Insecticidal Activity of Ethyl Acetate Extracts from Culture Filtrates of Mangrove Fungal Endophytes

Abstract

In the search for novel potent fungi-derived bioactive compounds for bioinsecticide applications, crude ethyl acetate culture filtrate extracts from 110 mangrove fungal endophytes were screened for their toxicity. Toxicity tests of all extracts against brine shrimp (Artemia salina) larvae were performed. The extracts with the highest toxicity were further examined for insecticidal activity against Spodoptera litura larvae and acetylcholinesterase (AChE) inhibition activity. The results showed that the extracts of five isolates exhibited the highest toxicity to brine shrimp at 50% lethal concentration (LC₅₀) values of 7.45 to 10.24 ppm. These five fungal isolates that obtained from Rhizophora mucronata were identified based on sequence data analysis of the internal transcribed spacer region of rDNA as Aspergillus oryzae (strain BPPTCC 6036), Emericella nidulans (strains BPPTCC 6035 and BPPTCC 6038), A. tamarii (strain BPPTCC 6037), and A. versicolor (strain BPPTCC 6039). The mean percentage of S. litura larval mortality following topical application of the five extracts ranged from 16.7% to 43.3%. In the AChE inhibition assay, the inhibition rates of the five extracts ranged from 40.7% to 48.9%, while eserine (positive control) had an inhibition rate of 96.8%, at a concentration of 100 ppm. The extracts used were crude extracts, so their potential as sources of AChE inhibition compounds makes them likely candidates as neurotoxins. The high-performance liquid chromatography profiles of the five extracts differed, indicating variations in their chemical constituents. This study highlights the potential of culture filtrate ethyl acetate extracts of mangrove fungal endophytes as a source of new potential bioactive compounds for bioinsecticide applications.

Safety and environmental issues surrounding the use of chemical insecticides have led to the development of alternative insect control measures, including bioinsecticides. A bioinsecticide is a formulation of naturally occurring substances that control pests through nontoxic mechanisms and in an ecofriendly manner. They can be animal, plant, or microorganism derived, and exploit living organisms (natural enemies), their products (phytochemicals, microbial products), and their byproducts (semiochemicals) in the management of insects [1]. While the marine environment is considered as a prolific resource for the isolation of less frequently exploited microorganisms [2], recent studies have focused mainly on the application of marine microorganisms in human drug development, with limited information regarding their insecticidal activities having been reported to date [2].

Mangrove fungi, a well-known marine-derived source of bioactive compounds, have recently been used in the isolation of new chemical compounds [3]; the mangrove ecosystem exhibits unique conditions that influence the fungal metabolite. Mangrove-associated fungi provide a broad variety of bioactive secondary metabolites with unique structures, including alkaloids, benzopyranones, chinones, flavonoids, phenolic acids, quinones, steroids, terpenoids, tetralones, and xanthones, among others [3]. The symbiotic nature of endophytic microorganisms favors metabolic interactions with their host plants and their environment, thereby increasing the production of bioactive compounds [4]. In particular, it has been demonstrated that the bioactive compounds produced by endophytic microorganisms contribute to natural plant defenses by preventing herbivory and invasion from superficial pathogens [4]. Several natural products (e.g., microbial polyketides) derived from microorganisms, such as avermectins and milbemycins, have been reported as potent insecticides against various insects and parasites [2]. In addition, they are believed to be the biggest selling and, arguably, most effective acaricides and anthelmintics currently available [2]. Nevertheless, the use of fungal secondary metabolites as bioinsecticides remains limited; most secondary metabolite-based bioinsecticides are dominated by bacteria [5].

Mangrove plants, such as Rhizophora apiculata, Rhizophora annamalayana, and Rhizophora mucronata, have been identified as potential sources of fungal isolates [6,7,8,9,10,11]. Many studies have explored the active compounds from mangrove fungal secondary metabolites for pharmaceutical applications, but research regarding their agrochemical applications remains scarce [12]. In Indonesia, mangrove fungal research has also focused on the exploration of mangrove fungal secondary metabolites for pharmaceutical applications [9,10,11,13], especially antibiotics; however, no research on their agrochemical applications has been reported to date. Many mangrove fungi isolated from mangrove species or genera have been used to extract biologically active compounds, several of which have exhibited insecticidal activities [14,15,16]. Recently, new active compounds, including 6-hydroxy-3-methylisochroman-5-carboxylic acid, mycoepoxydiene, 5-carboxylmellein, 5-methylmellein, pyrrole-2-carboxylic acid, and 3-methylhydantoin, were isolated from mangrove fungal culture media and shown to exhibit insecticidal activity against Helicoverpa armigera and Sinergasilus sp. [17].

