Influences of phosphate addition on fungal weathering of carbonate in the red soil from karst region

Simulation of organic acid migration and transformation in Mangrove soils based on soil column experiments

The practice of directly applying unfermented and decomposed organic matter to plants is rare in the growth process of terrestrial plants. The organic matter content at the discharge outlet of shrimp ponds is usually high. Therefore, it is necessary to collect soil from the discharge outlet of shrimp ponds and simulate the migration and transformation pathways of organic acids and related metabolic microorganisms in soil of mangrove wetlands through laboratory soil columns and the HYDRUS-1D model. Results showed that the content of oxalic acid remained relatively stable in the soil column at different depths, citric acid settled downward along the vertical direction, the concentration of acetic acid in the depth range of 30-50 cm increased. The organic acids formed insoluble or slightly soluble precipitates in the form of organic acid calcium, the organic acids in 40-50 cm were completely neutralized on the 18th day. The abundance of acid-producing Acinetobacter Johnsonii increased during the later stages of anaerobic acidification and disappeared after the addition of Ca(OH)2. The results of HYDRUS-1D simulation showed that the adsorption, deposition and transport of organic acids in the mangrove wetland were poor, the results of vertical infiltration modelling were in agreement with the soil column experiments.

Native mixed microbe inoculants (M1H) optimize soil health to promote Cajanus cajan growth: the soil fungi are more sensitive than bacteria

Microbial inoculant is widely used in plant growth and crop production. However, the effect of native mixed microbial inoculants on soil microbiota and plant growth remain to be elucidated. Here, we used pot experiment for 5 months to determine the microbial inoculants treatments with growth-promoting effect on Cajanus cajan, such as M1P (Serratia marcescens) treatment and M1H treatment: the mixture of M1P and M45N (Paenibacillus polymyxa), and investigate the effect of these inoculants on the capacity of soil nutrients and rhizosphere microbiomes in promoting C. cajan growth. Further, the adaptability of these strains to environmental stress (temperature and pH) was determined by using stress-resistant growth experiment. The results showed that M1H treatment resulted in soil nutrients consumption and led to substantial alterations in the microbial community that were more effective in promoting C. cajan growth. The enhanced plant growth observed with M1H inoculation may be due to its impact on the soil micro-environment, particularly through increasing beneficial genera (e.g., Cunninghamella, Mortierella, Chryseolinea, and Bacillus) and decreasing potential genera (e.g., Zopfiella and Podospora). In addition, at the genus level (top 10), the effect of M1H inoculation on soil fungal community was higher than that of bacteria, which shows that the change of soil fungal community after M1H inoculation was more sensitive than that of bacteria. Spearman correlation analysis further revealed that the abundance of Cunninghamella, Mortierella, Chryseolinea, Zopfiella and Podospora were the key factors affecting C. cajan growth. Moreover, FUNGuild function prediction clearly indicated distinct differences in the fungal functions of CK, MIP and M1H treatment, in which a lower relative abundance of saprotroph fungi in M1H treatment compared to CK, these results may confirmed the possibility of decreasing the abundance of Zopfiella and Podospora under M1H treatment. Taken together, our findings highlight the role of M1H inoculant in promoting C. cajan growth and ameliorating soil health, and providing valuable insight of using native mixed microbial inoculants to cultivate C. cajan and optimize soil micro-environment.

Stability of lead immobilization by Aspergillus niger and fluorapatite under different pH conditions

The combination of Aspergillus niger (A. niger) and fluorapatite (FAp) has been applied in lead (Pb) immobilization. However, the different pH can affect the stability of the immobilized Pb minerals. This experiment explored the stability of Pb immobilization by A. niger and FAp under different pH conditions. A. niger can grow normally in a pH range from 2.5 to 6.5 conditions. Meanwhile, more than 99 % of Pb cations were removed by A. niger and FAp under a pH range from 3.5 to 6.5. More importantly, only less than 1.23 % Pb was released again under pH 3.5-5.5. The strong acidic conditions (pH 1.5) inhibited the growth of A. niger and caused a lower Pb removal ratio of 37.86 %. In addition, the formed minerals of lead oxalate and lead/calcium oxalate coprecipitate dominate the Pb remediation by A. niger and FAp. The pH > 5.5 condition could decrease the stability of Pb immobilization via the low calcium/lead ratios. The combination of A. niger and FAp shows great potential in Pb remediation at a pH range from 3.5 to 5.5, suitable for fungal growth and Pb minerals stability. This research provides new insight into Pb remediation by phosphate-solubilizing fungi and FAp in various acidic environments.

