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Adaptations to nitrogen availability drive ecological divergence of chemosynthetic symbionts [1]

['Isidora Morel-Letelier', 'Eco-Evolutionary Interactions Group', 'Max Planck Institute For Marine Microbiology', 'Mpimm', 'Bremen', 'Benedict Yuen', 'A. Carlotta Kück', 'Yolanda E. Camacho-García', 'Centro De Investigación En Ciencias Del Mar Y Limnología', 'Cimar']

Date: 2024-06 

Bacterial symbionts, with their shorter generation times and capacity for horizontal gene transfer (HGT), play a critical role in allowing marine organisms to cope with environmental change. The closure of the Isthmus of Panama created distinct environmental conditions in the Tropical Eastern Pacific (TEP) and Caribbean, offering a “natural experiment” for studying how closely related animals evolve and adapt under environmental change. However, the role of bacterial symbionts in this process is often overlooked. We sequenced the genomes of endosymbiotic bacteria in two sets of sister species of chemosymbiotic bivalves from the genera Codakia and Ctena (family Lucinidae) collected on either side of the Isthmus, to investigate how differing environmental conditions have influenced the selection of symbionts and their metabolic capabilities. The lucinid sister species hosted different Candidatus Thiodiazotropha symbionts and only those from the Caribbean had the genetic potential for nitrogen fixation, while those from the TEP did not. Interestingly, this nitrogen-fixing ability did not correspond to symbiont phylogeny, suggesting convergent evolution of nitrogen fixation potential under nutrient-poor conditions. Reconstructing the evolutionary history of the nifHDKT operon by including other lucinid symbiont genomes from around the world further revealed that the last common ancestor (LCA) of Ca. Thiodiazotropha lacked nif genes, and populations in oligotrophic habitats later re-acquired the nif operon through HGT from the Sedimenticola symbiont lineage. Our study suggests that HGT of the nif operon has facilitated niche diversification of the globally distributed Ca. Thiodiazotropha endolucinida species clade. It highlights the importance of nitrogen availability in driving the ecological diversification of chemosynthetic symbiont species and the role that bacterial symbionts may play in the adaptation of marine organisms to changing environmental conditions.

Funding: This project received funding from the Max-Planck-Gesellschaft to LGWE. ACK, BY, LGWE and IML received salaries from Max-Planck-Gesellschaft. Part of the sequencing was carried out by the DNA Technologies and Expression Analysis Core at the UC Davis Genome Center, supported by NIH Shared Instrumentation Grant 1S10OD010786-01 and salary to LGEW. JMP’s and JTO’s contributions and LGEW's salary were supported by the ERC Starting Grant EvoLucin (grant number 802494), and a Vienna Research Grant for Young Investigators from the Vienna Science and Technology Fund (WWTF, VRG14-021). LGEW was supported with salary and a postdoctoral fellowship by a Marie Curie individual postdoctoral fellowship “MSCA-IF-EF-RI” for project #Pansymbiosis with grant number SEP-210693430. We thank the Gordon and Betty Moore Foundation ( https://www.moore.org ) through Grant GBMF5603 to JAE. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

We used high-throughput metagenomic sequencing to recover metagenome-assembled genomes (MAGs) of bacterial symbionts associated with Codakia and Ctena sister species from both sides of the Isthmus of Panama. Our primary objective was to investigate how differing environmental conditions on either side of the Isthmus have influenced the selection, diversity, and functional traits of symbionts associated with lucinid clams. We found that all lucinid symbionts from the Caribbean had the potential to fix nitrogen and assimilate nitrate, but these functions were absent in all symbionts from the TEP. To further explore the evolutionary origins of nitrogen fixation in different lucinid symbiont lineages, we compared the genomes of symbionts across the Isthmus of Panama with other lucinid symbiont genomes from around the world. Using phylogenetic reconciliation, we reconstructed the evolutionary history of the nifHDKT operon and identified two distinct HGT events that led to nif gene acquisition and correlated with the colonization of nutrient-poor environments. Within the globally distributed Ca. Thiodiazotropha endolucinida clade, populations in nutrient-poor environments possessed nitrogen fixation genes, whereas those closely related populations in nutrient-rich environments did not. Despite their high Average Nucleotide Identity (ANI; >95%) and evidence of homologous recombination between geographically distant populations, our findings indicate that these Ca. Thiodiazotropha endolucinida populations have diverged ecologically and occupy separate niches that differ in nitrogen availability. We hypothesize that the diversification of these symbionts was facilitated by the acquisition of the nitrogen fixation genes through HGT. Our results provide valuable insights into the dynamic interplay between environmental factors and genetic exchange that shape the ecology, evolution, and diversification of host-associated microorganisms.

