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Article

Loofah Sponge Has a Potential Multifunctional Role for Enhanced Tetracycline Biodegradation: Carrier, Putative Nutrient Releaser and Solubilizer

by
Lei Yu
1,
Yujing Zheng
2 and
Jing Liang
2,*
1
The Open University of Jilin, Changchun 130022, China
2
College of Life Science, Jilin Agricultural University, Changchun 130118, China
*
Author to whom correspondence should be addressed.
Processes 2025, 13(11), 3567; https://doi.org/10.3390/pr13113567
Submission received: 29 September 2025 / Revised: 3 November 2025 / Accepted: 3 November 2025 / Published: 5 November 2025
(This article belongs to the Section Chemical Processes and Systems)
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Abstract

The microbial removal of antibiotics is an environmentally friendly solution to antibiotic contamination in water. However, the main limitations for its application are the difficulty of direct utilization of antibiotics by bacteria and incomplete removal. In this study, a strain of Bacillus thuringiensis ZY that removed tetracycline (TC) as a sole carbon source was applied. Strain ZY was able to remove 50 mg/L TC at an efficiency higher than 70%, while the removal efficiency was increased to 100% after the immobilization by Loofah (Lfr). Meanwhile, the removal time was shortened from 6 to 4.5 d. Compared with the free ZY, the TC removal efficiency of Lfr-ZY was significantly improved under various conditions (temperature, pH and NaCl concentration). The removal efficiency of Lfr-ZY was still higher than 50% after 11 cycles, with strong removal ability and stability. In addition, the enhancement of TC bio-removal by Lfr-ZY involved the combination of the protection, adsorption, detoxification, putative nutrient release and solubilization effects of Lfr. The promising results suggest that the Lfr-based strategy has the potential for solving the problems of a lack of nutrient substrate for TC removal and the inability to remove it completely.

1. Introduction

Tetracycline (TC) is an antibiotic with broad-spectrum antimicrobial activity and it has been widely used in pharmaceutical factories, livestock and aquaculture industries [1,2]. However, misuse of antibiotics can cause a range of serious environmental pollution impacts and pose potential risks to human health [3,4,5]. Therefore, there is an urgent need for a feasible and effective method to remove residual TC. The current methods for TC removal mainly include physical, chemical and biological techniques [6]. Among them, the biological method is an excellent means of green TC removal, because of its advantages of low product toxicity and economic and environmental protection [7]. For example, Arthrobacter nicotianae OTC-16 can remove TC in Luria-Bertani medium, and all of its removal products were found to be less toxic than the parent by toxicity assessment [8]. Alcaligenes sp. T17 can also remove TC to less toxic substances in LB medium [9]. Pankaj et al. also found that Serratia marcescens strain WW1 removes TC by biodegradation and the product is less toxic [10]. Similarly, strain Pycnoporus sp. SYBC-L10 has also been noted to biodegrade TC and attenuate its toxicity by secreting laccase [11]. However, due to the high toxicity of TC, it is difficult for microorganisms to directly consume it as a carbon source. Currently, most studies have been conducted to realize the degradation of antibiotics in LB medium or medium with extra added nutrients [12,13,14]. The additional provision of rich nutrients to maintain the survival and degradation ability of the strains would increase the cost of TC bioremediation. In addition, the lack of nutrients in wastewater makes it become difficult for microorganisms to survive, leading to extremely low TC biodegradation efficiency in practical applications. In addition, due to the complex structure of TC, microorganisms are not able to completely degrade it, which causes secondary pollution in the environment and difficulty in pursuing microbial reuse to remove pollutants [15,16,17]. Therefore, it is crucial to solve the problems of nutrient sources and complete degradation of TC in the process of microbial degradation.
Immobilization of microorganisms on carriers allows them to remain active and reusable. It was shown that the loading of Bacillus subtilis on biochar increased the degradation efficiency of tunicamycin from 37.39% to 56.16%, as compared to free bacteria [18]. The immobilized bacteria maintained high removal efficiency after several reuses. Pseudomonas stutzeri and Shewanella putrefaciens loaded on biochar for the removal of doxycycline have also pointed out that immobilization altered the growth of strains and increased the expression of resistance genes, improving the biodegradation efficiency. The well-developed porous structure of biochar provides excellent adsorption capacity for antibiotics, as well as space for microbial growth and reproduction, which is more conducive to increasing cell density and enhancing microbial degradation [19,20]. In addition, strain Alcaligenes sp. R3 immobilized by carboxymethylcellulose and polydopamine has increased the TC-degraded efficiency from 71.62% to 91.16% [21]. Currently, most studies usually focus on the adsorption of pollutants and protection of microorganisms by the carrier itself; few studies have excavated the other features of the carriers. However, microbial degradation of TC is an urgent need for solving the problem of nutrient lack and inability to remove TC completely. Therefore, the search for a multifunctional carrier that can supply nutrients to strains while protecting them, improving microbial viability and thus enhancing antibiotic removal is expected to be a realistic and meaningful topic.
Biomass is biocompatible and widely available at a low cost. Its surface is rich in oxygen- and nitrogen-containing functional groups [22], which can be used as adsorption sites for pollutants. In addition, biomass usually precipitates a large number of microbially available substances such as sugars, alcohols, and acids in liquids [23], which have the potential to be used as high-quality carriers. For example, the removal efficiency of Pseudomonas citronellolis for phenol was increased from less than 46% to 99% after biochar immobilization. It was proposed that, in addition to the role of adsorption, biochar can dissolve alkaline substances to neutralize the acids produced by metabolism and improve the degradation ability of the strain by maintaining the stability of the environment [24]. It has been suggested that humic substances in corn kernel extract could increase the expression level of the electron transport pathway, which in turn could increase the reduction rate of Cr(VI) [25]. Previously, studies have mainly focused on the application of these materials as physical carriers or scaffolds. This study proposed and preliminarily inferred that Lfr can serve as a triple-functionalized platform integrating “physical carrier”, “slow-release source of nutrients”, and “biosurfactants”. The objectives in this study were (1) to investigate the influence of strains that remove TC as the sole carbon source; (2) to immobilize the strain using Loofah (Lfr) as a carrier to remove TC, so that the removal efficiency could reach 100%; and (3) to explore and verify the potential of Lfr as an integrated platform with active functions to remove TC. This study could provide a theoretical basis for the efficient bioremediation of TC.