Heavy metal phytoremediation potential of Cosmos bipinnatus for use in chromium-contaminated regions of Dravyavati River, Jaipur, Rajasthan, India

Due to their toxicity and permanence, heavy metals pose a significant threat as pollutants. Metals leach into soil from human activities including mining, manufacturing, and farming. Phytoremediation involves removing contaminants from soils using herbaceous plants and trees; it is a cost-effective, non-invasive, and aesthetically pleasing technique. The current research investigated the phytoremediation capacity of Cosmos bipinnatus in soil that had been polluted with chromium (Cr) along the Dravyavati River in Jaipur. The plant uptake of heavy metals was studied for 20, 40, 60, and 80 days of growth in pot and field experiments using atomic absorption spectrophotometry (AAS) to analyze residual heavy metal concentrations in the plant's shoots and roots. Chromium exhibited variation in total absorption by plants depending on the type of treatment. Using the translocation factor, it was observed that after 80 days in Nala soil 80% + wheat husk 20% (H2), the plant's capacity to absorb Cr was equivalent and considerably higher. Nola soil 80% + wheat husk 20%; H2, and Nala soil 70% + wheat husk 30%; H3 were among the treatments with TFs more than 1; however, the BCF values were all lower than 1. Additionally, root uptake was greater than shoot uptake. Thus, at H3 and H2 treatments, C. bipinnatus was determined to be an efficient phytoextractor for Cr, but at other concentrations, it acted as an excluder.

Phosphate-solubilizing fungus (PSF) - mediated phosphorous solubilization and validation through Artificial intelligence computation

Phosphate-solubilizing fungus (PSF) strain alaromyces funiculosus was investigated for phosphorus solubilization, utilizing a range of pH levels and phosphate sources, followed by data confirmation through artificial intelligence modeling. T. funiculosus strain was exposed to five different phosphate sources [Ca3(PO4)2, FePO4, CaHPO4, AlPO4, and phytin] at different pH levels (4.5, 5.5, 6.5, 7.0, and 7.5). ANOVA, Pareto charts, and normal plots were used for analyzing the data. Artificial intelligence-based multilayer perceptron (MLP), random forest (RF) and extreme gradient boosting (XGBoost) models were used for data validation and prediction. Five-fold more phosphate (P) solubility by T. funiculosus was registered as compared to the control. The maximum soluble P was found at pH 4.5 (318324 ppb) and CaHPO4 (444045 ppb). Combination of phytin × 4.5 pH yielded the highest dissolved phosphorus (1537988 ppb), followed by 127458 ppb from the control × 4.5 pH. Pareto chart and normal plot analysis showedthe negative impact of pH (B), pH × F/C (fungus/control) × P-Source (ABC), and F/C (A) factor. Whereas pH × P-Source (AC) and P-Source (C) has positive impact on P solubility. The maximum R2 scores showed the order of RF (0.944) > MLP (0.938) > XGBoost (0.899). T. funiculosus strain has a grain potential for sustainable use for different types of phosphate sources. Application AI/ML models based on different performance metrics predicted the validated the attained results. In future research, it is recommended to check the efficacy of developed strategy under field conditions and to check the impact on soil and plant.

Lead remediation by geological fluorapatite combined with Penicillium Oxalicum and Red yeast

Phosphate solubilizing fungi Penicillium oxalicum (POX) and Red yeast Rhodotorula mucilaginosa (Rho) have been applied in Pb remediation with the combination of fluorapatite (FAp), respectively. The secretion of oxalic acid by POX and the production of extracellular polymers (EPS) by Rho dominate the Pb remediation. In this study, the potential of Pb remediation by the fungal combined system (POX and Rho) with FAp was investigated. After six days of incubation, the combination of POX and Rho showed the highest Pb remove ratio (99.7%) and the lowest TCLP-Pb concentration (2.9 mg/L). The EPS combined with POX also enhanced Pb remediation, which has a 99.3% Pb removal ratio and 5.5 mg/L TCLP-Pb concentration. Meanwhile, Rho and EPS can also stimulate POX to secrete more oxalic acid, which reached 1510.1 and 1450.6 mg/L in six days, respectively. The secreted oxalic acid can promote FAp dissolution and the formation of lead oxalate and pyromorphite. Meanwhile, the EPS produced by Rho can combine with Pb to form EPS-Pb. In the combined system of POX + Rho and POX + EPS, all of the lead oxalate, pyromorphite, and EPS-Pb were observed. Our findings suggest that the combined application of POX and Rho with FAp is an effective approach for enhancing Pb remediation.

