Phosphorus (P) and nitrogen (N) are essential nutrients for microbial growth, playing crucial roles in regulating the biological productivity of marine ecosystems. Over the last decades, the relatively higher increase in anthropogenic N compared to P inputs is causing a continuous increase in the N:P supply ratio to the global biosphere. The high N:P ratio of riverine discharge may seasonally cause P limitation in estuaries and river-dominated continental shelf waters. We conducted a mesocosm experiment simulating a P-deplete and a P-replete riverine discharge to coastal waters in NW Spain to assess the functional response of marine microplankton using a metatranscriptomic approach. By examining the expression of 40 well-documented genes related to P-metabolism in prokaryotic and eukaryotic gene expression, we uncovered pronounced changes in microbial P-metabolism induced by riverine N:P ratio in this productive system. Remarkably, heterotrophic bacteria and eukaryotic phytoplankton exhibited contrasting phosphate metabolism strategies in response to P deficiency, with the former mostly expressing genes coding for high-affinity transporters and the latter mostly transcribing genes related with low-affinity transporters. Our results also highlight distinct regulatory and adaptive mechanisms across different members of the prokaryotic and eukaryotic communities when exposed to varying P concentrations. Our findings shed light on the broader ecological and functional roles of these genes in nutrient cycling within aquatic ecosystems, with potential application for the design of diagnostic tools for P status in coastal productive systems.

The growth of metagenomics-derived amino acid sequence data has transformed our understanding of protein function, microbial diversity, and evolutionary relationships. However, the vast majority of these proteins remain functionally uncharacterized. Grouping the millions of such uncharacterized sequences with the few experimentally characterized ones allows the transfer of annotations, while the inspection of conserved residues with multiple sequence alignments can provide clues to function, even in the absence of existing functional information. To address the challenges associated with this data surge and the need to group sequences, we present a scalable, open-source, parametrizable Nextflow pipeline (nf-core/proteinfamilies) that generates nascent protein families or assigns new proteins to existing families. The computational benchmarks demonstrated that resource usage scales approximately linearly with input size, and the biological benchmarks showed that the generated protein families closely resemble manually curated families in widely used databases.

Background: Identifying the microbial taxa that structure assemblages in response to local disturbances along the Chilean Patagonian coast is critical for understanding community reorganization and ecological risk in this vulnerable marine environment. Here, we examined benthic microbial interactions across sixteen sites spanning 42–44°S in the Inner Sea of Chiloé and adjacent fjord systems. Using co-occurrence network analysis, we characterized spatial interaction patterns and identified keystone taxa whose relationships with environmental variables may serve as ecological indicators. The presence of keystone taxa supports the concept of microbial “seed banks,” which sustain community stability and ecosystem functionality under stress. Networks revealed structured communities dominated by niche-specialist taxa, consistent with ecological processes of niche differentiation and environmental filtering.

Results: The benthic environment was found to harbor niche-specialist taxa, including Flavobacteriaceae, Gemmataceae, Humimicrobiaceae, Clostridiaceae, and Sulfurovaceae, typically associated with organic enrichment and anthropogenic influence. These microbial families showed strong correlations with environmental gradients such as nitrate, phosphate, silicate, pH, salinity, and phaeopigments. Alpha-diversity analyses revealed greater microbial richness in FZ compared to the ISC, likely reflecting higher environmental heterogeneity and long-term adaptation to dynamic and fluctuating conditions. Beta-diversity analyses further supported the presence of distinct microbial assemblages between the two zones. Multivariate statistics and co-occurrence network analyses demonstrated that FZ communities display enhanced taxonomic interactions and greater network complexity, suggesting increased ecological stability. In contrast, microbial networks in the ISC exhibited reduced connectivity, potentially indicative of environmental stressors disrupting microbial spatial organization and network resilience. Moreover, the identification of indigenous bacterial isolates provided evidence for the presence of opportunistic host-associated pathogens, highlighting the utility of microbial biomarkers in detecting early ecological responses to human impacts.