In Indonesia, one of the countries with the highest diversity of mangrove plants [18,19], there has been a limited number of studies on the prospecting of mangrove fungi for agrochemical applications. In an attempt to discover a novel substance of insecticidal importance against agricultural insects, the present study evaluates the cytotoxicity and insecticidal activities of ethyl acetate extracts from culture filtrates of endophytic fungal isolates obtained from the Indonesian mangrove plants R. mucronata, Sonneratia alba, and Avicennia marina.

MATERIALS AND METHODS

Plant samples, fungal isolates, and test organisms

Plant samples were collected from the Prof. Dr. Sedyatmo Angke Kapuk Mangrove Rehabilitation and Ecotourism, Jakarta, Indonesia (106°75'48.09'' N, 6°12'19.51'' E), by Silva Abraham on August 14, 2012. Samples of healthy mature living leaves, twigs, roots, and leaves litter from one mature tree of three mangrove plants (R. mucronata, S. alba, and A. marina) were chosen at random. Those samples were brought to the lab in sterile bags and processes within a few hours after sampling.

Fungal isolates obtained from the plant samples were deposited in the Badan Pengkajian dan Penerapan Teknologi Culture Collection (BPPTCC), Tangerang, Indonesia. Fungal isolates were preserved in a 2 mL CryoTube containing 1 mL of 10% glycerol solution (v/v) and 5% lactose (w/v) at -80℃.

Brine shrimp (Artemia salina) used for the lethality bioassay were hatched from brine shrimp eggs (Pfizer Consumer Inc., New York, NY, USA). One gram of A. salina eggs were hatched in 1,000mL of artificial seawater [20] with air bubbling and artificial illumination for 36 hr [21]. The phototropic nauplii (larvae) were collected with a pipette from the lighted side and concentrated in a 7-mL test tube. Spodoptera litura larvae were obtained from laboratory colonies maintained by the Center for Bioindustrial Technology, Badan Pengkajian dan Penerapan Teknologi, Tangerang, Indonesia.

Fungal isolation

The plant samples (twigs, roots, leaves, and leaf litter) were washed with sterile artificial seawater [20], then surface sterilized using the method described by Ananda and Sridhar [22]. All of the plant samples were placed in sterile tissue paper and dried in laminar airflow for 24 hr. The effectiveness of the sterilization procedure was evaluated following the method developed by Schulz et al. [23]. Briefly, sterilized tissue segments (approximately 1 cm × 1 cm) were pressed onto the surface of potato dextrose agar medium (Merck, Whitehouse Station, NJ, USA). The absence of growth of any fungi on the medium, other than endophytic fungi from the internal tissue of the plant samples confirmed that the sterilization procedure was effective in removing the surface fungi [23].

A combination of five isolation methods, namely direct plating [22], filtration using Whatman filter paper [24], filtration using a Millipore membrane [24], particle washing [25], and a moist chamber [26], were used to isolate the endophytic fungi. Further, six media were used for the isolation, namely Beauveria spp. isolation medium [27], a modification of the Tubaki medium (1 g peptone, 1 g KH₂PO₄, 0.5 g yeast extract, 0.5 g MgSO₄ · 7H₂O, 0.02 g FeSO₄, 2 g agar, 500mL artificial seawater) [28], Metarhizium spp. isolation medium [28], chitin medium [29], extract of mangrove leaf medium (following the method of Delalibera et al. [30]), and LC Miura's agar medium [31]. Antibiotics, 0.5 mL from 0.6 g/mL streptomycin (Kimia Farma, Jakarta, Indonesia), 0.5 mL from 0.05 g/mL tetracycline (Kimia Farma), 0.5 mL from 0.1 g/mL dodine (Sigma-Aldrich, St. Louis, MO, USA), and 2.5mL from 0.05 g/mL cycloheximide (Sigma-Aldrich) [27], were added to each of the isolation media. All fungal isolates grown on isolation media were purified and preserved in a 2 mL CryoTube containing 1 mL of 10% glycerol solution (v/v) and 5% lactose (w/v) at -80℃.