2. Materials and Methods

2.1. Materials

TC (99%) was purchased from Rhawn Chemical Reagents Co., Ltd. (Shanghai, China). KH2PO4, K2HPO4, MgSO4, NH4Cl, CaCl2, NaOH and HCl were purchased from China National Medicines Co., Ltd. (Beijing, China). Tryptone and yeast extract were from Hangzhou Best Biotechnology Co., Ltd. (Hangzhou, China). Agar was purchased from Beijing Obox Biotechnology Co. Ltd. (Beijing, China). NaCl was the product of Xilong Scientific Co., Ltd. (Chengdu, China). Lfr was purchased from Jiangxi Meier Loofah Co., Ltd. (Fuzhou, China). The TC-degrading strain Bacillus thuringiensis was provided by Jilin Agricultural University (Changchun, China).

2.2. Preparation of Lfr–ZY Complex

Lfr was washed with sterile water three times to fully remove impurities and then dried in an oven. After that, Lfr was cut into 1 cm × 1 cm × 1 cm, and immersed and stirred in 1 mol/L NaOH aqueous solution for 30 min. Then, the Lfr was rinsed with sterile water to neutrality, and dried. The strain was added into metal salt culture medium (MSM: 0.5 g/L MgSO4, 0.01 g/L CaCl2, 1.0 g/L KH2PO4, 1.0 g/L K2HPO4, 2 g/L NH4Cl) containing 50 mg/L TC, and incubated at 37 °C, pH 7, and 160 rpm. The suspension of the strain in the logarithmic growth phase was collected, and 1 g Lfr was sterilized at 121 °C for 15 min. After that, ZY suspension (100 mL) and Lfr were mixed and fixed at 30 °C and 120 rpm for 6 h to prepare the Lfr–ZY complex.
Different concentrations of bacterial solution were prepared and coated on the surface of agar. The number of colonies corresponding to each agar plate was recorded. The functional relationship between biomass and colony number was obtained. According to the decrease in biomass in the bacterial solution before and after fixation, the total amount of strain fixation on Lfr was calculated.

2.3. Removal of TC

The removal of TC through strain ZY and Lfr-ZY was investigated with the change in TC concentrations (50, 100, 150, and 200 mg/L) under 37 °C and 160 rpm. In addition, the effects of temperature (15, 28, 37, and 48 °C), pH (3, 5, 7, 9, and 11), and NaCl concentration (1, 3, 5, and 7%) on TC removal were examined. All treatments were replicated three times. The same medium was utilized without the addition of strain ZY to test the hydrolysis of TC. The optical density value (OD600) of cell suspensions was determined by the turbidimetric method and TC concentration was detected by UV spectrophotometry.

2.4. Determination of Antioxidant Enzymes

Superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) contents were determined by the reported method [26]. In the Supplementary Materials, see the antioxidant enzyme assay method for details.