The Lucinidae, one of the most species-rich bivalve families, thrive in a wide array of marine environments and is the most diverse group of chemosymbiotic animals [ 11 ]. Lucinids house endosymbiotic sulfide-oxidizing Gammaproteobacteria intracellularly within specialized gill cells, where they use the energy derived from oxidizing reduced sulfur compounds to synthesize organic carbon [ 11 – 13 ]. This partnership is obligate for lucinids because they rely on their symbionts for a significant portion of their carbon nutritional requirements [ 14 , 15 ]. The bacterial symbionts are acquired from free-living populations in the environment during the early developmental stages of each new generation [ 16 , 17 ]. Recent studies have begun to unveil the metabolic and genomic diversity among symbionts from vastly different environments across the globe, including differences in their abilities to metabolize carbon and inorganic nitrogen [ 18 – 20 ]. Notably, some symbiont clades of the genus Ca. Thiodiazotropha were the first documented example of nitrogen-fixing chemosynthetic symbionts [ 21 , 22 ]. However, the precise processes governing the diversification and adaptation of lucinid symbionts to changing environmental parameters, which may lead to the divergence of local populations of globally distributed taxa, remains poorly understood. Sister species of lucinids from the genera Codakia and Ctena have diverged on either side of the Isthmus of Panama [ 1 , 11 ]. This unique relationship allows for a comparative study of symbiont adaptation, free from the confounding effects of host evolutionary history.

The closure of the Isthmus of Panama about 2.8 million years ago had a profound impact on oceanic conditions, altering environmental factors such as ocean currents, salinity, temperature, and nutrient availability on both sides of the Isthmus [ 7 ]. The Tropical Eastern Pacific (TEP) continued to experience regular nutrient input due to seasonal upwelling, coupled with increased primary productivity, variable temperatures, and strong tides [ 7 ]. In contrast, the Caribbean coast became characterized by stable and warmer temperatures, higher salinity, and a notably low availability of organic nutrients [ 7 ]. Animal populations that were once connected became separated by the closure of the Isthmus, ultimately resulting in the emergence of sister species on separate evolutionary trajectories, diverging in response to the markedly different environmental conditions on either side of the Isthmus [ 8 ]. The adaptation strategies enabling these sister species to thrive in their respective, contrasting environments have been extensively studied, but these studies have primarily focused on the animals themselves, largely overlooking the potential influence of host-associated microorganisms (reviewed in [ 1 , 8 , 9 ]). The Isthmus of Panama presents a unique opportunity to investigate the drivers of diversification and adaptation through a "natural experiment” running for millions of years with a taxonomically replicated set of animal-microbe assemblages [ 10 ]. This offers valuable insights into the interplay between hosts and their microbial symbionts in the context of environmental change.

Global change gives rise to new environmental conditions and niches that organisms can adapt to and exploit, and the extent and mechanisms of adaptation in marine species may be significantly influenced by their microbiomes [ 1 ]. Bacterial symbionts of animals can potentially adapt to changing environments more rapidly and more flexibly than their hosts due to traits such as shorter generation times, enhanced recombination capabilities, and the potential for horizontal gene transfer (HGT) between distantly-related organisms [ 2 ]. In addition, horizontally acquired bacterial symbionts—those obtained from the environment in each generation—have access to a larger genetic pool for genetic exchange during their free-living phase, which can lead to faster adaptive responses [ 3 ]. Consequently, symbionts can serve as a source of ecological innovation, enabling the symbiosis to tap into novel resources and adapt to novel habitats [ 4 – 6 ]. Understanding the mechanisms enabling microbial symbionts to adapt to new environmental conditions will provide novel insights into how animal-microbe symbioses respond to changing environments.