Fungal identification

The fungal isolates were identified based on sequence data of the internal transcribed spacer (ITS) rDNA (including ITS1, 5.8S rDNA, and ITS2). Nuclear rDNA from the fungal isolates was extracted using a PrepMan Ultra kit (Applied Biosystems, Foster City, CA, USA). The ITS rDNA of the fungal isolates were amplified with primers ITS1 (5'-TCCGTAGGTGAACCTGCGG-3') and ITS4 (5'-TCCTCCGCTTATTGATATGC-3') as described by White et al. [32]. PCR thermal cycling was carried out in a PCR thermal cycler using the following parameters: 95℃ for 1 min, followed by 40 cycles at 94℃ for 1 min, 60℃ for 1 min, and 72℃ for 1 min, with a final extension cycle of 72℃ for 5 min [33]. Purified PCR products were sequenced using an automated DNA sequencer (ABI Prism 310 Genetic Analyzer; Applied Biosystems). The fungal isolates were identified based on sequence homology with fungal sequences obtained from the GenBank DNA database hosted by NCBI (http://blast.ncbi.nlm.nih.gov), using the BLAST search tool. The identification of the isolates was based on the sequence similarity cutoff point for fungal species delimitation of at least ≥ 97% according to Brock et al. [34] and with E-value cut-off 0.01. The sequences of ITS rDNA of fungal isolates were aligned with other sequences retrieved from GenBank using ClustalX [35]. Phylogenetic tree were constructed using the neighbor-joining method [36] with bootstrap values based on 1,000 replications [37]. The evolutionary distances were computed using the Kimura 2-parameter method [38]. The sequence data of ITS rDNA of the fungal strains were deposited into GenBank under the following accession numbers: KP165432 (Emericella nidulans BPPTCC 6035), KP165433 (Aspergillus oryzae BPPTCC 6036), KP165434 (Aspergillus tamarii BPPTCC 6037), KP165435 (E. nidulans BPPTCC 6038), and KP165436 (Aspergillus versicolor BPPTCC 6039).

Fermentation and extraction of culture filtrates

The fungal cell suspension (1 mL) from a seven-day-old culture grown on an malt extract agar (MEA) slant was inoculated into 100mL Erlenmeyer flasks containing 19mL of malt extract broth (MEB) medium [20], incubated for seven days on a rotary shaker at 65 rpm at room temperature, and used as a starter culture. This starter culture was then inoculated into a 500-mL Erlenmeyer flask containing 180 mL of fresh MEB medium and incubated for 14 days on a rotary shaker at 65 rpm. The fungal mycelium on each flask was collected using Whatman #1 filter paper, after which the filtrate was extracted with 100mL of ethyl acetate, using a separation funnel. The water fraction (upper layer) was collected and re-extracted (three times) with ethyl acetate. The ethyl acetate fraction (bottom layer) was collected and evaporated using a rotary evaporator.

Brine shrimp lethality bioassay

A brine shrimp lethality bioassay using larvae of A. salina was performed for the preliminary evaluation of toxicity from all fungal extracts. The bioassay was conducted to determine the 50% lethal concentration (LC₅₀) of the fungal culture ethyl acetate extracts. For preliminary screening, five concentrations of culture filtrate crude extracts (1,000, 500, 250, and 125 ppm) were tested against 20 larvae of A. salina for each concentration and repeated three times. The ethyl acetate extracts (10 µL) were evaporated in a 7-mL test tube and dissolved in 5 mL of artificial seawater; then, 10 µL of dimethyl sulfoxide (DMSO) were added to completely dissolve the extracts. The negative control consisted of 10 µL of pure ethyl acetate (evaporated) and 10 µL of DMSO in 5mL of artificial seawater. The mortality rate was recorded after exposing 20 larvae for 24 hr. The concentration of the testing ethyl acetate extract was gradually decreased following recording of the larvae mortality rate. The fungal extracts that exhibited the highest percentage mortality rates (≥ 90%) were chosen as candidates for the next test and tested in a decreased concentration. After a series of tests (decreasing gradually from 1,000 ppm to 500, 250, and 125 ppm), the fungal extracts was tested against A. salina larvae to determine LC₅₀. The extract concentrations used in this test were 80, 40, 20, 10, and 5 ppm.