2.5. Effects of Extract of Lfr on TC Removal

Lfr (10 g) was boiled in distilled water (1 L) for 50 min, and then filtered out. The remaining solution was the extract of Lfr. Strain ZY was inoculated into MSM containing Lfr extract and without MSM but only extract, respectively, then cultured at 37 °C and 160 rpm to remove 50 mg/L of TC. In addition, to confirm the role of extract in TC removal, Lfr extract was added to MSM without carbon source or nitrogen source, respectively, to monitor the growth of strains.
The composition and relative extract content: Lfr (1 g) was crushed and soaked in distilled water (100 mL), before being boiled for 50 min. After cooling, the extracts containing Lfr with and without strains were incubated at 37 °C and 160 rpm. Samples were collected at 1, 2, 3, 4, 5, 6, and 7 d. At each predetermined time point, we used a 0.45 μm filter membrane to filter the extract. Potassium hydrogen phthalate was used as the standard solution. The samples of extract solution after filtration were analyzed using a total organic carbon (TOC) analyzer. In addition, the supernatant of the extract was collected for analysis using High-Performance Liquid Chromatography–Mass Spectrometry (HPLC-MS).
Oleic acid (1%, 2%, 5%, 10%, 20% v/v) was added to MSM containing 50 mg/L TC; then, strain ZY was inoculated to explore the effect of oleic acid on TC removal.
MSM containing 50 mg/L TC was supplemented with malic acid, citric acid and oxalic acid as additional carbon sources. The strain ZY2 was inoculated to degrade TC under the above-mentioned condition. Samples were taken every 1 d to detect the remaining concentration of TC, and the growth of the strain was monitored. Three replicates were set for each group of experiments.

2.6. Adsorption of TC by Lfr

An amount of 1 g Lfr was put into MSM containing 50 mg/L of TC at pH 7, 37 °C, and 160 rpm. The samples were taken at regular intervals to monitor the change in TC content. All experiments were performed three times and the results were statistically processed.

2.7. Reusability

Lfr-ZY was added to MSM with 50 mg/L TC as the sole carbon source and incubated at pH 7, 37 °C, and 160 rpm. After complete removal of TC, the Lfr-ZY was fished out and re-put into MSM containing 50 mg/L of TC for removal, until the removed efficiency was decreased significantly. Each experiment was repeated three times.

2.8. Measurements

Scanning electron microscopic (SEM) images were obtained by applying a Hitachi Regulus 8220 field emission scanning electron microscope (Tokyo, Japan). FTIR data were collected on a Bruker IFS66 V FTIR spectrometer equipped with a DGTS detector (32 scans), using KBr pellets, and the spectra were recorded with a resolution of 4 cm−1 (Karlsruhe, Germany). The L5S UV-Vis spectrophotometer was used to measure the TC content and biomass of strains from Shanghai Yidian Analytical Instrument Co., Ltd. (Shanghai, China). Liquid chromatography–tandem mass spectrometry (LC-MS/MS) analysis was conducted using an AB Triple TOF 6600 mass spectrometer (AB SCIEX) and Agilent 1290 Infinity LC ultra-high-performance liquid chromatography system (Waldbronn, Germany).

