Transformation of redox-sensitive to redox-stable iron-bound phosphorus in anoxic lake sediments under laboratory conditions.

Phosphorus (P) can be retained in mineral association with ferrous iron (Fe) as vivianite, Fe(II)3(PO4)2 ∙ 8 H2O, in lake sediments. The mineral is formed and remains stable under anoxic non-sulphidogenic conditions and, therefore, acts as a long-term P sink. In laboratory experiments under anoxic conditions, we investigated whether P adsorbed to amorphous Fe(III)-hydroxide functioned as a precursor phase of vivianite when added to different sediments as a treatment. The untreated sediments served as controls and were naturally Fe-rich (559 µmol/g DW) and Fe-poor (219 µmol/g DW), respectively. The solid P binding forms analysed by sequential extraction and X-ray diffraction were related to coinciding pore water analyses and the bacterial community compositions of the sediments by bacterial 16S rRNA gene amplicon sequencing. In the treatments, within a period of 40 d, 70 % of the redox-sensitive Fe(III)-P was transformed into redox-stable P, which contained vivianite. The mineral was supersaturated in the pore water, but the presence of Fe(III)-P functioning as a precursor was sufficient for measurable vivianite formation. The composition of the microbial community did not differ significantly (PERMANOVA, p = 0.09) between treatment and control of the naturally Fe-rich sediment. In the naturally Fe-poor sediment, the microbial community changed significantly (PERMANOVA, p = 0.001) in response to the addition of Fe(III)-P to the sediment. The freshly formed redox-stable P was not retransferred to a redox-sensitive compound by aeration for 24 h until 90 % O2 saturation was reached in the sediment slurry. We conclude that 1) Fe(III)-hydroxide bound P, resulting from oxic conditions at the sediment-water interface, is immobilised during anoxic conditions and stable even after re-oxygenation; 2) the process is feasible within the time scales of anoxic lake stratification periods; and 3) in relatively Fe-poor lakes, Fe dosing can provide excess Fe to form the precursor.

Evaluating diurnal variations in retinal perfusion using optical coherence tomography angiography.

BACKGROUND: Optical coherence tomography angiography (OCTA) is a non-invasive tool for imaging and quantifying the retinal and choroidal vasculature as well as perfusion state in healthy eyes. Choroidal perfusion is subject to diurnal variation, showing lowest perfusion in the morning and highest in the afternoon. In this index study, OCTA was used to investigate diurnal changes of the retinal perfusion in healthy adult eyes and to identify impacting factors since retinal perfusion is known to be mainly determined by autoregulatory mechanisms. METHODS: A prospective study was conducted on healthy volunteers, each of whom underwent repeated measurements of mean arterial pressure (MAP), intraocular pressure (IOP), macular volume (MV), subfoveal choroidal thickness (SFCT), foveal avascular zone (FAZ) and retinal perfusion of the superficial capillary plexus (SCP), deep capillary plexus (DCP) and full retina (FR) slab at 7 a.m. and 4 p.m. Possible influence of MAP or IOP on the retinal perfusion was evaluated. RESULTS: A total of 22 eyes of 22 participants (mean age 55.91 ± 14.84) were analysed. Significant diurnal changes from 7 a.m. to 4 p.m. were observed for MAP (p < 0.001) and SFCT (p = 0.017). The perfusion of SCP, DCP and FR as well as the size of the FAZ and the MV did not show significant fluctuation during the day. No significant correlation between MAP or IOP and retinal perfusion values were detectable. CONCLUSION: OCTA-based analysis of the retina in healthy adults demonstrated a steady perfusion of both plexus during the day, independently of changes in MAP. These findings support the theory of autoregulatory mechanisms of the retinal blood flow.

Chironomid larvae enhance phosphorus burial in lake sediments: Insights from long-term and short-term experiments.