Topical application of culture filtrate extracts to S. litura larvae

The larvicidal activity of the fungal ethyl acetate extracts was evaluated using topical application to S. litura III instar larvae, as described by Supriyono [39]. Briefly, 5 µg of fungal extract were dissolved in 2 µL of acetone (analytical grade; Merck, Darmstadt, Germany) and applied to the dorsum of S. litura III instar larvae using a micropipette. Ten S. litura III instar larvae (10 to 14 mg in weight and 1 to 1.3 cm in length) were used for each extract treatment, and each treatment was performed in triplicate. Acetone (2 µL) was used as a negative control and the commercial insecticide deltamethrin (5 µg/2 µL; Decis 25 g/L; PT Bayer, Jakarta, Indonesia) was used as a positive control. Following treatment, the larvae were introduced into a 100-mL plastic cup and covered with cotton sheets. An artificial diet was prepared according to the method of Supriyono [39] for consumption by the treated larvae, consisting of 150 g of soybeans (soaked in 460 mL of distilled water for 24 hr), 3 g of L-(+)-ascorbic acid, 3 g of nipagine (p-hydroxybenzoic acid ethyl ester), 11 g of dried yeast powder, 180 mg of gentamicin sulfate, 1 mL of paraformaldehyde, 10 g of agar, and 315 mL of distilled water. The experiment was performed for 5 days and mortality was assessed on the fifth day. A larva was considered dead if it was unable to make a coordinated movement when gently prodded. The mortality percentage of each treatment was recorded. The larval percentage mortality rate was calculated according to the formula of Ladhari et al. [40]: % mortality = (Number of dead larvae/Total number of larvae) × 100.

Acetylcholinesterase (AChE) inhibition assay

Fungal extracts were evaluated spectrophotometrically for AChE inhibition potential using AChE iodide (AChI; Sigma) as a substrate, following Ellman's assay [41]. Fungal extracts (25 µL; 100 ppm concentration) were added to a 96-well microplate (Axygen BioScience, Inc., Union City, CA, USA) containing 50 µL bovine serum albumin (0.1%; Sigma) and 25 µL of AChE enzyme (0.28 U/mL; Sigma), after which 5,5-dithiobis-(2-nitrobenzoic acid) (DTNB; 125 µL, 3 mM; Sigma) were added to the mixture and incubated for 15 min in a water bath at 30℃. To start the reaction, 25 µL of 15 mM AChI was added to the mixture. The reaction between thiocholine and DTNB was indicated by the formation of the 5-thio-2-nitrobenzoate anion, indicated by a yellow color appearance. Absorbance was measured using an enzyme-linked immunosorbent assay reader (Elx800; BioTek Instruments, Inc., Winooski, VT, USA) at a wavelength of 400 nm for 8 min. The fungal extract-free reaction mixture was taken as a negative control and with the standard drug eserine (Sigma) as a positive control. Each assay was performed in triplicate and the AChE inhibitory value was taken as the average of three independent experiments. The percentage of enzyme activity and percentage of inhibition were calculated according to the equation in Ellman's assay [41].

High-performance liquid chromatography (HPLC) analysis of the ethyl acetate extracts from the fungal extracts

The active ethyl acetate extracts were analyzed by HPLC (HPLC-UV Vis Detector; Waters Co., Milford, MA, USA). All ethyl acetate extracts were loaded onto a C18 column in a 20-µL injection volume. Elution was performed using a linear gradient consisting of double distilled water and acetonitrile; an isocratic step was initially carried out for 3 min at 85% water, followed by a moderate increase in acetonitrile to reach 100% in 20min, at a flow rate of 1mL/min [42]. The second isocratic step was carried out for 5 min with 100% acetonitrile. The elution profiles of the secondary metabolite extracts were observed using absorption at 254 nm.

Statistical analyses

One-way analysis of variance (ANOVA) was used to compare the treatment means of brine shrimp lethality bioassay, topical application to S. litura larvae, and AChE inhibition assay. A post-hoc Tukey's honestly significant difference (HSD) test, with a significance level of α = 0.05, was performed when a significant difference between treatment means was detected. The mortality rates of brine shrimp and S. litura larvae were corrected for control mortality using Abbott's formula [43]. The corrected mortality rates were normalized by an arcsine square-root transformation before the ANOVA test was conducted. The concentration at which 50% of brine shrimp died (LC₅₀) for each fungal extract was determined by probit analysis [44]. All statistical analyses were performed using IBM SPSS Statistics ver. 21 software (IBM Corp., Armonk, NY, USA).