3. Results and Discussion

3.1. Removal of TC by ZY and Lfr-ZY

Among the current methods for antibiotic removal, microbial removal has been promoted, because of its low-toxicity removal products, lack of secondary pollution, and low cost. Therefore, in this study, strain Bacillus thuringiensis ZY was used to remove TC as the sole carbon source. Strain ZY was able to remove 50 mg/L TC, as the sole carbon source, at an efficiency of 73% within 6 d (Figure 1a). TC presents a complex structure and high toxicity, meaning that most microorganisms cannot directly utilize TC and need to be removed in an environment containing other abundant carbon sources. It has been reported that Bacillus clausii T could remove 10 mg/L TC in LB medium [27]. Also, Pandoraea sp. XY-2 could remove TC only in LB medium [17]. In addition, it has been found that Ochrobactrum sp. KSS10 removes 25 mg/L oxytetracycline in an environment containing large amounts of sodium acetate [28]. The addition of extra nutrients would make bioremediation costly and difficult to apply practically.
Lfr is a cheap and readily available renewable biomass material. Lfr presents a network with interconnected voids, which facilitates microbial attachment to its surface [29]. The Lfr had a specific surface area and pore volume of 4.3 m2g−1 and 2.4 × 10−4 cm3g−1 [30], which was beneficial for strain immobilization and pollutant adsorption. In addition, the LFr surface has many hydroxyl functional groups, as shown in Figure S1, which can be used to fix microorganisms through hydrogen bonds and other interactions. After immobilization of ZY on Lfr, the OD600 of bacterial suspension was decreased by about 0.1 at 6 h (Figure S2a), which indicated that a certain amount of ZY had been immobilized on Lfr. In addition, the appearance of new peaks around 1743 cm−1, 1412 cm−1 and 1373 cm−1 in FIIR spectra, which belong to t–NH2 and C=O groups on strains, showed the successfully assembly of ZY on Lfr (Figure S1) [31,32]. The load amount of bacteria on Lfr was calculated to be about 1.21 × 109 cfu/g, according to the relationship between biomass and colony (Figure S2b).
TC is one of the common pollutants in the environment. Studies have reported that a variety of microbial species (e.g., bacteria, fungi, and algae) are successfully isolated to remove TC. However, numerous studies have shown that despite the abundance of microbial species, most of them are inhibited on removal of TC, under the stress of high concentrations of TC. The protection of microorganisms to further enhance their removal capacity is an effective strategy. In this study, the removal of LFr–ZY complex on TC was investigated. The effect of initial concentrations on the removal of TC by free ZY and Lfr-ZY is exhibited in Figure 1, showing that the removal efficiency was significantly promoted by Lfr-ZY compared with free ZY. Lfr-ZY could completely remove TC, whose removal time was shortened from 6 d to 4.5 d, at 50 mg/L TC. Currently, biological treatments of antibiotics have faced the problem of incomplete removal. For example, the removal efficiency of 30 mg/L TC by strain Klebsiella sp. SQY5 was 76.56% [33]. In the study of removal of TC, it has also been reported that strain Achromobacter (SG-4 and SA-1-2) could only remove 30 mg/L TC at efficiencies of 55.8% and 58.3%, neither of which completely remove TC [34]. In addition, it has been found that Penicillium commune (KU298470), Epicoccum nigrum (KC164754), Trichoderma harzianum (KU198601), Aspergillus terreus (KU214241), and Beauveria bassiana (KU198598) could remove oxytetracycline at efficiencies of 64.8–78.3% [35]. A removal efficiency of 53.72% for 37.3 mg/L TC has been achieved using a membrane reactor [36]. Similarly, it has been suggested that 70% TC could be removed by pig manure composting [37]. Herein, Lfr-ZY totally removed TC with a shorter removal time, which avoided residual TC. In addition, Lfr-ZY could remove TC from 100 mg/L to 10.5 mg/L, whose removal efficiency was 2.25 times than that of free ZY (Figure 1b). Upon further increasing the TC concentration to 150 and 200 mg/L, the free strain could not grow and remove TC (Figure 1a). The removal efficiency of TC by Lfr-ZY reached 77.8% and 64.1% at 150 and 200 mg/L, respectively, which were 7.7 and 6.4 times than that of free ZY (Figure 1b), suggesting that Lfr-ZY could improve the removal efficiency. Similarly, a novel immobilized microbial particle based on Alcaligenes sp. R3 and carboxymethyl cellulose and polydopamine have been constructed to remove TC at a removal efficiency of 91.16% for 50 mg/L TC, while the free bacteria were only 71.62% [38]. The removal of TC by IMPS was negatively correlated with TC concentration, when TC exceeds 150 mg/L. Compared with free bacteria, Lfr-ZY can not only remove high concentrations of TC, but also accelerate the removal rate. The good biocompatibility and rough surface of Lfr enable bacteria to better combine with it, which is conducive to the accumulation of local bacteria to remove TC. This study suggests that TC removal by Lfr-ZY is a potential route for biological treatment of TC wastewater.