Tube-dwelling macrozoobenthos can affect lake ecosystems in myriad ways, including changes in nutrient fluxes across the sediment-water interface. The pumping activity of chironomid larvae reinforces the transport of solutes between sediment and water. The transport of oxygen into the area surrounding the burrows generates oxidized compounds such as iron(oxy)hydroxides, which results in an additional phosphorus (P) sorption capacity similar to that of oxidized sediment surfaces. In the present study, the effect of the oxidized burrow walls of Chironomus plumosus on P binding capacity and P binding forms was tested in the laboratory using sediments with differing iron contents and varying numbers of chironomid larvae. In an additional long-term experiment, lake sediment naturally rich in iron was incubated under oxic conditions for 165 days, followed by a 3.5-year anoxic period. These experiments showed that: (1) Under oxic conditions the cumulative P uptake by sediments was dependent on larval densities. (2) The P that accumulated both at the sediment-water interface and in the oxidized burrow walls was mainly present as reductive soluble P (iron-bound P). Surprisingly, the amount of P released during the anoxic period in the long-term experiment was independent of the amount of P previously taken up during the oxic period since a portion of P was permanently retained in the sediment. The increase in alkaline soluble metal-bound P (NaOH-SRP) in formerly colonized sediments is a strong indication that the excessive P fixation by reductive soluble iron triggers the subsequent formation of stable iron phosphate minerals such as vivianite. Our study shows that P fixation that is induced by chironomid larvae is not always a completely reversible phenomenon, even after the emergence of the larvae and the re-establishment of anoxic conditions in the sediment.

Evaluating diurnal changes in choroidal sublayer perfusion using optical coherence tomography angiography.

PURPOSE: To investigate diurnal changes of choroidal sublayer perfusion in normal eyes and to identify influencing factors using optical coherence tomography angiography (OCTA). METHODS: A prospective study was conducted on healthy volunteers, each of whom underwent repeated measurements of subfoveal choroidal thickness (SFCT) via enhanced depth imaging (EDI) optical coherence tomography (OCT) as well as perfusion of choroidal vascular sublayers using OCTA at 7 a.m., 12 p.m., 4 p.m. and 8 p.m. Possible interactions between diurnal variations and other factors, such as mean arterial pressure (MAP), gender and age, were evaluated. RESULTS: A total of 22 eyes from 22 participants were analysed. Mean age of participants was 56 years. A significant pattern of diurnal variation was observed for SFCT (p < 0.001) as well as perfusion of Sattler's layer (SLP; p = 0.009) and Haller's layer (HLP; p = 0.003). SFCT demonstrated a linear decrease, being thicker in the morning (348 µm) and thinner in the evening (310 µm). Both, SLP and HLP showed a quadratic relation to time of day, increasing from morning (64% and 76%) to afternoon (66% and 77%), before decreasing again in the evening (64% and 76%). HLP changes were significantly associated with fluctuations of MAP (B = 0.0007; CI 0.0001-0.0014; p = 0.047). No significant differences with regard to gender were detectable. However, older participants (≥60 years) had fewer diurnal changes (p = 0.042). CONCLUSION: Optical coherence tomography angiography-based analysis of choroidal sublayer perfusion demonstrated significant diurnal variations. Therefore, it is important to account for time of day, when comparing longitudinal OCTA data.

Mapping diurnal variations in choroidal sublayer perfusion in patients with idiopathic epiretinal membrane: an optical coherence tomography angiography study.