To investigate whether homologous recombination might play a role in maintaining the genetic connectivity of the Ca. T. endolucinda populations from different geographic locations, we measured the relative rates of recombination to mutation events from core genome alignments of all the Ca. T. endolucinda lineages. The R/θ of the Ca. T. endolucinda core genome—a 2,710,509 base pairs (bp) alignment—was 0.0999 while the r/m ratio was 0.589 ( Table 2 ), which is significantly higher than the value previously measured for this species (0.082) [ 19 ]. This notable difference could be explained by a lack of resolution in the data, as Osvatic and colleagues analyzed Ca. T. endolucinda MAGs from a single site (Bocas del Toro, Panama) [ 19 ]. Furthermore, we explored how these rates of homologous recombination compared with those observed in Ca. T. gloverae, a globally-distributed symbiont inhabiting deep-water or temperate environments, as well as the rates previously reported for Ca. T. taylori, a symbiont found in tropical oligotrophic shallow-water environments around the world [ 18 , 19 ]. The R/θ of the Ca. T. gloverae core genome (1,619,656 bp) was 0.121 while the r/m ratio was 1.138 ( Table 2 ). The rates of recombination to mutation events in Ca. T. endolucinda was therefore the lowest of the three globally distributed lucinid symbiont species and highest in Ca. T. gloverae ( Table 2 ).

We investigated the ancestral states of nifHDKT presence or absence to understand the evolutionary processes that could explain the incongruence between the symbiont phylogenomic tree and the nifHDKT tree ( Fig 4B ). According to this analysis, the last common ancestor (LCA) of the Sedimenticolaceae, as well as the LCAs of both Ca. Thiodiazotropha and Sedimenticola, did not possess the potential to fix nitrogen (Figs S4 and 4B ); independent gene gain or loss events explain the sporadic distribution of this metabolic function across the symbiont tree. The last common ancestor (LCA) of Ca. Sedimenticola endophacoides was inferred to possess nifHDKT genes, indicating a subsequent loss of the nitrogen fixation potential in the Florida symbiont lineage. Conversely, we identified three well-supported instances of nifHDKT horizontal gene transfer (Figs 4B and S3 ) to and from the Ca. Thiodiazotropha endolucinida clades. The LCA of the Ca. T. endolucinida clade lacked nifHDKT genes but the genes were subsequently acquired from an ancestral node of the Ca. Sedimenticola “PEGO” lineage before the LCA Ca. T. endolucinida HAW and CAR lineages diverged (Figs 4B and S3 ). Additionally, the LCA of the Ctena4 lineage acquired the ability to fix nitrogen from an ancestral node of the Ca. T. endolucinida Hawaii lineage, while a Monitilora ramsayi symbiont (Monit1), acquired the nifHDKT genes from an ancestral node of Ca. T. endolucinida CAR and HAW (Figs S3 and 4B ). The ancestral reconstruction and gene reconciliation analyses were, however, unable to resolve the patterns of nitrogenase gene loss and gain across the deep-branching nodes of the other Ca. Thiodiazotropha symbiont clades (e.g. the LCA of Ca. T. taylori, Ca. T. lotti and Ca. T. weberae).

A The symbiont nifHDKT genes formed three major clades that were incongruent with the phylogenetic relationships of the symbionts. Pruned maximum likelihood phylogenetic tree depicting the clades of lucinid symbionts’ nifHDKT genes. The tree was inferred using the best fit model GTR+F+I+R6 from a concatenated alignment of these genes. B The last common ancestor of the Ca. Thiodiazotropha genus lacked nitrogen fixation genes, which were subsequently independently acquired through horizontal gene transfer by different symbiont lineages. Ancestral reconstruction of the presence/absence of nifHDKT (orange—nifHDKT present, blue—nifHDKT absent, black—ambiguous state) mapped onto a pruned cladogram based on a maximum likelihood phylogenomic tree inferred using the best fit model Q.insect+F+R9 from a GTDB’s multiple sequence alignment from dereplicated genomes (at 99.5% ANI). Symbiont clades are annotated as in Fig 3B and the clades of their corresponding nifHDKT genes are annotated according to A. Robust horizontal gene transfer events inferred from the reconciliation of the gene tree with the phylogenomic tree were superimposed and are depicted as pink arrows.