RESULTS AND DISCUSSION

Preliminary lethality test of ethyl acetate extracts from fungal culture filtrates

To investigate the toxicity of 110 ethyl acetate extracts produced by 110 fungal culture filtrates (hereinafter referred to as extracts), a brine shrimp (A. salina) lethality test was used as the preliminary screening method. According to Harwig and Scott [21], extracts that cause more than 90% larval mortality are rated as highly toxic. As shown in Table 1, the serial decrease in concentration of extracts from 1,000 to 500, 250, and 125 ppm was followed by decreased in number of extracts of fungal isolates that exhibited toxic activities that cause more than 90% larval mortality. Table 1 highlighted only five extracts that displayed the highest toxicity at a concentration of 125 ppm.

Table 1. Preliminary brine shrimp (using larvae of Artemia salina) lethality bioassay of 110 ethyl acetate extracts from 110 fungal isolates at serially decreasing concentrations.

Identification of five fungal isolates that produced toxic extracts

The results of the sequence analysis by the BLAST homology search (http://blast.ncbi.nlm.nih.gov/Blast.cgi) based on the sequence data of ITS rDNA of the five fungal isolates that produced toxic extracts indicated a high homology with their closely related species, as obtained from GenBank (99%~100%). According to Brock et al. [34], the homology of ITS rDNA sequences of organisms belonging to the same species is ≥ 97%. Consequently, based on the sequence similarity cutoff point for fungal species [34], the five fungal isolates were identified as E. nidulans (isolate ARM1M1 = strain BPPTCC 6035 and isolate DRM3M3 = BPPTCC 6038); A. versicolor (isolate SRM6T4 = strain BPPTCC 6039); A. oryzae (isolate ARM3L1 = strain BPPTCC 6036); and A. tamarii (isolate DRM2KS1 = strain BPPTCC 6037). The phylogenetic tree (Fig. 1) shows that the position of each fungal strain used in this study is closely related with that of other strains from the same species retrieved from GenBank; they are clustered together, with high bootstrap support (86%~98%). As shown in Fig. 1, A. versicolor BPPTCC 6039 isolated from R. mucronata litter is closely related to four A. versicolor strains from GenBank, with very strong bootstrap support (98%). Two strains of E. nidulans isolated from roots and leaves of R. mucronata are clustered together with type strain E. nidulans and two strains from GenBank, with strong support (86%). A. tamarii BPPTCC 6037, isolated from leaves of R. mucronata, is located within the same cluster as two strains of A. tamarii from GenBank, with very strong bootstrap support (99%). A. oryzae BPPTCC 6036, isolated from roots of R. mucronata, is clustered together with type strain A. oryzae and other strains from GenBank, with strong bootstrap support (88%). The list of fungal strains, their origins, and their accession numbers of ITS rDNA sequence data used to draw the phylogenetic tree are shown in Table 2. As shown in Table 2, Aspergillus strains retrieved from GenBank were isolated from various environments, including soil, metal corrosion, milk, and rice wine starter. Aspergillus is a cosmopolitan fungus found in diverse environments and a wide range of habitats [45]. In marine and estuarine environments, Aspergillus is one of several fungal genera often found in different hosts, from marine organisms to mangrove plants [3,28]. Aspergillus species (A. flavus, A. niger, and A. nidulans) have been isolated from leaves, twigs, and roots of several Rhizophoraceae mangroves such as R. mucronata, R. stylosa, and R. apiculata [14,46]. It has been reported that Aspergillus fungi from those mangrove species produce a wide variety of bioactive secondary metabolites that display activities against human microbial pathogens and cancer Hep2 and MCF7 cell lines [47]. Other fungal species that have been isolated from Rhizophora are Phomopsis spp. from R. apiculata [6], Meyerozyma guilliermondii from the leaves of R. mucronata [48], Fusarium oxysporum from R. annamalayana [49], and two species of Pestalotiopsis from the twigs of two mangrove plants, R. apiculata and R. mucronata [7]; these fungal species have also produced secondary metabolites with various activities. Other Aspergillus isolated from various mangrove species include A. flavipes, from the inner leaves of Acanthus ilicifolius [50], Aspergillus sp., from the leaves of Ipomoea pes-caprae [51], A. flavus, from the roots of Hibiscus tiliaceus [52], and nine Aspergillus species, namely A. niger, A. fumigatus, A. aureoles, A. candidus, A. chevalier, A. flavus, A. glaucus, A. ochraceus, and A. smithii, from decomposing leaves of Ceriops tagal [53].