Conclusion: Microbial co-occurrence patterns in northern Patagonian sediments reveal shifts in community assembly and increased network complexity that may enhance resilience to perturbations. The detection of strains harboring opportunistic pathogen biomarkers highlights risks under intense anthropogenic pressures. Monitoring keystone taxa thus offers early warning of functional decline. Broader spatio-temporal assessments remain essential to clarify microbial dynamics and their implications for ecosystem health and antimicrobial resistance.

Background: While oligotrophic bacteria are known to dominate most marine microbial habitats, under certain conditions, such as during phytoplankton blooms, copiotrophs can dramatically increase in abundance and reach towering proportions of the bacterial communities. We are uncertain whether the bacteria exhibiting this capacity, which we denote as "bloomers," have specific functional characteristics or if, instead, they are randomly selected from the broader pool of copiotrophs. To explore the genomic determinants of this ecological trait, we conducted a comparative genomic analysis of bacterial genomes from microcosm experiments where grazer and viral presence was reduced and nutrient availability was increased, conditions that triggered bacterial blooms.

Results: We tested which functional genes were overrepresented in the bacteria that responded to the treatments, examining a total of 305 genomes from isolates and metagenome-assembled genomes (MAGs) that were categorized as copiotrophs or oligotrophs according to their codon usage bias (CUB). The responsive bacteria were enriched in genes related to transcriptional regulation in response to stimuli (mostly via two-component systems), transport, secretion, cell protection, catabolism of sugars and amino acids, and membrane/cell wall biosynthesis. These genes confer on them capabilities for adhesion, biofilm formation, resistance to stress, quorum sensing, chemotaxis, nutrient uptake, and fast replication. They were overrepresented mainly in copiotrophic genomes from the families Alteromonadaceae, Vibrionaceae, Rhodobacteraceae, Sphingomonadaceae, and Flavobacteriaceae. Additionally, we found that these responsive bacteria, when abundant, could affect biogeochemical cycling, particularly the phosphorus cycle.

Conclusions: In this study, we provide insights into the functional characteristics that enable certain bacteria to rapidly respond to changes in the environment and bloom. We also hint at the ecological meaning and implications of these phenomena that could affect biogeochemical cycles in the oceans.

Knowledge of Earth's microbiomes' capacity to degrade aromatic compounds is limited by the lack of accurate tools for identifying degrading genes and their associated taxa. Additionally, these estimates are hardly compared to in situ background concentrations of polycyclic aromatic hydrocarbons (PAHs), particularly in oceanic waters. This knowledge is important for assessing the persistence of the widespread and abundant PAHs in the environment and their interactions with microbes. Here, we present a new tool to identify aromatic ringhydroxylating dioxygenase alpha-subunit (arhdA) gene sequences by combining profile-based search with phylogenetic placement in a reference phylogeny. We identified arhdA-harboring taxa in both the Genome Taxonomy Database and the Malaspina Vertical Profiles Gene Database, a gene catalog derived from metagenomes collected during the Malaspina expedition. We found that multiple ubiquitous taxa in tropical and temperate oceans harbor arhdA. The comparison of arhdA gene abundances in seawater metagenomes with the field PAH concentrations showed that higher abundances of arhdA gene copies per cell were negatively correlated with 2-4 ring PAHs, consistent with the known degradation of lighter PAHs. Gene abundances were significantly higher in the particle-associated fraction than in the free-living fraction, suggesting particulate matter as a relevant reservoir of PAH degraders. Finally, we show that PAHs, together with other environmental variables, modulate the structure of oceanic microbial communities.

Background: Vitamin B1 (thiamin) is essential to life; yet little is known of the regulation of its availability in marine environments or how it varies seasonally. Since microbes are the key synthesizers of the vitamin in marine environments, we here used metatranscriptomics to examine the seasonal dynamics of B1 acquisition strategies (including both uptake and synthesis pathways) in Baltic Sea bacterioplankton.

