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Purpose: The main factors governing iron-bound organic carbon (Fe-bound OC) formation under different environmental conditions have been less well identified. In this study, we investigated the distribution of Fe-bound OC and the influencing factors in soils of a typical subtropical mountain ecosystem. Materials and methods: Soil samples at different altitudes (402–1653 m) and depths (0–20 cm, 20–40 cm, and 40–60 cm) in the Nanling Mountains of southern China were collected. The influence of soil property indicators and microbial diversity characteristics on the formation of Fe-bound OC was explored. Results: The Fe-bound OC content in the soils ranged from 1.40 ± 1.32 g/kg to 15.09 ± 3.57 g/kg, accounting for an average of 23.49 ± 13.03% of the total organic carbon (TOC), and generally increased with altitude and exhibited significantly higher values at the soil depth of 0–20 cm, while its proportion to TOC (fFe-OC) gradually increased as soil depth increased. The more significant indicators in explaining the variation of Fe-bound OC include TOC, particulate organic carbon (POC), free iron oxides (Fed), amorphous iron oxides (Feo), complexed iron oxides (Fep), Feo/Fed, pH, Ca, sand and silt. Feo/Fed was strongly linearly correlated with Fe-bound OC, thus can be a good predictor of Fe-bound OC content. RDA analysis showed that Fe-bound OC/fFe-OC had a relatively closer correlation with bacterial phyla including Proteobacteria, Actinobacteriota, Chloroflexi and Firmicutes. Conclusions: The attitude gradient and soil depth significantly influence the content of Fe-bound OC. The distribution of Fe-bound OC exhibited a certain correlation with both biotic and abiotic factors. [ABSTRACT FROM AUTHOR]

To clarify dynamic changes of organelle microstructures in Chlorella pyrenoidosa cells during photosynthetic growth with CO2 fixation, three-dimensional (3D) organelle microstructures in three growth periods of meristem, elongation, and maturity were quantitatively determined and comprehensively reconstructed with focused ion beam scanning electron microscopy (FIB-SEM). The single round-pancake mitochondria in each cell split into a dumbbell and then into a circular ring, while the barycenter distance of mitochondria to chloroplast and nucleus was reduced to 45.5% and 88.3% to strengthen energy transfer, respectively. The single pyrenoid consisting of a large part and another small part in each chloroplast gradually developed to a mature state in which the two parts were nearly equal in size. The nucleolus progressively became larger with euchromatin replication. The number of starch grains gradually increased, but the mean grain volume remained nearly unchanged. [ABSTRACT FROM AUTHOR]