Endozoicomonas is a core bacterial group in the microbiota of coral and has been functionally associated with coral health in many studies over the past decade. However, their ecological functions are mostly unknown. In the present study, we discovered a novel, ubiquitous Endozoicomonas species, 8E, which possesses potent DMSP degradation activity via the DddD protein-mediated cleavage pathway, compared to E. acroporae (11) and the common oceanic DMSP utilizer, Ruegeria atlantica (41). DMSP cleavage activity was previously thought to be rare in Endozoicomonas, but the identification and proof of function in this study revealed that DMSP degradation is a common ecological function of Endozoicomonas species in coral reefs. Using comparative transcriptomic analysis, we uncorved variation in the regulation of dddD gene mediated DMSP metabolism in Endozoicomonas. This included pathways directly related and indirectly related, as well as other bacteria. These findings suggest that the underlying utilization mechanism of DMSP in bacteria is more complex, diverse, and crucial than previously thought.
Divergence in metabolic activities of DMSP degradation in two Endozoicomonas species
Within 12 hours of incubation, we clearly observed different profiles of DMSP degradation and uptake between 8E and E. acroporae. 8E displayed rapid and efficient DMSP cleavage, produced the highest DMS concentration (79.92 nmol/ml) within 6 hours of incubation, and consumed less than 35% of the supplied DMSP (Fig. 4a). Contrastingly, E. acroporae consumed more than 70% of the supplied DMSP in the first 8 hours and produced only a small amount of DMS (14.55 nmol/ml) (Fig. 4b). The underlying molecular mechanism causing the metabolic differences is still unclear, but three possible causes based on these results include: (i) Molecular variations at the gene and enzyme levels (i.e., variations in the dddD gene cluster and differentiations in enzymatic kinetics and activity); (ii) Variations in the physiological purpose of DMSP metabolism; (iii) More than one DMSP lyase gene participated in the DMSP degradation.
The DddD protein in the two Endozoicomonas species shares relatively low sequence identities with M. zhenjiangenesis and Pseudomonas sp. SJZ079 (Table S3), indicating the presence of molecular variations at the gene and enzyme levels. Furthermore, the phylogenetic relationship of the DddD protein and operon arrangement shows incongruence with bacterial taxonomy and phylogeny (Fig. 3a), indicating that the dddD gene clusters in both bacteria were horizontally acquired from different sources. Therefore, the different profiles of DMSP degradation may be caused by primary variations in the metabolic features of uniquely-sourced dddD genes or operons. Furthermore, the enzymatic kinetics and activity of the DddD protein between the two bacteria would be variable. We propose that the DddD enzyme might play a key role in the different metabolic profiles of the bacteria. The efficiency of DMSP cleavage activity was high in 8E (confirmed by the NanoSIMS results) which produced a DMS concentration 5 times higher than that of E. acroporae with only half the DMSP consumption (Fig. 4). This difference in observed efficiency suggests that the DddD lyase in 8E might have a high Vmax value (indicating higher DMSP cleavage activity) and a low Km value (indicating higher affinity to DMSP) than that in E. acroporae. Further investigation is required to confirm enzymatic characters. Cloning the purified DddD protein into a clear system such as E. coli and observing heterogeneous expression may be a direction for future study.
Apart from variations in the operons or genes, we also propose that variations in the physiological purpose of DMSP metabolism could lead to varying profiles of DMS production and DMSP uptake in the two bacteria. The upregulation of genes involved in the TCA cycle and amino acid metabolism in 8E following the addition of DMSP likely indicates that DMSP is used as a substitute carbon source for energy generation in poor nutrient conditions. On the other hand, the energy-related genes were commonly downregulated when DMSP was added in E. acroporae, suggesting that DMSP is used for non-energy production metabolisms. Similarly, we found that the proliferation efficiency of 8E was two times higher than E. acroporae with the addition of DMSP in the specific minimum medium, suggesting that DMSP may be used as an alternative energy source for 8E to propagate. The rate of proliferation of E. acroporae did not increase regardless of whether DMSP was added to the medium, indicating that the bacteria was under stressful conditions. Hence, E. acroporae likely uses DMSP to cope with stressful conditions. The bacterium might store DMSP and gradually cleave it to produce DMS. This strategy is common for DMSP users. Microorganisms accumulate DMSP for its beneficial antioxidant (24), osmolytic and cryoprotectant (42) properties, which might help soothe stressful physiology. Overall, though DMSP might mainly act as an antioxidant or other beneficial anti-stress molecule in E. acroporae, it could act as an energy source in 8E. Many studies have discussed how DMSP plays different physiological roles in different organisms (24, 29, 43). However, details comparative studies on the subject are rare, especially concerning only dddD-mediated DMSP utilization in closely related organisms.