3.2. Influences

pH is an extremely important indicator of the growth of microorganisms. Excessive acidity or alkalinity can cause changes in the charges carried by bio-macromolecules such as proteins and nucleic acids, thus affecting their biological activity [39]. In addition, TC would be converted to its isomers under some extreme pH values, increasing the toxicity of the environment, which contributes to the damage of microorganisms [40]. Therefore, certain protection for microorganisms under extreme pH conditions, such as loading on carriers, is more conducive to microbial removal of TC. In this study, the pH effect on TC removal by Lfr–ZY complex was explored. As shown in Figure 2a, the removal efficiency of TC was much higher than that of free ZY, under pH 3-11, suggesting that Lfr-ZY was more adaptable to pH changes. As shown in Figure 2b, the TC removal efficiencies of free ZY were 73.2% and 59.4% at pH 7 and 9, respectively, both of which were unable to completely remove TC. However, Lfr-ZY could totally remove TC under these conditions. It is worth noting that free ZY could not grow under extreme pH (pH 3, 5, and 11) to remove TC, the decrease of which only depended on hydrolysis. Interestingly, after immobilization by Lfr, it endowed ZY with the ability to remove TC under extreme pH values, which reduced the TC concentration from 50 mg/L to 26.8 mg/L, 21.4 mg/L, and 16.2 mg/L, at pH 3, 5, and 11, with removal efficiencies 2.01, 3.14, and 1.43 times higher than that of free ZY, respectively. The advantage of the rough surface of Lfr can encourage ZY to grow in large numbers on its surface, promoting the formation of biofilm as well as the expression of TC-degrading enzymes, so that the microorganisms can resist the damage caused by acidic and alkaline conditions.
Temperature is one of the most important factors in microbial growth. Unsuitable temperatures can disrupt cell membranes and enzyme activities, hindering nutrient uptake and metabolite secretion. In this study, the effect of temperature on the removal of TC by Lfr-ZY was investigated. As shown in Figure 2, the removal efficiency of TC was only 38.6% for free ZY at 15 °C. After immobilization, Lfr-ZY increased the removal efficiency to 59.1%, suggesting its capacity for low-temperature tolerance (Figure 2d). As the temperature was raised to 28 and 37 °C, the TC removal efficiency of free ZY was increased to 53.9% and 73.2%, respectively, whereas the removal efficiency of TC by Lfr-ZY was increased to 100%. This indicated that Lfr-ZY was more adaptable to temperature changes, as the carrier provided attachment space for strains, which was more favorable for biofilm formation and aggregation to avoid temperature change-induced enzyme damage [41]. It was also found that the removal efficiency of COD in oilfield wastewater by immobilized microorganisms is significantly higher than that of free microorganisms as the temperature changes [42].
TC-polluted wastewater is often accompanied by a large number of inorganic salts during the discharge process, which affects the removal of TC [43]. Therefore, the effect of NaCl concentrations on TC removal by Lfr-ZY was studied. Lfr–ZY complex had a higher NaCl tolerance than free ZY (Figure 2). Lfr-ZY increased the TC removal efficiency to 100% at 1% NaCl, which was 1.6 times higher than free ZY (61.1%). It is worth mentioning that after immobilization, ZY could bear a NaCl concentration of 3% with 79.3%TC removal efficiency, while free ZY could not survive. This could be attributed to the adsorption of salt ions by Lfr, which protected the bacteria from salt stress. Similarly, it has also been found that immobilized cells have better phenol removal efficiency under salt stress compared to free cells [44].

3.3. Cycling

Cycling is important when assessing the feasibility of the removal process for organic pollutants. In order to investigate the reusability of TC removal by Lfr-ZY, cycling was carried out. As shown in Figure 3, the removal efficiency of Lfr-ZY on 50 mg/L TC was still 100% after seven cycles; the total removal time was only delayed for 4 d. Further, the removal efficiency of TC was still higher than 50% after eleven cycles, indicating the excellent recycling performance of Lfr-ZY. To assess the structural stability of the Lfr during the initial and middle phases of the long-term test, SEM characterization was performed after three cycles. The SEM images showed that the Lfr remained largely intact after three cycles of operation (Figure S3), compared with the reported structure [45]. While the wrinkles of Lfr were decreased, there was no evidence of significant structural collapse or removal. This structural integrity is consistent with its ability to continue functioning as a stable microbial carrier and reaction platform. It is important to note that while the degradation performance was sustained over 11 cycles, the direct structural confirmation via SEM is limited to the 3-cycle time point. It has been investigated that the stability of honeysuckle residue-derived biochar-immobilized Bacillus cereus (HBCM) for chlortetracycline removal remains above 70% for three cycles [18]. In addition, a biochar loading Bacillus sp. SDB4 could remove 20 mg/L sulfamethoxazole with 100% removal efficiency in the first two cycles, while the removal efficiency of the fourth and fifth cycles decreases significantly [46]. Herein, Lfr-ZY presents high recycling potentials for TC removal.

