isotope ratios that are indistinguishable from co-existing non-cycad plants and generally inconsistent with microbial N2 fixation, pointing to nitrogen assimilation from soils and not through symbiosis. This pattern indicates that, rather than being ancestral within cycads, N2-fixing symbiosis arose independently in the lineages leading to living cycads during or after the Jurassic. The preferential survival of these lineages may therefore reflect the effects of competition with angiosperms and Cenozoic climatic change.

Carbon cycling: How much life has ever existed on Earth?

Carbon has cycled through Earth's biosphere for billions of years. New work estimates that life has recycled the equivalent of almost 100 times the Earth's entire carbon reservoir through the biosphere. This highlights life's global impact, providing a benchmark for habitable planets.

Biological diversification linked to environmental stabilization following the Sturtian Snowball glaciation.

The body fossil and biomarker records hint at an increase in biotic complexity between the two Cryogenian Snowball Earth episodes (ca. 661 million to ≤650 million years ago). Oxygen and nutrient availability can promote biotic complexity, but nutrient (particularly phosphorus) and redox dynamics across this interval remain poorly understood. Here, we present high-resolution paleoredox and phosphorus phase association data from multiple globally distributed drill core records through the non-glacial interval. These data are first correlated regionally by litho- and chemostratigraphy, and then calibrated within a series of global chronostratigraphic frameworks. The combined data show that regional differences in postglacial redox stabilization were partly controlled by the intensity of phosphorus recycling from marine sediments. The apparent increase in biotic complexity followed a global transition to more stable and less reducing conditions in shallow to mid-depth marine environments and occurred within a tolerable climatic window during progressive cooling after post-Snowball super-greenhouse conditions.

Mercury abundance and isotopic composition indicate subaerial volcanism prior to the end-Archean "whiff" of oxygen.

Earth's early atmosphere witnessed multiple transient episodes of oxygenation before the Great Oxidation Event 2.4 billion years ago (Ga) [e.g., A. D. Anbar et al., Science 317, 1903-1906 (2007); M. C. Koehler, R. Buick, M. E. Barley, Precambrian Res. 320, 281-290 (2019)], but the triggers for these short-lived events are so far unknown. Here, we use mercury (Hg) abundance and stable isotope composition to investigate atmospheric evolution and its driving mechanisms across the well-studied "whiff" of O2 recorded in the ∼2.5-Ga Mt. McRae Shale from the Pilbara Craton in Western Australia [A. D. Anbar et al., Science 317, 1903-1906 (2007)]. Our data from the oxygenated interval show strong Hg enrichment paired with slightly negative ∆199Hg and near-zero ∆200Hg, suggestive of increased oxidative weathering. In contrast, slightly older beds, which were evidently deposited under an anoxic atmosphere in ferruginous waters [C. T. Reinhard, R. Raiswell, C. Scott, A. D. Anbar, T. W. Lyons, Science 326, 713-716 (2009)], show Hg enrichment coupled with positive ∆199Hg and slightly negative ∆200Hg values. This pattern is consistent with photochemical reactions associated with subaerial volcanism under intense UV radiation. Our results therefore suggest that the whiff of O2 was preceded by subaerial volcanism. The transient interval of O2 accumulation may thus have been triggered by diminished volcanic O2 sinks, followed by enhanced nutrient supply to the ocean from weathering of volcanic rocks causing increased biological productivity.

Mandated Kindergarten Eye Examinations in a US Suburban Clinic: Is It Worth the Cost?