After dereplication, we analyzed a total of 242 MAGs from the order Chromatiales (including Ca. Thiodiazotropha and Sedimenticola genera), from which we identified 139 complete nifHDKT operons. Trees constructed for each individual nif gene had consistent topologies for the strongly supported lucinid symbiont nif clades, suggesting that nifH, nifD, nifK and nifT are co-inherited ( S1 Fig ). The resulting nifHDKT tree revealed that all the lucinid symbiont nifHDKT sequences are closer to each other than to any non-symbiont relative (Figs S2 and 4A). Moreover, our analysis revealed the presence of three distinct major symbiont nifHDKT clades, which we have denoted as Clade A, Clade B, and Clade C (Figs S2 and 4A ). Clade A comprises genes from Ca. Sedimenticola endophacoides. Clade B consists of genes from Ca. Thiodiazotropha endolucinida, Ctena4, Monit1 (Monitilora ramsayi symbionts), and Pegophyseminae symbionts “PEGO”. Lastly, Clade C encompasses genes from Ca. Thiodiazotropha taylori, lotti, weberae, and fergusoni, as well as Ctena2 and Ctena3 ( Fig 4A and 4B ). The topology of these clades was inconsistent with the phylogenomic tree (Figs 3B and 4A ). Although the Ca. T. endolucinida, Ctena4, and Monit1 symbionts belong to the genus Ca. Thiodiazotropha, the nifHDKT genes from these symbionts formed a clade with the nifHDKT genes of the “PEGO”, which belongs to the Sedimenticola genus ( Fig 4A ).

A Geographic origins of the MAGs included in this analysis. Points were colored based on the clade they belong to. Different shapes indicate whether the samples were obtained in this study (circles) or in previous studies (triangles). The magnified map of the Isthmus—where the density of sampling sites was high—shows these sampling sites in detail. The map was generated with data from Natural Earth ( http://www.naturalearthdata.com/ ) using the R package "rnaturalearth" (v0.3.2) ( https://github.com/ropensci/rnaturalearth ). B Maximum likelihood phylogenomic tree inferred from GTDB’s multiple sequence alignment using the best fit model Q.plant+F+I+G4. The names of clades found in the host sister species across the Isthmus of Panama are in turquoise (Caribbean) or purple (TEP; Tropical Eastern Pacific) font, while globally-distributed symbiont clades are highlighted in gray. Previously described clades were collapsed and annotated in the same way as in the most recent phylogenetic analysis of lucinid symbionts [ 18 ] ( S2 Table ). New clades were collapsed based on ANI (>95%) and/or location. Clades with the potential for nitrogen fixation are indicated with a star and colored symbols match the symbology of the map. Black squares indicate UFB values above 95% and SH-aLRT values above 80%.

To further investigate the distribution of the nitrogen fixation pathway in lucinid symbionts, we generated a phylogenomic tree combining our newly obtained MAGs with those used in the most recent global tree of lucinid symbionts [ 18 ]. In addition, we added 23 new high-quality MAGs from both the Ca. Thiodiazotropha and Sedimenticola genera ( S2 Table ). These MAGs originated from specimens of 11 lucinid species collected at various shallow water and low latitude sites to include other oligotrophic and nutrient rich sites beyond the Isthmus ( Fig 3A and S1 Table ). Seven MAGs clustered within Ca. Sedimenticola endophacoides and three MAGs formed a Sedimenticola sister clade composed exclusively of symbionts from lucinids of the Pegophyseminae subfamily, which will be provisionally referred to as “PEGO”. Others clustered with Ca. T. boucheti, Ca. T. fergusoni and Ca. T. endolucinida. Three MAGs did not form clades with any other MAGs (Indoaustriella corrugata, Ctena bella Hawaii 21, Ctena bella French Poly symbionts), and thus represent novel species. The genomic potential for nitrogen fixation was widely, albeit heterogeneously, distributed across the lucinid symbiont tree and found in both Sedimenticola and Ca. Thiodiazotropha symbionts ( Fig 3B ). The presence/absence of nitrogen fixation genes varied even within the two species clades Ca. S. endophacoides and Ca. T. endolucinida. For example, despite their genetic similarity (>95% ANI, S1 Dataset ), nitrogen fixation genes were present in the MAGs of the Ca. Thiodiazotropha endolucinida lineage associated with Ctena bella from Hawaii (Ca. T. endolucinida HAW) but not in the Ca. Thiodiazotropha endolucinida MAGs retrieved from Lucina adansonia from Cape Verde ( Fig 3 and S6 Dataset). Similarly, Ca. Sedimenticola endophacoides associated with Phacoides pectinatus from Florida lacked nitrogen fixation genes, even though this ability was present in closely related lineages of Ca. Sedimenticola endophacoides associated with P. pectinatus from Guadeloupe (>96% ANI, S1 Dataset ) and Panama (>98% ANI, S1 and S6 Datasets).