The Regulation of Phosphorus Release by Penicillium chrysogenum in Different Phosphate via the TCA Cycle and Mycelial Morphology

Phosphate-solubilizing fungi (PSF) efficiently dissolve insoluble phosphates through the production of organic acids. This study investigates the mechanisms of organic acid secretion by PSF, specifically Penicillium chrysogenum, under tricalcium phosphate (Ca3(PO4)2, Ca-P) and ferric phosphate (FePO4, Fe-P) conditions. Penicillium chrysogenum exhibited higher phosphorus (P) release efficiency from Ca-P (693.6 mg/L) than from Fe-P (162.6 mg/L). However, Fe-P significantly enhanced oxalic acid (1193.7 mg/L) and citric acid (227.7 mg/L) production by Penicillium chrysogenum compared with Ca-P (905.7 and 3.5 mg/L, respectively). The presence of Fe-P upregulated the expression of genes and activity of enzymes related to the tricarboxylic acid cycle, including pyruvate dehydrogenase and citrate synthase. Additionally, Fe-P upregulated the expression of chitinase and endoglucanase genes, inducing a transformation of Penicillium chrysogenum mycelial morphology from pellet to filamentous. The filamentous morphology exhibited higher efficiency in oxalic acid secretion and P release from Fe-P and Ca-P. Compared with pellet morphology, filamentous morphology enhanced P release capacity by > 40% and > 18% in Ca-P and Fe-P, respectively. This study explored the strategies employed by PSF to improve the dissolution of different insoluble phosphates.

Shift in rhizospheric and endophytic microbial communities of dominant plants around Sunit Alkaline Lake

Alkaline lakes are a special type of extreme saline-alkali ecosystem, and the dominant plants store a large number of microbial resources with salinity-tolerant or growth-promoting properties in the littoral zones. In this study, high-throughput sequencing technology and molecular ecological networks were used to analyze the bacteria and fungi from different rhizocompartments of three dominant plants along the salinity gradient in the littoral zones of Sunit Alkali Lake. The study found that fungal communities were more tolerant of environmental abiotic stress, and salinity was not the main environmental factor affecting the composition of microbial communities. Mantel test analysis revealed that SOC (soil organic carbon) was the primary environmental factor affecting the rhizosphere bacterial community as well as the rhizosphere endophyte bacteria and fungi, while CO₃^2- (carbonate ions) had a greater impact on the rhizosphere fungal communities. In addition, keystones identified through the associated molecular network play an important role in helping plants resist saline-alkali environments. There were significant differences in the metabolic pathways of microorganisms from different rhizocompartments predicted by the PICRUSt2 database, which may help to understand how microorganisms resist environmental stress. This study is of great importance for understanding the salt environments around alkaline lakes and excavating potential microbial resources.

Machine learning: An effective technical method for future use in assessing the effectiveness of phosphorus-dissolving microbial agroremediation

The issue of agricultural pollution has become one of the most important environmental concerns worldwide because of its relevance to human survival and health. Microbial remediation is an effective method for treating heavy metal pollution in agriculture, but the evaluation of its effectiveness has been a difficult issue. Machine learning (ML), a widely used data processing technique, can improve the accuracy of assessments and predictions by analyzing and processing large amounts of data. In microbial remediation, ML can help identify the types of microbes, mechanisms of action and adapted environments, predict the effectiveness of microbial remediation and potential problems, and assess the ecological benefits and crop growth after remediation. In addition, ML can help optimize monitoring programs, improve the accuracy and effectiveness of heavy metal pollution monitoring, and provide a scientific basis for the development of treatment measures. Therefore, ML has important application prospects in assessing the effectiveness of microbial remediation of heavy metal pollution in agriculture and is expected to be an effective pollution management technology.

