To identify differences in the composition of the exometabolome of four functional benthic groups and their adaptation to different biodiversity treatments, we conducted a four-week experiment and sampled organism-surrounding seawater for untargeted metabolomics in June 2022 in the Ocean2100 coral aquarium facility of Justus Liebig University Giessen, Germany.
Species selection
To investigate the exometabolomes under different biodiversity scenarios for diverse benthic marine organismal groups, we studied two stony coral, two soft coral, two macroalgal, and one sponge species. We selected the species based on a series of productivity assays conducted in the Ocean2100 coral aquarium facility (Engelhardt et al. 2023; Vetter et al. 2024): Pocillopora verrucosa (Ellis & Solander, 1786) was found to increase photosynthetic productivity with increasing diversity of surrounding organisms (Engelhardt et al. 2023), while Montipora digitata (Dana, 1846) was less productive in polyculture (Engelhardt et al. 2023). The two soft coral species Sinularia sp. (May, 1899) and Xenia sp. (Lamarck, 1816) and the two macroalgal species Caulerpa sp. (Lamouroux, 1809) and Peyssonnelia sp. (Decaisne, 1841) were chosen based on their range of effects on the productivity of stony corals (Engelhardt et al. 2023). The sponge Haliclona cnidata (Schellenberg, Reichert, Hardt, Schmidtberg, Kämpfer, Glaeser, Schubert & Wilke, 2019) increased the photosynthetic productivity of stony corals (Engelhardt et al. 2023).
Fragmentation and acclimation
Stony coral fragments were produced with an angle grinder (Multitool 3000-15, Dremel, Netherlands). Soft corals and the macroalga Peysonnelia sp. were cut with a scalpel and glued to roughened glass slides. The macroalga Caulerpa sp. was tied together in small bundles with nylon string, and the sponge was threaded through the main tube with nylon string. All organisms were suspended in the water column. During the six weeks of healing and acclimation, we maintained all fragments in the main aquarium facility in controlled polyculture tanks (Fig. 1). At the start of the experiment, replicate fragments were distributed into three treatments: i) monoculture (each species held in isolation - no other species or sediments present), ii) controlled polyculture (all investigated species together in a clean tank), and iii) seminatural polyculture (all investigated species in a mesocosm tank with additional benthic organisms, fishes, shrimps, sea grass, and sediments). All organisms were cultured at 26 ˚C, a light intensity of 200 ± 20 µmol photons m− 2 s− 1, and salinity of 35. This set-up was maintained for four weeks until the incubations and sampling were performed (Fig. 1).
Water preparation and incubations
We conducted the incubations on four consecutive days. First, we sterile-filtered artificial seawater (ASW) from an empty tank within the same aquarium system: after pre-filtering the ASW through a fine-mesh net (65 µm), we used a peristaltic pump (MCP Process, Idex Health & Science GmbH) for serial filtration; first with an autoclaved 0.45 µm membrane filter (Qpore), then with an autoclaved 0.22 µm membrane filter (Qpore), and finally with a sterile 0.2 µm membrane filter (Whatman) using in-line filter holders (Sartorius, Göttingen).
We performed incubations in clean autoclaved 1-L glass jars filled with 625 ml of the sterile-filtered ASW. The organisms were rinsed with sterile-filtered ASW and separately incubated in the jars. On each measurement day, three control incubations without organisms were included to account for the metabolites already present in the incubation water. The incubations lasted 2 h on a multi-point stirring plate at 9 ± 1.5 cm s− 1 flow and 200 ± 20 photons µmol m− 2 s− 1 (Rades et al. 2022).
Sampling and sample processing
After incubations, 50 ml of the seawater was transferred with an autoclaved syringe through a 0.22 µl Sterivex filter (EMD Millipore Corporation, Burlington, USA) and directly loaded on HLB solid-phase extraction columns (60 mg, Waters, Massachusetts, US) at a flow-rate of 1 ml min− 1 (MCP Process, Idex Health & Science GmbH). Columns were preconditioned with 2 column volumes acetonitrile (HPLC grade, VWR, USA) and two column volumes ultra-pure water (Sartorius AG, Göttingen, Germany) on a SPE vacuum chamber (Macherey-Nagel, Düren, Germany). After sample loading, columns were rinsed with two column volumes of ultra-pure water, dried on a vacuum block for 15 minutes (Macherey-Nagel, Düren, Germany), and stored at 4°C until elution (max. 48 h). For elution, we used two column volumes of 100% methanol (LC-MS grade, Honeywell Riedel-de-Haën™, Germany), dried samples overnight in a vacuum centrifuge (GeneVac HT-12, SP Industries, USA), re-dissolved the dry metabolites with 75 µl methanol in a v-bottom 96-well microplate (Greiner bio-one, Germany), and closed the plate with a pierceable TPE capcluster (Micronic, Netherlands).