Results: Elevated B1-related gene expression was observed in summer, coinciding with increased temperatures and bacterial activity and decreased nutrient availability. Different bacterial taxa exhibited distinct B1 acquisition strategies. We identified filamentous Cyanobacteria of the order Nostocales as critical to sustaining B1 production during summer, potentially compensating for limited synthesis in heterotrophic bacteria, especially for 4-amino-5-hydroxymethylpyrimidine (HMP) synthesis. Also, Pelagibacterales accounted for major portions of the community transcription, primarily taking up and salvaging the B1 precursor HMP during summer. This study highlights the partitioning of B1 synthesis, salvage, and uptake among microbial taxa, underscoring that transcriptional activity was more dynamic over time than changes in the genomic potential.

Conclusions: We emphasize the influence of environmental conditions on microbial community dynamics and B1 cycling in general, and the potential implications of global change-induced increases in filamentous Cyanobacteria blooms on vitamin food web transfer in particular.

In recent years, new contributors to the marine silica cycle have emerged, including pico-sized phytoplankton (<2-3 μm in size) such as Synechococcus and picoeukaryotes. Their contribution and relevance to silica cycling are still under investigation. Field studies reporting the biogenic silica (bSi) standing stock in the pico-sized fraction are limited to silica-poor oligotrophic environments, and the mechanism of bSi accumulation in picoplankton remains unknown. We investigated the variability of bSi standing stocks in two size fractions (picoplankton, 0.22-3 mu m and microplankton, >3 μm) in the dissolved silica-replete Baltic Sea via biweekly time series samplings spanning 2 years. Time series data showed that the large changes in bSi standing stock in the Baltic Proper were primarily related to microplankton biomass and community composition. Meanwhile, picoplankton were, at times, surprisingly high contributors to total bSi year-round (up to 21.6%). Simultaneously, we performed microcosm incubation experiments with natural phytoplankton communities in each season to examine how nutrient additions affected bSi concentrations. In these experiments, increases in microplankton bSi were directly correlated to increases in diatom biomass, highlighting their influential role in the Baltic Sea silica cycle. Meanwhile, phosphorus additions triggered an increase in picoplankton bSi accumulation in all experiments. This uncovers a potential control of bSi accumulation in picoplankton, which can help identify the cellular mechanisms behind this process and uncover their role in silica cycling. The results link phytoplankton community composition and silica cycling, which is important for understanding the consequences of organism shifts due to climate change.

A few genera of diatoms that form stable partnerships with N₂-fixing filamentous cyanobacteria Richelia spp. are widespread in the open ocean. A unique feature of the diatom-Richelia symbioses is the symbiont cellular location spans a continuum of integration (epibiont, periplasmic, and endobiont) that is reflected in the symbiont genome size and content. In this study, we analyzed genomes derived from cultures and environmental metagenome-assembled genomes of Richelia symbionts, focusing on characters indicative of genome evolution. Our results show an enrichment of short-length transposases and pseudogenes in the periplasmic symbiont genomes, suggesting an active and transitionary period in genome evolution. By contrast, genomes of endobionts exhibited fewer transposases and pseudogenes, reflecting advanced stages of genome reduction. Pangenome analyses identified that endobionts streamline their genomes and retain most genes in the core genome, whereas periplasmic symbionts and epibionts maintain larger flexible genomes, indicating higher genomic plasticity compared with the genomes of endobionts. Functional gene comparisons with other N₂-fixing cyanobacteria revealed that Richelia endobionts have similar patterns of metabolic loss but are distinguished by the absence of specific pathways (e.g., cytochrome bd ubiquinol oxidase and lipid A) that increase both dependency and direct interactions with their respective hosts. In conclusion, our findings underscore the dynamic nature of genome reduction in N₂-fixing cyanobacterial symbionts and demonstrate the diatom-Richelia symbioses as a valuable and rare model to study genome evolution in the transitional stages from a free-living facultative symbiont to a host-dependent endobiont.