Lead remediation is promoted by phosphate-solubilizing fungi and apatite via the enhanced production of organic acid

Lead (Pb) is one of the most common heavy metal pollutants in the environment, which can indirectly or directly threaten human health. Lead immobilization by apatite can reduce the effectiveness of Pb cations via the formation of pyromorphite (Pyro). However, the formation of Pyro is always depending on the release of phosphorus (P) from apatite. Phosphate-solubilizing fungi (PSF) can secrete large amounts of organic acid to promote the release of P from apatite. Although the combination of PSF and apatite has shown a huge potential in Pb remediation, this pathway needs to be more attention, especially for organic acid secretion by PSF. This research mainly reviews the possible pathway to strengthen Pb immobilization by PSF and apatite. Meanwhile, the limitation of this approach is also reviewed, with the aim of a better stabilizing effect of Pb in the environment and promoting the development of these remediation technologies.

Evaluating the survival of Aspergillus niger in a highly polluted red soil with addition of Phosphogypsum and bioorganic fertilizer

Phosphate-solubilizing fungi (PSF) can enhance P release from phosphate minerals to immobilize heavy metals. However, this promotion substantially depends on their survival in highly polluted soils. The aim of this study was to investigate the survival of PSF after addition of phosphogypsum (PG) and bioorganic fertilizer (BF) in the soil with coexistence of multiple heavy metals, e.g., Pb, As, Cd, Sb, etc. Addition of typical PSF (Aspergillus niger) did not promote the formation of pyromorphite (the most stable form of Pb), possibly due to the buffering effect of the soil (the secreted oxalic acid was neutralized) and limited P supply. Meanwhile, despite that A. niger has high tolerance to heavy metal stress, its survival was significantly declined due to the deficiency of available P. It was also shown that PG, as the major by-product in phoschemical industry, still has relatively high available P compared with common natural soils. PG addition dramatically increased available P (up to 93.87 mg/kg) and the subsequent fungal growth. However, sole PG did not promote the formation of pyromorphite, probably as the abundant Fe²⁺ and Mn²⁺ prevented the contact between PO₄^3- and Pb²⁺ in the soil system. The enhanced soil respiration after addition of BF and PG confirmed the promoted microbial activity (elevated to 3465.58 μg C kg h^-1). This study showed PG's potential as P source for both microbial growth and heavy metal remediation in soil system. A combination of PG, A. niger, and BF can hence achieve long-term bioremediation of heavy metals.

Aspergillus niger Enhances Organic and Inorganic Phosphorus Release from Wheat Straw by Secretion of Degrading Enzymes and Oxalic Acid

To explore the mechanisms of crop straw degradation and phosphorus (P) release by phosphate-solubilizing fungi (PSF), a typical PSF Aspergillus niger (A. niger, ANG) was investigated for the degradation of wheat straw (WST) in this work. The results revealed that A. niger significantly increased wheat straw degradation (30%) compared with no A. niger treatment (7.7%). Meanwhile, more than 92% of total P was released from WST by A. niger, much higher than from WST treatment (69.5%). Although the ratios of inorganic P release between WST and WST + ANG treatments were similar (17.6 vs 19.7%), a significant difference occurred between their release of organic P, i.e., WST (51.9%) vs WST + ANG (72.5%). The high enzyme activity of β-1,4-glucanase and β-glucosidase produced by A. niger contributed to the wheat straw degradation and organic P release compared with no A. niger treatment. Oxalic acid secreted by A. niger dominated the release of inorganic P from WST. Our findings suggested that A. niger is an efficient microbial agent for crop straw degradation and P release, which could be a candidate in the pathway of straw return.

Soil bacterial communities of three types of plants from ecological restoration areas and plant-growth promotional benefits of Microbacterium invictum (strain X-18)