Mass spectrometric analysis
We used a quadrupole time-of-flight spectrometer (LC-QTOF maXis II Bruker Daltonik), which was equipped with an electrospray ionization source connected to an Agilent 1290 infinity LC system (Agilent) for all UHPLC-QTOF-HR-MS and MS/MS measurements (Brinkmann et al. 2022). At 45°C, we performed C18 RP-UHPLC (ACQUITY UPLC BEH C18 column [130 Å, 1.7µm, 2.1 x 100 mm]) with the following linear gradient: 0 min: 95% A; 0.30 min: 95% A; 18.00 min: 4.75% A; 18.10 min: 0% A; 22.50 min: 0% A; 22.60 min: 95% A; 25.00 min: 95% A (A: H2O, 0.1% formic acid; B: acetonitrile, 0.1% formic acid; flow rate: 0.6 mL/min). Scan range was 50-2000 m/z at 1 Hz for MS measurements. MS/MS analysis was carried out at 6 Hz with selection of top 5 intense ions in full spectrum for collisional induced dissociation (CID) using N2. Precursors were excluded after two spectra and released after 0.5 min and if their intensity increased by ≥ 1.5-fold.
Metabolomics data processing
MS data were processed with DataAnalysis 5.3 (Bruker) with "Recalculate Line Spectra" (Threshold 10000) and "Find Molecular Features" (SN = 0), with features given as unique combination of mass to charge ratio (m/z) and retention time (s). By bucketing (12 s, 5 ppm), using ProfileAnalysis 2.3 (Bruker) we checked which features from different samples describe the same ion. Based on the chromatograms, three outliers could be identified and were removed from analyses (Caulerpa sp. monoculture 1, M. digitata monoculture 3, Xenia sp. controlled polyculture 3). Features found at least twice within each species and treatment (twice per quadruplet) were considered 'valid'. This reduced the number of features from 873 (MS raw data) to 338 (curated feature table (CFT)).
Molecular networking
For annotation and visualization of the MS/MS data, we used the Global Natural Products Social Molecular Networking (GNPS) database to create a molecular network. For this, we uploaded .mzML files from the untargeted data to the MassIVE server (https://massive.ucsd.edu/) and used molecular networking of the GNPS database with default parameters for high resolution mass spectrometers (version release 30; Wang et al. 2016). The results are available at:
https://gnps.ucsd.edu/ProteoSAFe/status.jsp?task=a356c65ad38249379d64a23e4f39c6ca . Most of the parent ions and molecular families had no matches (comparison to > 220 000 metabolites), even though the database includes a marine natural product library (Wang et al. 2016).
To check the quality of annotations, we generated an Extracted Ion Chromatogram (EIC) (based on sum formula ± 0.02 Da) for all samples containing the node. The highest full scan spectrum of the EIC at the relevant retention time (spectrum at side of highest peak if saturated) was checked for the entry of the annotated formula as well as for isotope patterns and mass precision using ‘Chem-Formula manually’. The retention time (RT) was limited to 18:00 min to exclude lipophilic compounds in the washing step as they show high signal intensities and therefore dominate statistical analyses (details see Online resource 1 Fig. 1–27 and Online resource Table 1). Cytoscape (Version 3.10.01) was used for visualization and to link known and unknown compounds that belong to the same molecular family to create chemical inventories of the organisms.
Statistical analysis
We performed statistical analyses and visualizations with the curated feature table (CFT) and MSMS node table from GNPS (M2NT) in R statistical environment (v.4.3.2, R Core Team 2023) using the packages tidyverse (Wickham et al. 2019), decontam (Davis et al. 2017), vegan (Oksanen et al. 2022), lme4 (Bates 2016) and IndicSpecies (Cáceres and Legendre 2009). In the M2NT, all nodes that only appeared in one sample were removed. The CFT were decontaminated with the decontam package using the prevalence method with the default threshold of 0.1.
To investigate effects of species, organismal group, and treatments, as well as between species within each treatment and between treatments within each species, we created general mixed effects models (glmer) with the lme4 package. For the M2NT we set family = binomial for glmer creation. With the CFT, glmers were performed on log + 1 transformed data with family = Gamma. In both, species, organismal group, and treatment were set as continuous fixed factor (depending on the target effect group) and day of sampling as random factor, followed by a Tukey post-hoc comparison and ‘holm’ adjustment for multiple testing. For visualization, we plotted Bray-Curtis dissimilarities of the CFT and Binary-Jaccard similarities of the M2NT (presence/absence) as non-metric Multi-Dimensional Scaling ordinations. To detect representative metabolites for treatments, species, or combinations thereof, we used the ‘multipatt’ function from IndicSpecies on peak intensity data from the CFT at p ≤ 0.01.