All nitrogen-fixing root nodule symbioses of angiosperms-legume and actinorhizal symbioses-possess a common ancestor. Molecular processes for the induction of root nodules are modulated by phytohormones, as is the case of the first nodulation-related transcription factor NODULE INCEPTION (NIN), whose expression can be induced by exogenous cytokinin in legumes. The process of actinorhizal nodule organogenesis is less well understood. To study the changes exerted by phytohormones on the expression of the orthologs of CYCLOPS, NIN, and NF-YA1 in the actinorhizal host Datisca glomerata, an axenic hydroponic system was established and used to examine the transcriptional responses (RT-qPCR) in roots treated with the synthetic cytokinin 6-Benzylaminopurine (BAP), the natural auxin Phenylacetic acid (PAA), and the synthetic auxin 1-Naphthaleneacetic acid (NAA). The model legume Lotus japonicus was used as positive control. Molecular readouts for auxins and cytokinin were established: DgSAUR1 for PAA, DgGH3.1. for NAA, and DgARR9 for BAP. L. japonicus NIN was induced by BAP, PAA, and NAA in a dosage- and time-dependent manner. While expression of D. glomerata NIN2 could not be induced in roots, D. glomerata NIN1 was induced by PAA; this induction was abolished in the presence of exogenous BAP. Furthermore, the induction of DgNIN1 expression by PAA required ethylene and gibberellic acid. This study suggests that while cytokinin signaling is central for cortex-induced nodules of L. japonicus, it acts antagonistically to the induction of nodule primordia of D. glomerata by PAA in the root pericycle.

Through biosilicification, organisms incorporate dissolved silica (dSi) and deposit it as biogenic silica (bSi), driving the silicon (Si) cycle in aquatic systems. While Si accumulation in marine picocyanobacteria has been recently observed, its mechanisms and ecological implications remain unclear. This study investigates biosilicification in marine and brackish picocyanobacteria of the Synechococcus clade and two model freshwater coccoid cyanobacteria. Brackish strains showed significantly higher Si quotas when supplemented with external dSi (100 mu M) compared to controls (up to 60.0 +/- 7.3 amol Si.cell-1 versus 9.2 to 16.3 +/- 2.9 amol Si.cell-1). Conversely, freshwater strains displayed no significant differences in Si quotas between dSi-enriched treatments and controls, emphasizing that not all phytoplanktons without an obligate Si requirement accumulate this element. The Si-accumulating marine and brackish picocyanobacteria clustered within the Synechococcus clade, whereas their freshwater counterparts formed a distinct sister group, suggesting a link between phylogeny and silicification. Rapid culture growth caused increased pH and led to dSi precipitation, influencing apparent dSi uptake; this was mitigated by pH control through bubbling. This phenomenon has significant implications for natural systems affected by phytoplankton blooms. In such environments, pH-induced silicon precipitation may reduce dSi availability impacting Si-dependent populations like diatoms. Our findings suggest brackish picocyanobacteria could significantly influence the Si cycle through at least two mechanisms: cellular Si accumulation and biologically induced changes in dSi concentrations.IMPORTANCEThis work provides the first evidence of biogenic silica accumulation in brackish picocyanobacteria and uncovers a link between phylogeny and biosilicification patterns. Our findings demonstrate that picocyanobacterial growth induces pH-dependent silica precipitation, which could lead to overestimations of cellular Si quotas by up to 85%. This process may drive substantial silica precipitation in highly productive freshwater and coastal marine systems, with potential effects on silica cycling and the population dynamics of Si-dependent phytoplankton. The extent of biosilicification in modern picocyanobacteria offers insights into the rock record, shedding light on the evolutionary and ecological dynamics that influence sedimentary processes and the preservation of biosilicification signatures in geological formations. Overall, this research adds to the significant impact that microorganisms lacking an obligate silica requirement may have on silica dynamics.