Microbial-assisted phytoremediation promotes the ecological restoration of high and steep rocky slopes. To determine the structure and function of microbial communities in the soil in response to changes in soil nutrient content, the bacterial communities of rhizospheric soil from three types of plants, i.e., Robinia pseudoacacia, Pinus massoniana, and Cynodon dactylon, were analyzed using Illumina sequencing technology. High-quality sequences were clustered at the 97% similarity level. The dominant genera were found to be RB41, Gemmatimonas, Sphingomonas, Bradyrhizobium, and Ellin6067. The Tukey HSD (honestly significant difference) test results showed that the abundance of RB41 and Gemmatimonas were significantly different among three types of plants (p < 0.01). The relative abundances of RB41 (13.32%) and Gemmatimonas (3.36%) in rhizospheric soil samples from R. pseudoacacia were significantly higher than that from P. massoniana (0.16 and 0.35%) and C. dactylon (0.40 and 0.82%), respectively. The soil chemical properties analyses suggested that significant differences in rhizospheric soil nutrient content among the three plant types. Especially the available phosphorus, the content of it in the rhizospheric soil of R. pseudoacacia was about 280% (P. massoniana) and 58% (C. dactylon) higher than that of the other two plants, respectively. The soil bacterial communities were further studied using the correlation analysis and the Tax4Fun analysis. A significant and positive correlation was observed between Gemmatimonas and soil nutrient components. Except total nitrogen, the positive correlation between Gemmatimonas and other soil nutrient components was above 0.9. The outcomes of these analyses suggested that Gemmatimonas could be the indicator genus in response to changes in the soil nutrient content. Besides, the genes involved in metabolism were the major contributor to soil nutrients. This study showed that soil nutrients affect the soil bacterial community structure and function. In addition, pot experiments showed that Microbacterium invictum X-18 isolated from the rhizospheric soil of R. pseudoacacia significantly improved soil nutrient content and increased R. pseudoacacia growth. A significant increase in the numbers of nodules of R. pseudoacacia and an increase of 28% in plant height, accompanied by an increase of 94% in available phosphorus was measured in the M. invictum X-18 treatment than the control treatment.

Responses of Labile Organic Carbon and Extractable Cadmium Fractions in an Agricultural Soil Following Long-Term Repeated Application of Pig Manure and Effective Microbes

Long-term pig manure addition has been widely applied in red soil to improve soil fertility. However, the influence of combined utilization of pig manure and effective microbes (EM) on soil organic carbon (SOC) and Cd are not well understood. This study conducted a 23-year (1996-2019) long-term fertilization field trial to investigate the changes of different fractions of SOC and Cd under chemical fertilization (CF), pig manure (PM), and pig manure with effective microbes (PM + EM) treatments in an agricultural soil of Jiangxi Province, South China. The results showed that the pig manure addition significantly enhanced the contents of SOC and Cd in the soils compared with the CF treatments. Furthermore, with the increment of SOC, the PM + EM treatment significantly increased the contents of soil microbial biomass carbon, dissolved organic carbon and easily oxidizable carbon compared with the pig manure application alone. Meanwhile, compared with the CF treatments, the EM addition significantly enhanced the exchangeable and oxidizable fractions of Cd, thus the potential Cd environment risk due to pig manure application should be carefully assessed.

Water vapor adsorption by dry soils: A potential link between the water and carbon cycles

Water vapor adsorption (WVA) by soil is a potential contributor to the water cycle in drylands. However, continuous in-situ estimates of WVA are still scarce and the understanding of its coupling with carbon cycle and ecosystem processes remains at an incipient stage. Here we aimed to (1) identify periods of WVA and improve the understanding of the underlying processes involved in its temporal patterns by using the gradient method; (2) characterize a potential coupling between water vapor and CO₂ fluxes, and (3) explore the effect of soil properties and biocrusts ecological succession on fluxes. We assumed that the nocturnal soil CO₂ uptake increasingly reported in those environments could come from WVA enhancing geochemical reactions involving calcite. We measured continuously during ca. 2 years the relative humidity and CO₂ molar fraction in soil and atmosphere, in association with below- and aboveground variables, over the biocrusts ecological succession. We estimated water vapor and CO₂ fluxes with the gradient method, and cumulative fluxes over the study. Then, we used statistical modelling to explore relationships between variables. Our main findings are (1) WVA fluxes during hot and dry periods, and new insights on their underlying mechanisms; (2) a diel coupling between water vapor and CO₂ fluxes and between cumulative fluxes, well predicted by our models; and (3) cumulative CO₂ influxes increasing with specific surface area in early succession stages, thus mitigating CO₂ emissions. During summer drought, as WVA was the main water source, it probably maintained ecosystem processes such as microbial activity and mineral reactions in this dryland. We suggest that WVA could drive the nocturnal CO₂ uptake in those moments and discuss biogeochemical mechanisms potentially involved. Additional research is needed to monitor soil water vapor and CO₂ uptake and separate their biotic and abiotic components as those sinks could grow with climate change.