PURPOSE: The state of Illinois has required an examination by an optometrist or ophthalmologist prior to mandatory kindergarten since 2008. This requirement has allowed us to gather information regarding disease prevalence in a local suburban population. METHODS: A retrospective chart review was undertaken of kindergarten eye examinations performed at the Wheaton Eye Clinic between September 2008 and February 2017. Inclusion criteria included school eye examination as a reason for the visit, and a cycloplegic refraction was completed. RESULTS: Of 3612 patient charts identified, 1085 satisfied the inclusion criteria, of which 48% were female. The average age of the patients was 5.3 years (range, 3.8 to 6.7). Historical characteristics showed 143 (13%) were premature, 28 (3%) patients were autistic and 109 (10%) were developmentally delayed. On examination, 56 (5%) had <20/40 (WHO mild visual impairment) vision in better seeing eye, 34 (3%) had spherical equivalent refractive error in either eye > +3.50 diopters and one < -3.00 diopters. Fifty-eight patients (5%) were diagnosed with amblyopia and 101 patients (9.3%) were given glasses. Abnormal external exam finding was found in 218 (20%) while 16 (1.5%) had an abnormal fundus finding. Thirteen percent (146/1085) had a clinically relevant diagnosis in our examinations, with a cost of $1635/diagnosis. In total, our follow-up rate was 83% (78/94). CONCLUSION: Our rate of treatable ocular conditions discovered via state-mandated kindergarten eye examinations is similar to large-scale screening programs. The cost per condition found with full examinations was substantially higher; however, follow-up was more consistent.

Carbon cycle inverse modeling suggests large changes in fractional organic burial are consistent with the carbon isotope record and may have contributed to the rise of oxygen.

Abundant geologic evidence shows that atmospheric oxygen levels were negligible until the Great Oxidation Event (GOE) at 2.4-2.1 Ga. The burial of organic matter is balanced by the release of oxygen, and if the release rate exceeds efficient oxygen sinks, atmospheric oxygen can accumulate until limited by oxidative weathering. The organic burial rate relative to the total carbon burial rate can be inferred from the carbon isotope record in sedimentary carbonates and organic matter, which provides a proxy for the oxygen source flux through time. Because there are no large secular trends in the carbon isotope record over time, it is commonly assumed that the oxygen source flux changed only modestly. Therefore, declines in oxygen sinks have been used to explain the GOE. However, the average isotopic value of carbon fluxes into the atmosphere-ocean system can evolve due to changing proportions of weathering and outgassing inputs. If so, large secular changes in organic burial would be possible despite unchanging carbon isotope values in sedimentary rocks. Here, we present an inverse analysis using a self-consistent carbon cycle model to determine the maximum change in organic burial since ~4 Ga allowed by the carbon isotope record and other geological proxies. We find that fractional organic burial may have increased by 2-5 times since the Archean. This happens because O2 -dependent continental weathering of 13 C-depleted organics changes carbon isotope inputs to the atmosphere-ocean system. This increase in relative organic burial is consistent with an anoxic-to-oxic atmospheric transition around 2.4 Ga without declining oxygen sinks, although these likely contributed. Moreover, our inverse analysis suggests that the Archean absolute organic burial flux was comparable to modern, implying high organic burial efficiency and ruling out very low Archean primary productivity.

Exploring cycad foliage as an archive of the isotopic composition of atmospheric nitrogen.

Molecular nitrogen (N2 ) constitutes the majority of Earth's modern atmosphere, contributing ~0.79 bar of partial pressure (pN2 ). However, fluctuations in pN2 may have occurred on 107 -109  year timescales in Earth's past, perhaps altering the isotopic composition of atmospheric nitrogen. Here, we explore an archive that may record the isotopic composition of atmospheric N2 in deep time: the foliage of cycads. Cycads are ancient gymnosperms that host symbiotic N2 -fixing cyanobacteria in modified root structures known as coralloid roots. All extant species of cycads are known to host symbionts, suggesting that this N2 -fixing capacity is perhaps ancestral, reaching back to the early history of cycads in the late Paleozoic. Therefore, if the process of microbial N2 fixation records the δ15 N value of atmospheric N2 in cycad foliage, the fossil record of cycads may provide an archive of atmospheric δ15 N values. To explore this potential proxy, we conducted a survey of wild cycads growing in a range of modern environments to determine whether cycad foliage reliably records the isotopic composition of atmospheric N2 . We find that neither biological nor environmental factors significantly influence the δ15 N values of cycad foliage, suggesting that they provide a reasonably robust record of the δ15 N of atmospheric N2 . Application of this proxy to the record of carbonaceous cycad fossils may not only help to constrain changes in atmospheric nitrogen isotope ratios since the late Paleozoic, but also could shed light on the antiquity of the N2 -fixing symbiosis between cycads and cyanobacteria.