After findings indicating that DMSP is not only the precursor of atmospheric trace gas DMS, but also plays multiple roles in different marine organisms (21, 42), increasing numbers of DMSP lyase have been identified. To date, nine DMSP lyase (including algal Alma1) have been found and categorized into three different lysis pathways based on their byproducts. There is a possibility of a novel degradation enzyme participating in Endozoicomonas DMSP degradation, as more than half of the genes (~ 2500 genes) are hypothetical. A future dddD gene knock-out experiment could be applied to clarify the hypothesis despite the lack of a successful gene knockout experiment conducted on Endozoicomonas to date.
Differential gene expression in DMSP pathways and relevant metabolisms in Endozoicomonas
According to the biochemical assay and the distinct gene regulation in two Endozoicomonas species that is associated with energy- and carbohydrate-related metabolic pathways, we believe the two species use DMSP for different biological purposes. This difference is also found when examining the pathway not directly linked with DMSP metabolism (layer 3, Fig. 5, S3). Genes for flagella, chemotaxis and quorum-sensing related pathways were upregulated in 8E. We observed a similar trend of gene regulation when the coral mucus-associated bacteria Roseobacter clades were exposed to DMSP (44–48). A flagella-like structure was observed on 8E during incubation in 0.1 mM DMSP in minimal medium (Fig. 1b), suggesting that flagella related physiological responses including chemotaxis and quorum-sensing are coupled with DMSP metabolism. However, a similar coupling response was not observed in E. acroporae, indicating that the gene upregulation concurrence of flagella, chemotaxis and quorum-sensing only appears in some DMSP bacterial degraders.
Additionally, the upregulated genes involved in aerobactin (a bacterial iron chelating agent, also called siderophore) biosynthesis and the Fe (III) hydroxamate transporter may increase resistance to oxidative stress in E. acroporae but not in 8E. For example, siderophores might increase resistance to oxidative stress in E. coli (49) and in the human pathogen Yersinia pseudotuberculosis (50). We speculate that aerobactin, a citrate-hydroxamate siderophore, could be another way for E. acroporae to mitigate oxidative stress.
dddD -mediated DMSP metabolism in coral reefs is more ecologically crucial than previously thought
The dominant DMSP lyase in coastal seawater is DddD protein, which is mainly found in Oceanospirillales (class Gammaproteobacteria). However, the ecological roles and influences of Oceanospirillales in DMSP-abundant coral reefs are still unclear. Endozoicomonas, being one predominant group of Oceanospirillales in coral, has long been thought to participate in coral sulfur cycling via DMSP metabolism (19, 51). With the framework of additional Endozoicomonas genomes sequenced in recent years, we inspected the gene contents of our sample and identified 10 Endozoicomonas species that contain DMSP cleavage genes (Table S6), indicating that DMSP degradation is a common eco-physiological function in Endozoicomonas and suggesting that this bacterial group plays a more crucial role in the sulfur cycle of coral holobionts or coral reefs than previously thought.
The DMS concentration produced by both Endozoicomonas species detected in our experiment was higher than that of R. atlantica. In out experiment, the cleavage ability of Endozoicomonas were outcompete to the widely documented DMSP degraders Roseobacter. In contrast, Ruegeria can tolerate a high DMSP concentration (5 mM) incubation and displayed higher DMS production at this high DMSP concentration than at a low DMSP concentration (0.1 mM) feeding incubation (52). In most cases, DMSP concentration in coral is less than 0.1 mM (40, 53, 54), and the highest DMSP concentration of 0.177 mM was detected in the coral species Acropora millepora under thermal stress (55). Our observations of the low DMS production rate of R. atlantica matched results from previous studies showing that Ruegeria species can shift the DMSP metabolic pathway from demethylation to cleavage along with an increase in DMSP concentration (from 0.1 mM to 5 mM DMSP) (52). Hence, we suggest that the Ruegeria in the stressed corals mostly metabolize DMSP via a non-DMS generated demethylation pathway. Unlike Ruegeria, Endozoicomonas cannot tolerate high DMSP concentrations, but can cope with 0.1 mM DMSP (Fig. S6), suggesting that a species of Endozoicomonas, rather than Ruegeria, could be the main producer of DMS in stressed corals.
We noticed that most Endozoicomonas genomes (9/10) accommodate the dddD gene (Table S6) rather than other DMSP cleavage genes. Compared to other DMSP cleavage pathways, dddD is unique and advantageous in the DMSP cleavage pathway as it produces 3-hydroxiopropionate instead of cytotoxic compound acrylate (30). DMSP cleavage does not accumulate harmful compounds in the cell, and also releases the potent antioxidant DMS, indicating that the selection of the dddD gene in Endozoicomonas may be associated with the establishment of a bacteria-coral mutualism. We performed a simple distribution analysis of 8E across the Indo-Pacific Ocean (Fig. S7) and found that 8E are widely distributed and have a similar distribution with E. acroporae (11). We suspect that the ability to cleave DMSP, enabled by the dddD gene, might be one of the reasons for the widespread appearance of Endozoicomonas, especially in the Acropora coral.