BACKGROUND: Optical coherence tomography angiography (OCTA) is a non-invasive tool for imaging and quantifying the choroidal vasculature and perfusion state. In this index study, OCTA was used to investigate diurnal changes in choroidal sublayer perfusion in eyes with idiopathic epiretinal membrane (ERM) and to identify impacting factors. METHODS: A prospective study was conducted on volunteers with symptomatic ERM, each of whom underwent repeated measurements of subfoveal choroidal thickness (SFCT) using enhanced-depth imaging optical coherence tomography and perfusion of choroidal vascular sublayers using OCTA at 7 a.m., 12 p.m., 4 p.m., and 8 p.m. Possible interactions between diurnal variations and other factors, such as gender and age, were evaluated. RESULTS: A total of 21 eyes of 21 participants (mean age 72.43 ± 7.06 years) were analysed. A significant pattern of diurnal variation was observed for SFCT (p = 0.008) as well as perfusion of Haller's layer (HLP, p = 0.001). SFCT and HLP both demonstrated a quadratic relation to time of the day, decreasing from morning to afternoon, before increasing again in the evening. No significant differences with regard to gender or age were detectable. CONCLUSION: OCTA-based analysis of choroidal sublayer perfusion demonstrated significant diurnal variations in patients with symptomatic ERM, which are quite different from changes reported in healthy eyes. Therefore, it is important to account for time of day, when comparing longitudinal OCTA data.

Impact of correct anatomical slab segmentation on foveal avascular zone measurements by optical coherence tomography angiography in healthy adults.

PURPOSE: To evaluate the impact of correct anatomical slab segmentation on foveal avascular zone (FAZ) dimensions in the superficial capillary plexus (SCP) and deep capillary plexus (DCP) using optical coherence tomography angiography (OCTA). METHODS: Participants with healthy retinas were recruited, and 5 × 5 mm OCTA images were acquired using the Canon HS-100 Angio eXpert module. FAZ size was measured in automatically (AS, manufacturer-based) and manually (MS, anatomical-based) segmented OCTA slabs by two experienced graders. FAZ dimensions, inter-rater agreement, and correlation to demographic and retinal parameters were evaluated. RESULTS: A total of 38 eyes from 20 healthy adult subjects were included in this cross-sectional study. While in AS slabs, the FAZ in the SCP was smaller than in the DCP, in MS images, it was the opposite. MS had a relevant impact on inter-rater agreement of FAZ measurements in the SCP. The FAZ area in both plexus correlated inversely with the central retinal thickness (CRT), irrespective of the segmentation applied. Furthermore, an enlargement of FAZ size in the DCP with increasing age was found. Finally, the FAZ in female participants was significantly larger than in their male counterparts, regardless of the evaluated plexus and chosen segmentation. CONCLUSIONS: Correct anatomical slab segmentation has a significant impact on FAZ size measurements. Not adjusting the segmentation boundaries represents a significant source of error for measuring FAZ area and confounds comparisons across studies as well as OCTA devices.

Sedimentary Sulphur:Iron Ratio Indicates Vivianite Occurrence: A Study from Two Contrasting Freshwater Systems.

An increasing number of studies constrain the importance of iron for the long-term retention of phosphorus (P) under anoxic conditions, i.e. the formation of reduced iron phosphate minerals such as vivianite (Fe3(PO4)2⋅8H2O). Much remains unknown about vivianite formation, the factors controlling its occurrence, and its relevance for P burial during early sediment diagenesis. To study the occurrence of vivianite and to assess its relevance for P binding, surface sediments of two hydrologically contrasting waters were analysed by heavy-liquid separation and subsequent powder X-ray diffraction. In Lake Arendsee, vivianite was present in deeper sediment horizons and not in the uppermost layers with a sharp transition between vivianite and non-vivianite bearing layers. In contrast, in lowland river Lower Havel vivianite was present in the upper sediment layers and not in deeper horizons with a gradual transition between non-vivianite and vivianite bearing layers. In both waters, vivianite occurrence was accompanied by the presence of pyrite (FeS2). Vivianite formation was favoured by an elevated iron availability through a lower degree of sulphidisation and was present at a molar ratio of total sulphur to reactive iron smaller than 1.1, only. A longer lasting burden of sediments by organic matter, i.e. due to eutrophication, favours the release of sulphides, and the formation of insoluble iron sulphides leading to a lack of available iron and to less or no vivianite formation. This weakening in sedimentary P retention, representing a negative feedback mechanism (P release) in terms of water quality, could be partly compensated by harmless Fe amendments.