3.4. Mechanism of Lfr Assisting ZY to Enhance TC Removal

3.4.1. Protection of ZY by Lfr

To realize the immobilization of microorganisms, bacteria with adhesion or carriers that are easily adhered to are required in order to enable bacteria’s colonization on the carrier. To verify the morphology of the strains on Lfr, SEM was applied. The free ZY presented as short rod-like shapes of 3–4 μm in size (Figure 4a,b). As shown in Figure 4c,d, the surface of Lfr was plicated, which ensured that strain ZY could firmly immobilize on it and obtain a high biomass concentration. This suggested that Lfr provided a suitable environment for strain ZY to survive, which guaranteed the stability of strain ZY to effectively remove TC. In the study of microorganisms immobilized on chitosan–biochar to bioremediate crude oil, it was pointed out that the bacteria adhered to the surface and the interior of the material weakened the toxicity and improved the removal efficiency of the crude oil [37].
Under TC stress, cells produce excessive reactive oxygen species (ROS) [47]. Meanwhile, microorganisms have developed complex systems to prevent oxidative damage. One of these protective mechanisms is the antioxidant enzymatic system, which consists of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) [48]. Herein, the enzymatic activities of CAT, POD and SOD of free ZY were 2.89, 1.70 and 1.32 times higher than those of Lfr-ZY, respectively, under TC stress at a concentration of 50 mg/L (Figure 5a). The synergistic effect of SOD, POD and CAT can keep the concentration of free radicals in a relatively low state, thus reducing ROS’ damage to the bacterial cell membranes [49]. In order to alleviate the oxidative stress, cells tend to increase the expression of antioxidant enzymes. Therefore, a higher expression of antioxidant enzymes implies that the cells are under greater stress. Lfr gave microorganisms protection against ROS damage, which in turn reduced the expression level of antioxidant enzymes.
Lfr not only acted as carrier for microorganisms, but also played the role of adsorbent for TC. As shown in Figure S4, the removal efficiencies of TC (50 mg/L) by Lfr and sterile Lfr were about 10% and 12% within 6 d. It was indicated that Lfr had a certain adsorption capacity on TC, attenuating its toxicity to microorganisms, which enabled ZY to adapt to the environment quickly and improved the efficiency of TC removal. The ZY could not remove 150 mg/L and 200 mg/L TC, but the removal of TC in the situation was much higher than its adsorption. This demonstrated that the removal of TC was due to the synergy of Lfr, Lfr extract and strains, not only the adsorption of Lfr.

3.4.2. Potential of Nutrient Supplement by Lfr

Biomass can release some nutrients to ensure the normal growth of strains. For example, corncob has released organic matter in aqueous solution, which can be used as a carbon source for microbial growth [50]. In addition, biochar has dissolved alkaline organic matter to keep the removal environment at a stable pH [24]. Interestingly, Lfr is rich in composition and can release a large amount of organic matter in water. Herein, the TOC of Lfr extract was measured. As shown in Figure S5, the data exhibited a quick carbon source release in the beginning due to the heat process; then, the release became continuous and slow, suggesting the possibility of nutrient supplement for strains. With the addition of strains, the TOC was gradually decreased, suggesting that the strains likely utilized the carbon source of Lfr extract. To investigate whether the Lfr releaser promoted the removal of TC, in MSM solutions with and without Lfr releaser were monitored. As shown in Figure 5b, strain ZY could remove 89.2% TC in MSM containing Lfr releaser, which was significantly higher than that without releaser, indicating that the releaser of Lfr has the potential to promote the removal of TC. Further, strain ZY was placed into the solution only containing Lfr releaser without MSM. The results showed that strain ZY could still remove TC with an efficiency of 68.7%, suggesting that Lfr releaser likely supplied the nutrients required for the growth of strain ZY and improved removal of TC.
Carbon sources are nutrients for the formation of microbial cells and metabolic products. Due to the toxicity of TC, its degradation rate is slow when microorganisms grow with TC as the sole carbon source. In this study, the effects of carbon sources, such as glucose, malic acid, citric acid and oxalic acid, on the degradation of TC by ZY were studied. As shown in Figure S6, the addition of 1% of the above carbon sources increased the degradation efficiency of TC by 18%, 11.5% and 6.6% for glucose, malic acid and citric acid, respectively. Oxalic acid was similar with the TC alone. The results indicated that additional carbon sources could improve the performance of the strain on TC degradation. The addition of high-quality carbon sources can provide energy for microorganisms during the biodegradation of TC, enhance their tolerance to TC, and facilitate the degradation rate of organic pollutants by microorganisms.
In order to confirm the role that Lfr releaser played in the growth of strain ZY, it was inoculated in MSM without carbon or nitrogen sources, respectively, to monitor the growth of ZY. As shown in Figure 5c,d, when strain ZY was in an environment without carbon source, it would utilize Lfr releaser for growth, which was about 0.6, indicating that the substances contained in the Lfr releaser could provide a carbon source for strain ZY growth. Meanwhile, it was found that strain ZY could also grow in an environment without a nitrogen source but with Lfr releaser, which suggested that the Lfr releaser likely provides a nitrogen source for bacterial growth. The strain was nutritionally deficient during the removal process, while Lfr releaser may provide sufficient nutrients for strains to help them continue to grow. Thus, this process ensures that the strains do not die due to the toxicity of TC or a lack of nutrients, achieving continuously efficient removal of TC.
What are the specific nutrients provided by Lfr for strain growth? In this study, the composition of Lfr releaser was therefore examined using LC-MS/MS. As shown in Figure 6, the Lfr releaser contained a large amount of sugar. As shown in Table S1, Lfr releaser contained lots of sugars, which could be used as carbon sources for microbial growth. The highest-percentage saccharide was gentianose, with a relative content of 3.25%, followed by stachyose with 0.58%. Raffinose, palatinose, 1,4-D-xylobiose, D-psicose, laminaritetraose, and D-turanose were also detected. It has been reported that these saccharides, such as gentianose, stachyose and raffinose, could be used as metabolic substrates for biotransformation in microorganisms [51,52,53,54,55]. Therefore, strain ZY likely utilized these saccharides as an additional carbon source to rapidly increase biomass and remove TC through co-metabolism. In the removal of p-chloroaniline by strain Brevibacillus S-618, the addition of carbon sources such as sodium succinate and glucose resulted in co-metabolism, which promotes the removal of pollutants [56]. Also, the removal of TC by Klebsiella sp. SQY5 could be enhanced through the addition of additional carbon sources [57]. In addition, it has been suggested that additional serine as a carbon source could increase the production of EPS for TC biosorption, which improves the TC removal efficiency [58]. Thus, it can be expected that Lfr has the potential as a carbon source with long-lasting carbon release to efficiently remove TC with low risk of secondary contamination.
In addition, the Lfr releaser also contained a large number of alkaloids and amino acids, which were rich in variety, as shown in Figure 6b and Table S2. These mainly included trigonelline, which accounted for 3.86% of the total substance, as well as arginine (2.36%), proline (0.78%), and histidine (0.22%). Therefore, when the environment lacks nitrogen, the variety of nitrogenous species in the Lfr releaser likely provides sufficient nitrogen sources for microorganisms to grow and increase the removal efficiency of TC through co-metabolism. It has been reported that the removal efficiency of petroleum by strains is increased from 45.06% to 82.29% after the addition of a nitrogen source [59]. It has also been found that the microbial biomass is increased considerably, enhancing the removal efficiency of pollutants by strains [60].