Transient surface ocean oxygenation recorded in the ∼2.66-Ga Jeerinah Formation, Australia.

Many paleoredox proxies indicate low-level and dynamic incipient oxygenation of Earth's surface environments during the Neoarchean (2.8-2.5 Ga) before the Great Oxidation Event (GOE) at ∼2.4 Ga. The mode, tempo, and scale of these redox changes are poorly understood, because data from various locations and ages suggest both protracted and transient oxygenation. Here, we present bulk rock and kerogen-bound nitrogen isotope ratios as well as bulk rock selenium abundances and isotope ratios from drill cores sampled at high stratigraphic resolution through the Jeerinah Formation (∼2.66 Ga; Fortescue Group, Western Australia) to test for changes in the redox state of the surface environment. We find that both shallow and deep depositional facies in the Jeerinah Formation display episodes of positive primary δ15N values ranging from +4 to +6‰, recording aerobic nitrogen cycling that requires free O2 in the upper water column. Moderate selenium enrichments up to 5.4 ppm in the near-shore core may indicate coincident oxidative weathering of sulfide minerals on land, although not to the extent seen in the younger Mt. McRae Shale that records a well-documented "whiff" of atmospheric oxygen at 2.5 Ga. Unlike the Mt. McRae Shale, Jeerinah selenium isotopes do not show a significant excursion concurrent with the positive δ15N values. Our data are thus most parsimoniously interpreted as evidence for transient surface ocean oxygenation lasting less than 50 My, extending over hundreds of kilometers, and occurring well before the GOE. The nitrogen isotope data clearly record nitrification and denitrification, providing the oldest firm evidence for these microbial metabolisms.

Biomass recycling and Earth's early phosphorus cycle.

Phosphorus sets the pace of marine biological productivity on geological time scales. Recent estimates of Precambrian phosphorus levels suggest a severe deficit of this macronutrient, with the depletion attributed to scavenging by iron minerals. We propose that the size of the marine phosphorus reservoir was instead constrained by muted liberation of phosphorus during the remineralization of biomass. In the modern ocean, most biomass-bound phosphorus gets aerobically recycled; but a dearth of oxidizing power in Earth's early oceans would have limited the stoichiometric capacity for remineralization, particularly during the Archean. The resulting low phosphorus concentrations would have substantially hampered primary productivity, contributing to the delayed rise of atmospheric oxygen.

Selenium isotopes record extensive marine suboxia during the Great Oxidation Event.

It has been proposed that an "oxygen overshoot" occurred during the early Paleoproterozoic Great Oxidation Event (GOE) in association with the extreme positive carbon isotopic excursion known as the Lomagundi Event. Moreover, it has also been suggested that environmental oxygen levels then crashed to very low levels during the subsequent extremely negative Shunga-Francevillian carbon isotopic anomaly. These redox fluctuations could have profoundly influenced the course of eukaryotic evolution, as eukaryotes have several metabolic processes that are obligately aerobic. Here we investigate the magnitude of these proposed oxygen perturbations using selenium (Se) geochemistry, which is sensitive to redox transitions across suboxic conditions. We find that δ82/78Se values in offshore shales show a positive excursion from 2.32 Ga until 2.1 Ga (mean +1.03 ± 0.67‰). Selenium abundances and Se/TOC (total organic carbon) ratios similarly show a peak during this interval. Together these data suggest that during the GOE there was pervasive suboxia in near-shore environments, allowing nonquantitative Se reduction to drive the residual Se oxyanions isotopically heavy. This implies O2 levels of >0.4 µM in these settings. Unlike in the late Neoproterozoic and Phanerozoic, when negative δ82/78Se values are observed in offshore environments, only a single formation, evidently the shallowest, shows evidence of negative δ82/78Se. This suggests that there was no upwelling of Se oxyanions from an oxic deep-ocean reservoir, which is consistent with previous estimates that the deep ocean remained anoxic throughout the GOE. The abrupt decline in δ82/78Se and Se/TOC values during the subsequent Shunga-Francevillian anomaly indicates a widespread decrease in surface oxygenation.