Fig. 1. Phylogenetic position of mangrove fungal endophytes based on internal transcribed spacer rDNA sequence data. Strains used in this study are indicated in bold.

Table 2. Strains and their accession numbers of ITS rDNA sequences data used to draw the phylogenetic tree.

Nucleotide sequences determined in this study are indicated in bold.

ITS, internal transcribed spacer; NRRL, Agricultural Research Service Culture Collection, National Center for Agricultural Utilization Research, Peoria, IL; UOA/HCPF, University of Athens/Hellenic Collection of Pathogenic Fungi, National and Kapodistrian University of Athens; ATCC, American Type Culture Collection, Manassas, VA; IFM, Research Center for Pathogenic Fungi and Microbial Toxicoses, Chiba University, Japanese Federation of Culture Collections; SRRC, Southern Regional Research Center, Agricultural Research Service, US Department of Agriculture, New Orleans, LA; BPPTCC, Badan Pengkajian dan Penerapan Teknologi (Agency for The Assessment and Applications of Technology) Culture Collection, Tangerang, Indonesia; CBS, Centraalbureau voor Schimmelcultures, Utrecht; other abbreviations are not registered.

Brine shrimp lethality bioassay of five extracts from fungal culture filtrates

The brine shrimp lethality bioassay of the five selected fungal extracts at lower concentrations (2.5, 5, 10, 20, 40, and 80 ppm) showed that, as the extract concentration increased, so did the degree of toxicity. All five extracts exhibited a similar pattern. One-way ANOVA showed that different concentrations of the extracts from A. oryzae [F(5, 12) = 45.69, p < 0.001], E. nidulans BPPTCC 6035 [F(5, 12) = 66.86, p < 0.001], E. nidulans BPPTCC 6038 [F(5, 12) = 72.71, p < 0.001], A. tamari [F(5, 12) = 22.16, p < 0.001], and A. versicolor [F(5, 12) = 43.17, p < 0.001] had significant effects on the mortality rates of brine shrimp larvae. Post-hoc comparisons using Tukey's HSD test indicated significant differences between the means of some pairs of concentrations but not the means of other pairs. Consequently, the mean mortality rates of the brine shrimp bioassay for the five extracts using six concentrations resulted in a different number of homogeneous subsets of means. So, the toxicity rating of each extract corresponded to its subset of means. The rating of the toxicity of the five extracts is shown in Table 3. Of the five extracts tested, four extracts (produced by A. oryzae BPPTCC 6036, E. nidulans BPPTCC 6038, A. tamari BPPTCC 6037, and A. versicolor BPPTCC 6039) were rated as highly toxic, and one extract (produced by E. nidulans BPPTCC 6035) was rated as toxic at a concentration of 80 ppm. Two extracts (produced by A. oryzae BPPTCC 6036 and A. tamari BPPTCC 6037) were even highly toxic at concentrations of 40 ppm. Harwig and Scott [21] rated the toxicity of filtrates and extracts based on the mortality rate against A. salina larvae as follows: non-toxic (0%~9%), slightly toxic (10%~49%), toxic (50%~89%), and very/highly toxic (90%~100%). The mortality rates of the five fungal extracts in this experiment ranged from 75% to 98.3% at a concentration of 80 ppm. Qiao et al. [54] reported that six indoloditerpene derivatives from the endophytic fungus A. oryzae, which were isolated from the marine red algae Heterosiphonia japonica, displayed a mortality rate of 31.4%~ 74.2% against A. salina at a concentration of 100 µg/mL (0.1 ppm).

Table 3. Toxicity of ethyl acetate extracts from five fungal strains, based on brine shrimp lethality test using larvae of Artemia salina.

BPPTCC, Badan Pengkajian dan Penerapan Teknologi (Agency for The Assessment and Applications of Technology) Culture Collection, Tangerang, Indonesia.

^aAverage of three replicates, standard error (SE); means within the column of each fungal extract followed by the same letter are not significantly different in Tukey's honestly significant difference means comparisons test at α = 0.05.

^bToxicity of extracts are rated following Harwig and Scott [21]: non-toxic (0%~9%); slightly toxic (10%~49%); toxic (50%~89%) and highly toxic (90%~100%).