The metabolic enrichment analysis across the Isthmus revealed that nitrogen fixation (found in 99% of Caribbean MAGs), nitrate assimilation (which includes nitrate transport), and assimilatory nitrate reduction (both found in 96% of Caribbean MAGs) pathways were enriched in the MAGs of all three symbiont clades found in the Caribbean ( Fig 2 ). All three Caribbean symbiont clades (Ca. T. endolucinda CAR, Ca. T. taylori, and Ca. T. fergusoni) consistently encoded these three nitrogen metabolic pathways that were absent in symbiont MAGs from the TEP, even though Ca. T. endolucinida Caribbean clade was more closely related to clades from the TEP (Ca. T. larai, Ca. T. boucheti and Ca. T. endolucinida TEP) than the two other Caribbean symbiont clades ( Fig 2 and S3 Dataset ). These enrichment patterns therefore do not correlate with the phylogenetic relationships of the lucinid hosts or their symbionts. The genes enriched in the Caribbean MAGs belonged to the same nitrogen metabolic pathways that were identified through the module enrichment analysis ( Table 1 and S3 and S4 Datasets ), namely nitrogen fixation and nitrate assimilation. Besides the minimum gene set for nitrogen fixation [ 28 ] (nifHDKENB), we observed enrichment of a varied repertoire of genes involved in the process, which included predicted functions in regulation, biosynthesis, assembly and structure. Additionally, a gene annotated as an H 2 O-forming NADH oxidase was enriched in Caribbean MAGs and was often located in the same genomic region as the nitrogen fixation genes. Both the mapping of the metagenomic reads to the nif genes and an HMM search of the nitrogenase against the metagenome assemblies supported the conclusion that the TEP symbionts did not have the potential to fix nitrogen ( S5 Dataset ). No metabolic modules were found to be enriched in TEP MAGs ( Fig 2 and S3 Dataset), but genes encoding an electron-transferring-flavoprotein dehydrogenase and genes involved in gamma-polyglutamate biosynthesis were enriched in all three symbiont clades from the TEP, a pattern which also did not correlate with symbiont or host phylogeny ( Table 1 and S4 Dataset).

The MAGs constituted six distinct clades—three from each side of the Isthmus, but all were taxonomically assigned to the genus Ca. Thiodiazotropha ( Fig 1C and 1D ). Two clades were identified as previously described Caribbean lucinid symbionts Ca. T. taylori and Ca. T. endolucinida [ 19 , 22 ]. Three of the remaining four clades are previously undescribed symbiont species, based on an ANI threshold of 95% for species delimitation [ 24 – 26 ] ( S1 Dataset ). We discovered one new bacterial species clade from the Caribbean (Ca. Thiodiazotropha fergusoni), for which we propose the name after Walter Ferguson, a Panamanian-born calypso singer and songwriter based in Cahuita, Costa Rica (1919–2023). We designate the two new species clades from the TEP as Ca. Thiodiazotropha larai and Ca. Thiodiazotropha boucheti. These names honor Minor Lara for his contributions to marine research and conservation in the Guanacaste region of Costa Rica, and Dr. Philippe Bouchet for his extensive work on lucinids. The third clade from the TEP had an ANI of ~95.5% to Ca. T. endolucinida, suggesting its inclusion within this species, but it formed a distinct monophyletic sub-clade unique to the TEP. We shall hereafter refer to this clade as Ca. T. endolucinida TEP (Tropical Eastern Pacific) to distinguish it from the originally described Caribbean clade, which we will hereafter refer to as Ca. T. endolucinida CAR. Two of the MAGs classified as Ca. T. fergusoni and one classified as Ca. T. endolucinida TEP, from the samples sequenced with PacBio, were circularized. No symbiont clade was present on both sides of the Isthmus ( Fig 1 ). However, unlike their hosts, the symbionts on either side of the Isthmus did not share a sister lineage relationship, indicating the absence of co-diversification in the host and symbiont phylogenies ( Fig 1B and 1C ). MAGs classified as Ca. T. fergusoni, Ca. T. endolucinida CAR and Ca. T. endolucinida TEP were recovered from both Codakia and Ctena specimens, while Ca. T. taylori, Ca. T. boucheti and Ca. T. larai MAGs were only recovered from Ctena specimens ( Fig 1C and 1D ).