3.4.3. Potential Solubilizing Effect of Lfr

TC dissolves more slowly in water, resulting in a slower TC removal rate. Herein, lots of surfactants were detected in the Lfr releaser; the highest content was seen for oleic acid, which accounted for 20%, followed by betaine at 1.95%, as shown in Table 1. To confirm the solubilizing effect of Lfr, different oleic acids were added to study the removal of TC by strain ZY. As can be seen in Figure S7, strain ZY showed enhanced removal efficiency of TC in MSM containing oleic acid, with a removal efficiency of 83.3%, which was 10% (5% oleic acid) higher compared to in the absence of oleic acid. In addition, to some extent, the increase in oleic acid enhanced the removal efficiency of TC. The enhancement in degradation efficiency observed with oleic acid supplementation, a key component identified in Lfr, provides indirect support for the hypothesis that surfactant-induced solubilization is a likely mechanism contributing to the improved bioavailability of TC. However, this mechanism remains inferred rather than directly proven, as solubility was not quantitatively measured and potential turbidity effects could not be fully decoupled. It has been found that the addition of linoleic acid could significantly promote the removal of benzopyrene by strains) [61]. Betaine is a kind of bio-surfactant. It has been suggested that the addition of betaine improves the removal capacity of toluene to a greater extent than other treatments, due to its solubilizing effect. As TC dissolved slowly in water, the presence of surfactants accelerated the dissolution, promoting the bio-removal process [27].
In summary, the mechanism by which Lfr enhances TC removal is mainly as follows (Figure 7): (1) Lfr adsorbs some TC, which reduces the toxicity of TC on ZY; meanwhile, Lfr carriers provide a suitable living space, ensuring the normal growth of microorganisms and preventing oxidative stress damage, which guarantees highly efficient TC removal; (2) Lfr releases additional nutrients, likely enabling strain ZY to utilize them to co-metabolize; (3) Lfr releases surfactants, likely accelerates the dissolution of TC, and promotes the removal of TC by strains.

