Environmental regulations on bacterial abundance in the South China Sea inferred from regression models.

Bacteria play a critical role in carbon cycling and nutrient remineralization. To reveal potential mechanisms controlling bacterial abundance in the upper 200 m of the South China Sea (SCS), the generalized linear model (GLM), generalized additive model (GAM) and generalized boosted model (GBM) were constructed to address the relationship between bacterial abundance and environmental factors, including geographical variables, biotic variables and water chemistry. GAM and GBM were found suitable for modeling bacterial abundance in the SCS. The predictive performance of GBM was superior to GLM and GAM for bacterial distribution. In addition, bacterial abundance predicted by GBM from environmental parameters was highly consistent with the observations, indicating that GBM was robust to predict bacterial abundance from environmental parameters. Furthermore, the key environmental factors modulating the horizontal and vertical distribution of bacteria were determined based on models. Horizontally, surface bacterial abundance decreased from onshore to offshore, which was primarily regulated by salinity and chlorophyll-a. Vertically, bacterial abundance decreased with depth. Chlorophyll-a was primarily responsible for vertical variability in bacterial abundance in the upper 100 m, where temperature was higher than the optimum temperature (21 °C) for bacterial growth. In contrast, temperature was a dominant factor regulating bacterial abundance below 100 m, where temperature was below 21 °C and positively correlated with BA. Viruses and nutrients played less important roles in regulating bacterial abundance than chlorophyll-a and temperature in the SCS. Our models elucidated environmental regulations on bacterial abundance, which was helpful for us to understand bacterial carbon cycling in the SCS.

Effect of Ocean Acidification on Bacterial Metabolic Activity and Community Composition in Oligotrophic Oceans, Inferred From Short-Term Bioassays.

Increasing anthropogenic CO2 emissions in recent decades cause ocean acidification (OA), affecting carbon cycling in oceans by regulating eco-physiological processes of plankton. Heterotrophic bacteria play an important role in carbon cycling in oceans. However, the effect of OA on bacteria in oceans, especially in oligotrophic regions, was not well understood. In our study, the response of bacterial metabolic activity and community composition to OA was assessed by determining bacterial production, respiration, and community composition at the low-pCO2 (400 ppm) and high-pCO2 (800 ppm) treatments over the short term at two oligotrophic stations in the northern South China Sea. Bacterial production decreased significantly by 17.1-37.1 % in response to OA, since bacteria with high nucleic acid content preferentially were repressed by OA, which was less abundant under high-pCO2 treatment. Correspondingly, shifts in bacterial community composition occurred in response to OA, with a high fraction of the small-sized bacteria and high bacterial species diversity in a high-pCO2 scenario at K11. Bacterial respiration responded to OA differently at both stations, most likely attributed to different physiological responses of the bacterial community to OA. OA mitigated bacterial growth efficiency, and consequently, a larger fraction of DOC entering microbial loops was transferred to CO2.

Response of Bacterial Metabolic Activity to the River Discharge in the Pearl River Estuary: Implication for CO2 Degassing Fluxes.

Bacterial production (BP), respiration (BR) and growth efficiency (BGE) were simultaneously determined along an environmental gradient in the Pearl River Estuary (PRE) in the wet season (May 2015) and the dry season (January 2016), in order to examine bacterial responses to the riverine dissolved organic carbon (DOC) in the PRE. The Pearl River discharge delivered labile dissolved organic matters (DOM) with low DOC:DON ratio, resulting in a clear gradient in DOC concentrations and DOC:DON ratios. BP (3.93-144 µg C L-1 d-1) was more variable than BR (64.6-567 µg C L-1 d-1) in terms of the percentage, along an environmental gradient in the PRE. In response to riverine DOC input, BP and the cell-specific BP increased; in contrast, the cell-specific bacterial respiration declined, likely because labile riverine DOC mitigated energetic cost for cell maintenance. Consequently, an increase in bacterial respiration was less than expected. Our findings implied that the input of highly bioavailable riverine DOC altered the carbon portioning between anabolic and catabolic pathways, consequently decreasing the fraction of DOC that bacterioplankton utilized for bacterial respiration. This might be one of the underlying mechanisms for the low CO2 degassing in the PRE receiving large amounts of sewage DOC.

Spatiotemporal Variability in Phosphorus Species in the Pearl River Estuary: Influence of the River Discharge.

Phosphorus was the stoichiometrically limiting nutrient in the Pearl River Estuary (PRE). In order to examine how the river discharge regulates phosphorus dynamics in the PRE, the concentrations of dissolved inorganic phosphorus (DIP) and organic phosphorus (DOP), particulate inorganic phosphorus (PIP) and organic phosphorus (POP) in the water column were determined in May 2015 (spring), August 2015 (summer) and January 2016 (winter). Our results showed that all types of phosphorus were significantly lower in spring and summer than in winter. The Pearl River discharge input played an important role in regulating phosphorus dynamics. Strong vertical mixing in winter resulted in high levels of total particulate phosphorus (1.50 ± 0.97 µM) and dissolved phosphate (DIP: 1.44 ± 0.57 µM, DOP: 0.58 ± 0.42 µM) at the surface. On the other hand, the river discharge input created stratification in spring and summer, favoring the settlement of suspended particulate matter and enhancing light levels. This promoted phytoplankton growth, which was responsible for a DIP drawdown of 0.43 ± 0.37 µM in May and 0.56 ± 0.42 µM in August at the surface. Additionally, stratification restricted the bottom phosphorus replenishment. Our findings provided an insight into processes causing stoichiometric P limitation in the PRE.

Boosting Catalytic Performance of Metal-Organic Framework by Increasing the Defects via a Facile and Green Approach.

The control of defects in crystalline materials has long been of significance since the defects are correlated with the performances of the materials. Yet such control remains a challenge for metal-organic frameworks (MOFs), which are usually well-crystallized under hydro-/solvothermal conditions. In this contribution, we demonstrate for the first time how to increase the defects of MOF via a facile and green approach as exemplified in the context of solvent-free synthesis of UiO-66(Zr). Such increase of defects leads to drastic enhancement of catalysis performance when compared to UiO-66(Zr) prepared from conventional hydro-/solvothermal synthesis. Our work therefore not only opens a new door for boosting the catalytic activities of MOFs but also contributes a new approach to control the defects in crystalline materials for various applications.