Anisometropia and amblyopia in nasolacrimal duct obstruction.

PURPOSE: To investigate an association of childhood nasolacrimal duct obstruction (NLDO) with anisometropia and amblyopia. METHODS: The medical records of patients from newborn to 6 years of age with a diagnosis of NLDO seen from 2000 to 2010 were retrospectively reviewed. Data collected included age at onset of NLDO, laterality of NLDO, cycloplegic refractive error, determination of clinically significant anisometropia (defined as ≥1 D), and diagnosis of amblyopia with amblyopia subtype (anisometropic vs other). RESULTS: A total of 1,218 patients with NLDO were included. Of these, 887 cases (72.8%) were unilateral; 331 (27.2%), bilateral. Anisometropia was found in 67 (7.6%) unilateral cases and 12 (3.6 %) bilateral cases on initial examination (χ2=5.48, P < 0.01). Same-sided unilateral NLDO was significantly associated with greater hyperopia in the anisometropia patients (χ(2) = 33.01, P < 0.001). Follow-up data were available for 482 NLDO patients and revealed an additional 26 patients with anisometropia for a total of 105 of 1218 (8.6%). Of the 482 patients, 28 (5.8%) developed amblyopia, 16 cases of which were purely attributable to anisometropia. Of 41 patients with anisometropia who were in the follow-up, 9 (22%) developed amblyopia. CONCLUSIONS: Anisometropia occurred at a greater rate in unilateral NLDO patients compared with bilateral NLDO patients and occurred at a greater rate in this NLDO cohort than expected in the general pediatric population. Several patients with anisometropia developed clinical amblyopia. Measurement of cycloplegic refraction in all NLDO patients at initial examination should be considered. Periodic follow-up is appropriate for patients with NLDO and anisometropia.

Childhood glaucoma.

The childhood glaucomas represent of variety of glaucoma subtypes that can cause serious and permanent visual damage. Early detection and treatment of infantile glaucoma is essential to preserve vision. The clinical signs and symptoms of infantile glaucoma are recognizable by nonophthalmologists but may be confused with other common clinical entities. Surgical and occasionally medical interventions can control raised intraocular pressure and thus prevent permanent glaucoma damage to the eye. Amblyopia is a common reason for permanent visual loss in this disease and needs to be treated aggressively as well.

A case of Moebius syndrome in association with Klinefelter syndrome.

PURPOSE: To describe an infant affected by Klinefelter syndrome, who also demonstrated clinical signs of Moebius syndrome. METHODS: A clinical case report. RESULTS: A male infant was born full-term to a healthy couple after an unremarkable pregnancy. Several dysmorphic features and generalized hypotonia were noted at birth. Chromosome study revealed a 47, XXY chromosome pattern, which is consistent with Klinefelter syndrome. The patient also demonstrated clinical findings of Moebius syndrome: bilateral horizontal gaze palsy, bilateral cranial nerve seven palsy, pointed tongue, pectoral muscle hypoplasia, and clubfeet. CONCLUSION: We report the first clinical case of a patient with Klinefelter syndrome who was also affected by Moebius syndrome. Although clinically intriguing, coexistence of the two syndromes most likely represents a chance occurrence.