LC₅₀ values of the ethyl acetate extracts from five fungal strains

Table 4 shows the LC₅₀ values of the ethyl acetate extracts from five fungal strains based on the brine shrimp lethality bioassay. The linear regression of the probit analysis was used to calculate the LC₅₀ value for each extract, as depicted in Fig. 2. As shown in Table 4, the LC₅₀ values of the five fungal extracts against A. salina larvae ranged from 7.45 ppm to 10.24 ppm. The extract that showed the lowest LC₅₀ value was from A. tamarii (7.45 ppm) which, as mentioned, was rated as highly toxic, causing 96.67% mortality of A. salina larvae. Miao et al. [55] found that the LC₅₀ value of 6-O-methylaverufin produced by the endophytic fungus A. versicolor and isolated from the brown algae Sargassum thunbergii was 0.5 µg/mL, or 5 × 10^-4 ppm. Another study reported that 4-phenyl-3,4-dihydroquinolone and aflaquinolone A derivatives produced by A. nidulans which was isolated from fresh leaves of the mangrove plant Rhizophora stylosa exhibited toxic activities against A. salina, with LC₅₀ values ranging from 4.5 to 7.1 µM [46]. In the present study, the LC₅₀ values of A. versicolor and E. nidulans extracts were 8.89 and 9.96 ppm, respectively, more toxic than the results of the two studies discussed [46,55]. Of note, their studies used single compounds, whereas the present study used crude extracts. According to the standard which classifies active plant compounds and extracts as toxic if their LC₅₀ value is < 1,000 µg/mL or < 1,000 ppm [56], all extracts produced by the five fungal strains in this study are classified as toxic.

Table 4. LC₅₀ values of ethyl acetate extracts from five fungal strains, based on brine shrimp lethality test using larvae of Artemia salina.

BPPTCC, Badan Pengkajian dan Penerapan Teknologi (Agency for The Assessment and Applications of Technology) Culture Collection, Tangerang, Indonesia.

Fig. 2. Linear regression of PROBIT mortality against log concentration of ethyl acetate extracts of culture filtrate from Aspergillus oryzae BPPTCC 6036 (A), Emericella nidulans BPPTCC 6035 (B), Emericella nidulans BPPTCC 6038 (C), Aspergillus tamarii BPPTCC 6037 (D), and Aspergillus versicolor BPPTCC 6039 (E), based on brine shrimp lethality test using larvae of A. salina. BPPTCC, Badan Pengkajian dan Penerapan Teknologi (Agency for The Assessment and Applications of Technology) Culture Collection, Tangerang, Indonesia.

Topical application of the five extracts onto S. litura III instar larvae

The results of the topical application of the five extracts onto S. litura III instar larvae (Table 5) indicate that the extract produced by A. versicolor resulted in the highest mortality rate of S. litura larvae (43.3%) at a 5 µg/mL concentration. The percentage mortality caused by the extract is higher than that of the positive control, the commercial insecticide deltamethrin (36.7%). However, an ANOVA of the larval mortality rates found no significant difference among the treatment means [(F(5, 12) = 2.34, p = 0.106]. Using a topical application bioassay, several studies have demonstrated that Aspergillus exhibits insecticidal activity against S. litura larvae [57,58]. The 4-(N-methyl-N-phenyl amino)-butan-2-one compound produced by A. gorakhpurensis yielded a 50% mortality rate of S. litura IV instar larvae at a concentration of 330.69 µg/mL [57]. Another study also has reported that the kojic acid produced by A. funiculosus resulted in a 50% mortality rate of S. litura IV instar larvae at a concentration of 693.52 µg/mL [58]. In the search for new insecticides, acute toxic effects as demonstrated by topical application are usually the yardstick by which products are measured, yielding a spectrum of fast-acting, potent products [59]. The results of the topical application bioassay in this study indicate that the five extracts tested have acute toxic effects on S. litura larvae. As well, the degree of acute toxicity of the five extracts in this study is higher than reported in the two earlier studies [57,58]. This finding suggests that the five extracts have high potential as source of active compounds for bioinsecticides. The AChE inhibitor assay results for the five extracts and the standard drug eserine at a concentration of 100 ppm are summarized in Table 6. The inhibition rates of the five extracts ranged from 40.7% to 48.9%, while eserine had an inhibition rate of 96.8%. ANOVA found a highly significant difference in the inhibition rates of the extracts and eserine [F(5, 12) = 3,414.51, p < 0.001]. A post-hoc Tukey's HSD test showed that eserine had significantly higher AChE inhibitory activity than all the extracts. Among the five extracts, the extract produced by A. tamarii BPPTCC 6037 had the highest inhibition rate (48.9%), though it did not differ significantly from the extract produced by E. nidulans BPPTCC 6038 (47.6%). This study showed that the crude extracts produced by A. tamarii BPPTCC 6037, E. nidulans BPPTCC 6038, and A. versicolor BPPTCC 6039 exhibited a high AChE inhibitory activity at a concentration of 100 ppm. The extracts used were crude extracts, so their potential as sources of AChE inhibition compounds makes them likely candidates as neurotoxins. In a previous study, Qiao et al. [54] reported that the AChE inhibitory activity of six indoloditerpene derivatives produced by the endophytic fungus A. oryzae ranged from 4% to 16.4% at a concentration of 100 µg/mL (0.1 ppm).