A Sister species from the lucinid genera Codakia (circles) and Ctena (triangles) were sampled on the Caribbean (turquoise) and TEP (Tropical Eastern Pacific; purple) side of the Isthmus. The map was generated with data from Natural Earth ( http://www.naturalearthdata.com/ ) using the R package "rnaturalearth" (v0.3.2) ( https://github.com/ropensci/rnaturalearth ). B Schematic representation of the phylogenetic relationships of the Codakia and Ctena species collected, based on the most recent taxonomic study [ 23 ]. C Six symbiont lineages, four previously undescribed (bold text), were associated with either Codakia (circles) or Ctena (triangles) hosts across the Isthmus. Maximum likelihood phylogenomic tree of symbiont MAGs recovered from the gills of host sister pairs inferred from GTDB’s (Genome Taxonomy Database) multiple sequence alignment using the best fit model Q.plant+F+I+G4. MAGs of Monitilora ramsayi symbionts [ 18 ] were used as an outgroup. Blue squares indicate ultra fast bootstrap (UFB) values above 95% and SH-aLRT values above 80%. All monophyletic clades were collapsed by location to facilitate interpretation. D No symbiont clade was found on both sides of the Isthmus. Heatmap depicting average nucleotide identities (ANI) between the symbiont clades found across the Isthmus. The symbiont clades are colored in tones of green (Caribbean side “CAR”) and purple (TEP side) and the genus of the host from which the MAGs were recovered is indicated by the colors violet (Codakia) and pink (Ctena).

Discussion

Chemosymbiotic sister species separated by the Isthmus of Panama reveal symbiont adaptation to changing environments Studies on the adaptations enabling animals to cope with environmental changes resulting from the closure of the Isthmus of Panama (reviewed in [8]) have largely overlooked the role of host-associated microbes [1]. Our investigation focused on understanding how environmental conditions on either side of the Isthmus of Panama have influenced the distribution, diversity and metabolic functions of symbionts associated with lucinid sister species separated by the closure of the Isthmus. Codakia and Ctena sister species from either side of the Isthmus hosted distinct clades of symbionts from the genus Ca. Thiodiazotropha. Despite the absence of symbiont clade overlap across the Isthmus, we observed a lack of specificity between hosts and symbionts, which was evident in the incongruence of their respective phylogenetic relationships. We observed multiple instances where different host species of the Codakia and Ctena genera on the same side of the Isthmus share the same symbiont groups. This is consistent with horizontal symbiont acquisition from the environment and indicates that the environment is a key factor influencing symbiont selection and distribution [19,29,30]. We compared the metabolic potential of the symbionts from either side of the Isthmus to investigate how the vastly different environmental conditions of the Caribbean Sea (CAR) and TEP have shaped the evolution of lucinid symbionts and their metabolic capabilities. We discovered that nitrogen fixation genes were encoded in the MAGs of all Caribbean symbionts but absent in all the MAGs of symbionts from the TEP. Similarly, we observed that the capacity for assimilatory nitrate reduction was ubiquitous in symbionts from the Caribbean, but not in those from the TEP (Fig 2). Assimilatory nitrate reductases are similarly absent in many deep-water symbiont lineages, which also lack nitrogen fixation genes [18], suggesting that these two capabilities may be linked. The coastal waters of the TEP are frequently enriched with nutrients due to seasonal upwelling, leading to nitrate levels that are roughly ten times higher than those found in seagrass beds in the Caribbean [31,32]. Hence, the absence of nitrogen fixation genes in the symbiont MAGs from the nitrogen-rich TEP region is consistent with the hypothesis that lucinid symbiont diazotrophy has evolved as an adaptation to life in nitrogen-poor oligotrophic habitats like tropical coral reefs and seagrass beds [33]. Although the TEP symbionts did not possess the capacity to fix nitrogen, the MAGs of all three TEP symbiont lineages encoded unique accessory metabolic capabilities that were lacking in the Caribbean symbionts. Specifically, TEP symbionts had the genetic potential for synthesizing gammapolyglutamate (Table 1), a storage compound produced by bacteria during nutrient limitation [34]. Electron-transferring-flavoprotein (ETF) dehydrogenase genes were also enriched in the MAGs from the TEP symbionts (Table 1); this enzyme is upregulated in Pseudomonas aeruginosa and Pseudomonas syringae in response to low temperatures [35,36] and in Neisseria gonorrhoeae under anaerobic conditions [37]. The TEP environment is characterized by drastic seasonal changes in physical conditions and nutrient availability. For example, nitrate concentrations in the upper layer of water differ by approximately one order of magnitude between the wet and dry seasons [38–41]. It is intriguing to speculate that the gammapolyglutamate synthesis and ETF dehydrogenase genes are beneficial to the TEP symbionts during the seasonal environmental changes typical of upwelling regions, which include changes in nutrient levels, colder temperatures, or reduced oxygen levels [38–41].