Remediation of Lead-Contaminated Water by Red Yeast and Different Types of Phosphate

Rhodotorulamucilaginosa (Rho) can secrete large amounts of extracellular polymeric substances (EPS) to resist lead (Pb) toxicity. Phosphate is an effective material for the remediation of Pb. This study explored the Pb remediation by the combination of Rho and different types of phosphate in water. To do so, four phosphates, namely, ferric phosphate (FePO₄, Fe-P), aluminum phosphate (AlPO₄, Al-P), calcium phosphate [Ca₃(PO₄)₂, Ca-P], and phosphogypsum (PG) were employed along with Rho. Compared with Rho application, the addition of phosphate significantly promoted the secretion of EPS by Rho (21–25 vs 16 mg). The formed EPS-Pb contributes to the Pb immobilization in the combination of Rho and phosphate. After 6 days of incubation, Rho + phosphate treatments immobilized over 98% of Pb cations, which is significantly higher than Rho treatment (94%). Of all Rho + phosphate treatments, Ca-P and PG-amended Rho had higher secretion of EPS, resulting in higher Pb removal. Nevertheless, PG had the highest efficiency for Pb removal compared with other phosphates, which reached 99.9% after 6 days of incubation. Likewise, new Pb minerals, such as pyromorphite and lead sulfate, only appeared in Rho + PG treatment. Altogether, this study concludes on the combined application of Rho and phosphate as an efficient approach to promote Pb remediation, particularly using PG waste.

Penicillium simplicissimum NL-Z1 Induced an Imposed Effect to Promote the Leguminous Plant Growth

It is found effective for phytoremediation of the guest soil spraying method by adding microbes to promote the growth of arbor leguminous plant on a high and steep rock slope. However, its underlying mechanisms remain elusive. Here, some experiments were conducted to explore the multifunctions of Penicillium simplicissimum NL-Z1 on rock weathering, nodule growth, and beneficial microbial regulation. The results show that P. simplicissimum NL-Z1 significantly increased the release of phosphorus, potassium, calcium, and magnesium from the rock by 226, 29, 24, and 95%, respectively, compared with that of the control. A significant increase of 153% in Indigofera pseudotinctoria Matsum nodule biomass, accompanied by an increase of 37% in the leguminous plant biomass was observed in the P. simplicissimum NL-Z1 treatment than in the control treatment. Interestingly, even though P. simplicissimum NL-Z1 itself became a minor microbial community in the soil, it induced a significant increase in Mortierella, which, as a beneficial microbe, can promote phosphate-solubilizing and plant growth. The results suggest that P. simplicissimum NL-Z1 could induce an imposed effect to promote leguminous plant growth, which may be conducive to the development of the phytoremediation technique for high and steep rock slope. The study provides a novel thought of using the indirect effect of microbes, i.e., promoting other beneficial microbes, to improve soil environment.

A study of P release from Fe-P and Ca-P via the organic acids secreted by Aspergillus niger

Phosphate solubilizing fungi (PSF) have been widely applied to dissolve insoluble phosphates (IPs). However, the PSF usually demonstrates a different phosphate solubilizing capacity for various IPs. This study explored the mechanisms of Aspergillus niger for the dissolution of ferric phosphate (FePO₄, Fe-P), and tricalcium phosphate (Ca₃[PO₄]₂, Ca-P) regarding the tricarboxylic acid (TCA) cycle. Aspergillus niger has higher phosphorus (P) content released from Ca-P, reached the maximum value of 861 mg/L after seven days of incubation, compared with the 169 mg/L from Fe-P. Oxalic acid promoted the release of P from Ca-P through the formation of calcium oxalate. The presence of Fe-P can stimulate A. niger to secrete large amounts of citric acid, confirmed by the enhancement of citrate synthase (CS) activity. However, citric acid only promotes 0.5% of P released from Fe-P. Meanwhile, although oxalic acid still dominates the release of P from Fe-P, its abundance was significantly declined. In contrast, oxalic acid also shows a higher P release ratio in Ca-P than citric acid, i.e., 36% vs. 22%. This study points to the future usage of A. niger to dissolve IPs in soil required to enhance oxalic acid secretion.