Table 5. Percentage of larval mortality from topical application bioassay (5 µg/mL) of five fungal extracts on Spodoptera litura instar III larvae.

BPPTCC, Badan Pengkajian dan Penerapan Teknologi (Agency for The Assessment and Applications of Technology) Culture Collection, Tangerang, Indonesia.

^aAverage of three replicates, standard error (SE). There is no significant difference of means (p = 0.106) by one way ANOVA.

Table 6. Percentage of acetylcholinesterase (AChE) inhibition potential of five fungal extracts at concentration of 100 ppm.

BPPTCC, Badan Pengkajian dan Penerapan Teknologi (Agency for The Assessment and Applications of Technology) Culture Collection, Tangerang, Indonesia.

^aAverage of three replicates, standard error (SE); means followed by the same letter are not significantly different in Tukey's honestly significant difference means comparisons test at α = 0.05.

HPLC analysis of the extracts produced by five fungal strains

The HPLC profiles of the ethyl acetate extracts produced by five fungal strains (Fig. 3) show that each extract has a different peak characteristic profile. The results of the HPLC analysis of the extracts indicate that the five extracts are composed of different compounds. The profiles indicated differences between the chemical constituents and the secondary metabolites. Further studies, including fractionation, isolation, and characterization of each peak, are therefore required since each ethyl acetate extract peak might represent a single compound with a different toxicity. Additionally, assessing the toxicity degree of each fraction and elucidating the best toxic fraction may lead to the discovery of new active compounds. Through optimization of the fermentation and extraction process, the toxic fraction could be increased, consequently increasing the degree of toxicity, acute toxicity, and AChE inhibition activity of extracts.

Fig. 3. High-performance liquid chromatography profiles from ethyl acetate extracts of culture filtrates produced by Aspergillus oryzae BPPTCC 6036 (A), Emericella nidulans BPPTCC 6035 (B), Emericella nidulans BPPTCC 6038 (C), Aspergillus tamarii BPPTCC 6037 (D), and Aspergillus versicolor BPPTCC 6039 (E). BPPTCC, Badan Pengkajian dan Penerapan Teknologi (Agency for The Assessment and Applications of Technology) Culture Collection, Tangerang, Indonesia.

In the development of bioinsecticidal compounds, complex mixtures are likely to be more durable in terms of insects developing resistance and behavioral desensitization [59]. According to Hummelbrunner and Isman [59], a complex mixture of bioinsecticidal compounds usually has a synergistic effect; compounds lacking acute toxicity might still confer protection to crops by reducing the fitness of insect herbivores through the inhibition of larval growth, disruption of larval development, or failure in pupal eclosion. The complex mixture of compounds or crude extracts presented herein exhibited toxicity, acute toxicity, and neurotoxicity, demonstrating the synergistic effect required for bioinsecticidal development.

In summary, the ethyl acetate extracts of culture filtrates from five mangrove endophytic fungi exhibited high toxicities against brine shrimp and S. litura larvae as well as AChE inhibitors. The HPLC analysis demonstrated that the extracts had different profiles and peak characteristics. Thus, the extracts of culture filtrates of mangrove endophytic fungi might provide new bioactive compounds for use as potential bioinsecticides. Further study on the screening and characterization of bioactive compounds from the extracts are warranted.