Lucinid symbionts have convergently evolved the ability to fix nitrogen on multiple occasions We reconstructed the phylogenetic relationships of the nifHDKT genes to gain insights into the evolution of nitrogen fixation in the lucinid symbionts and the factors underlying the sporadic distribution of this metabolic capability across the lucinid symbiont tree. While clades A and C of the nifHDKT gene tree mirrored the phylogenetic relationships of the symbionts, the incongruence of clade B with symbiont tree topology suggests the nifHDKT genes of the Ca. T. endolucinida, Monitilora ramsayi symbionts (Monit1), and Ctena4 lineages have not co-evolved and/or co-diversified with the single-copy core genes in their respective genomes (Fig 4A). To further investigate this incongruence, we inferred the ancestral states and horizontal gene transfer events of the nifHDKT genes throughout the evolution of lucinid symbionts. This analysis indicated that diazotrophy was most likely not an ancestral trait of either the Sedimenticola or Ca. Thiodiazotropha genera, but was acquired independently by different symbiont lineages that inhabit nutrient-poor environments (Figs S4 and 4B). For example, nifHDKT genes were absent in the last common ancestor (LCA) of the Ca. T. endolucinida species clade (Fig 4B). Consequently, the TEP, Cape Verde L. adansoni and Cardiolucina cf. quadrata lineages, which either originate from nutrient-rich upwelling regions or deep waters (S2 Table), are incapable of fixing nitrogen, possibly due to the absence of selection pressure that would drive the acquisition and maintenance of this function. Our analysis further predicted that nifHDKT genes were later acquired by horizontal gene transfer from a Sedimenticola bacterium to the LCA of the Ca. T. endolucinida HAW and Ca. T. endolucinida CAR lineages. The source of this transfer could have been either an ancestral population of the Pegophyseminae symbionts “PEGO” or an unsampled closely related lineage. Our analyses also indicated a well-supported transfer event from an ancestral population of the Ca. T. endolucinida HAW clade to the Ctena4 clade, which comprises samples from the Florida Keys and the Caribbean (Fig 4B). These findings indicate that both these clades have convergently acquired nitrogen fixation capabilities in an oligotrophic environment, and further suggests the horizontal transfer of nitrogen fixation genes might be a major factor enabling lucinid symbiont adaptation to nitrogen-poor conditions. The third well-supported transfer event was inferred from an ancestral node of the clade consisting of Ca. T. endolucinida CAR and HAW to Monit1. Both the Monit1 clade and nif-lacking Monit2 clade each consisted of a single Monitilora ramsayi symbiont MAG. Given that both MAGs were obtained from samples of the same host species and location (Queensland, Australia) (Fig 3B), and no additional metadata is available, further investigation is required to understand evolution of nitrogen fixation in the Monitilora ramsayi symbiosis. The acquisition of nifHDKT genes by Ca. T. endolucinida from a Sedimenticola lineage, rather than one of the Ca. Thiodazotropha is unexpected, given that transfers are more likely within the same clade as gene flow tends to occur more frequently among genetically similar bacteria [42]. A possible explanation for this could be the higher abundance of Sedimenticola OTUs in sediment bacterial communities [43] compared to Ca. Thiodiazotropha-like OTUs [44]. This disparity in abundance could reduce the frequency of physical encounters between the different Ca. Thiodiazotropha lineages—thereby reducing the likelihood of genetic exchange—while increasing the likelihood of encounters with members of the Sedimenticola lineage. However, the abundance of free-living “PEGO” symbionts and their close relatives has not been measured, and further investigation of the free-living microbial communities in lucinid habitats is required to gain a more comprehensive understanding of these dynamics. Finally, the ambiguous ancestral states of the deeper branching nodes of the other Ca. Thiodiazotropha clades hinder our interpretation of gene loss and/or reacquisition events in these symbiont lineages. Filling the gaps in the lucinid symbiont phylogeny by including novel symbiont and/or free-living relative genomes may resolve the uncertainties in the ancestral state reconstruction.

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