4. Conclusions

In conclusion, a Lfr–ZY complex was prepared to completely remove TC, which was used as a sole carbon source without other energy supplementation. Lfr-ZY was able to completely remove 50 mg/L of TC within 4.5 d. Lfr-ZY maintained a high TC removal efficiency over a wide pH range compared to free bacteria. In addition, Lfr-ZY showed high TC removal efficiency after 11 cycles. The rough structure and various functional groups of Lfr could provide sites for bacterial growth and colonization, as well as for the adsorption of TC, which effectively avoided activity damage and TC toxicity to strain ZY. Lfr carrier has the potential to release nutrients and surfactants, which could supply carbon and nitrogen sources for microorganisms, enhancing biomass and TC removal efficiency through co-metabolism. The dissolved surfactants promoted the dissolution and transport of TC, which led to its more efficient removal. This study provides a new insight for biomass-enhancing pollutant removal, with potential for practical applications.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/pr13113567/s1: Figure S1: Biomass change in ZY2 immobilized on Lfr; Figure S2: Adsorption of TC by Lfr; Figure S3: Effect of oleic acid on TC removal; Figure S4: Adsorption of TC by (a) Lfr and (b) sterile Lfr (initial TC concentration 50 mg/L, pH 7, Lfr 1g); Figure S5: TOC of Lfr extract (a) without and (b) with the addition of ZY; Figure S6: Effects of additional carbon sources on the degradation of TC (a) and growth (b) by strain ZY; Figure S7: Effect of oleic acid on TC degradation by strain ZY; Table S1: Carbon source components and contents in Lfr; Table S2: Nitrogen source components and contents in Lfr releaser.

Author Contributions

Conceptualization, J.L. and L.Y.; methodology, L.Y. and Y.Z.; software, L.Y.; validation, Y.Z.; data curation, L.Y.; writing—original draft preparation, J.L.; writing—review and editing, J.L. and L.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article and Supplementary Materials. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Processes 13 03567 g001
Figure 1. Effect of initial TC concentration on (a) concentration and (b) removal efficiency of free ZY and Lfr-ZY.
Figure 1. Effect of initial TC concentration on (a) concentration and (b) removal efficiency of free ZY and Lfr-ZY.
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Figure 2. Effect of (a,b) initial pH, (c,d) temperature and (e,f) NaCl concentration on TC removal by free ZY and immobilized ZY.
Figure 2. Effect of (a,b) initial pH, (c,d) temperature and (e,f) NaCl concentration on TC removal by free ZY and immobilized ZY.
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Figure 3. Cycling of TC removal by Lfr-ZY.
Figure 3. Cycling of TC removal by Lfr-ZY.
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Figure 4. SEM images of (a,b) free ZY and (c,d) ZY immobilized on Lfr.
Figure 4. SEM images of (a,b) free ZY and (c,d) ZY immobilized on Lfr.
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Figure 5. (a) Enzymatic activities of SOD, CAT and POD in free and immobilized ZY and (b) effect of Lfr releaser on TC removal and the growth data of ZY, with the addition of Lfr releaser as (c) carbon and (d) nitrogen sources.
Figure 5. (a) Enzymatic activities of SOD, CAT and POD in free and immobilized ZY and (b) effect of Lfr releaser on TC removal and the growth data of ZY, with the addition of Lfr releaser as (c) carbon and (d) nitrogen sources.
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Figure 6. (a) Carbon source and (b) nitrogen source composition and content of Lfr releaser.
Figure 6. (a) Carbon source and (b) nitrogen source composition and content of Lfr releaser.
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Figure 7. Schematic diagram of TC removal by Lfr-ZY.
Figure 7. Schematic diagram of TC removal by Lfr-ZY.
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Table 1. Surfactant composition and content of Lfr releaser.
Table 1. Surfactant composition and content of Lfr releaser.
ComponentsRt(s)m/zRelative Content
Oleic acid43.16282.2920.00%
Betaine381.36160.141.95%
Erucamide42.24338.350.31%
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Yu, L.; Zheng, Y.; Liang, J. Loofah Sponge Has a Potential Multifunctional Role for Enhanced Tetracycline Biodegradation: Carrier, Putative Nutrient Releaser and Solubilizer. Processes 2025, 13, 3567. https://doi.org/10.3390/pr13113567

AMA Style

Yu L, Zheng Y, Liang J. Loofah Sponge Has a Potential Multifunctional Role for Enhanced Tetracycline Biodegradation: Carrier, Putative Nutrient Releaser and Solubilizer. Processes. 2025; 13(11):3567. https://doi.org/10.3390/pr13113567

Chicago/Turabian Style

Yu, Lei, Yujing Zheng, and Jing Liang. 2025. "Loofah Sponge Has a Potential Multifunctional Role for Enhanced Tetracycline Biodegradation: Carrier, Putative Nutrient Releaser and Solubilizer" Processes 13, no. 11: 3567. https://doi.org/10.3390/pr13113567

APA Style

Yu, L., Zheng, Y., & Liang, J. (2025). Loofah Sponge Has a Potential Multifunctional Role for Enhanced Tetracycline Biodegradation: Carrier, Putative Nutrient Releaser and Solubilizer. Processes, 13(11), 3567. https://doi.org/10.3390/pr13113567

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