Journal of Eukaryotic Microbiology

Heterotrophic flagellates play a major role in nutrient recycling and shaping aquatic microbial communities. Many motile species have evolved diverse predatory strategies to exploit microbial prey, from bacteria to small eukaryotes. Excavates, deep-branching eukaryotic lineages with distinctive biflagellate morphology and conserved cytoskeletal features, exhibit varied feeding behaviours, including grazing, raptorial feeding, and filter-feeding. However, the temporal dynamics of their motility, feeding, and spatial distribution in relation to bacterial population density remain under-explored. Here, we quantify the behavioural ecology of excavates using high-resolution, single-cell tracking and quantitative analysis. Focussing on two Discoba species - the marine Jakoba libera and freshwater Reclinomonas americana - we identify different behavioural patterns: J. libera swims, forages and feeds, while R. americana swims and feeds. Prey availability differentially affects motility: J. libera increases the number of swimming cells when food is scarce, while R. americana shows the opposite trend. Spatial statistics reveal fundamental differences in their feeding and redistribution strategies. J. libera acts as an active forager, whereas R. americana behaves as a strict filter feeder. A third species, the anaerobic Kipferlia bialata (from a different clade), adopts a foraging strategy similar to J. libera. Together, these results elucidate the adaptive relationship between prey availability, feeding modes, and motility in bacteriovorous excavate flagellates, offering new insights into the ecological and evolutionary dynamics of microbial predators.

Hydractiniidae is a diverse family of colonial hydrozoans exhibiting substantial variation in colony organization, reproduction and life-cycle strategies, including species with and without a medusa stage. Several hydractiniids are also established model organisms, yet species boundaries and distributions remain incompletely resolved, limiting comparisons of reproductive and developmental traits across taxa. Here, we combine morphological and molecular data to reassess hydractiniid diversity, species identities, and life-cycle variation in material from the North Atlantic and Arctic Ocean. We integrate detailed observations of colonies and reproductive structures with DNA barcoding and mitochondrial genomes to evaluate species boundaries and provide molecular support for taxonomic identifications. Our results confirm 10 species in the study area, occurring from shallow waters to 2600 m depth and from coastal fjords to deep-sea hydrothermal vents. We further show that some historical species identifications and distribution records require reassessment. We establish a new genus for Arctic hydractiniid species (Bonneviactinia gen. nov.) and reassign two species to Schuchertinia. Mapping reproductive strategies onto the phylogeny reveals a broad pattern of increasing retention of medusa characters, from fixed sporosacs through reduced and fixed medusoids and non-liberated eumedusoids to fully developed, free-swimming feeding medusae, suggesting a progressive evolutionary trajectory in life-cycle complexity across Hydractiniidae. Furthermore, we clarify the identity of Podocoryna carnea and, through rearing experiments from polyp stage to medusae, characterize its morphological variation and developmental stages. By linking morphologically documented specimens and vouchers to genetic data, we establish reference resources that will facilitate future taxonomic, phylogenetic and biodiversity studies, including molecular surveys of marine communities. Together, these results demonstrate the value of integrative taxonomy for resolving hydractiniid diversity and provide a comparative foundation for investigating the evolution of reproductive strategies and life-cycle complexity in Medusozoa.

Microsporidia are a group of parasites that exhibit vast morphological, ecological, and genetic diversity. To catalog this diversity, we previously generated a database of attributes for microsporidia species. Here we expand this database to include species described since 2021, previously missed descriptions, and provisional species defined by clustering GenBank 18S rRNA sequences, resulting in 1,795 species. Using these data, we describe when and where microsporidia have been reported, their hosts and host habitats, the tissues they infect, and their spore morphology. A phylogeny of 18S sequence clusters shows that closely related microsporidia tend to infect closely related hosts, such that sequence identity is predictive of host taxonomy. Clustering also reveals previously unappreciated genetic diversity within some named species, in particular the human pathogen Vittaforma corneae, suggesting that these may comprise multiple species. We have also generated a web portal through which new species information can be submitted, which will facilitate regular updates. This database provides a resource for systematic analysis of microsporidia properties and a reference for taxonomic descriptions.

Unlike most other heterotrophic protist lineages which synthesize glycogen, alveolates accumulate -glucan storage polysaccharides in the form of starch. Within Alveolata, ciliates do so by using a simple pathway analogous to that used for glycogen metabolism in eukaryotes. However, Myzozoa have evolved five additional enzymatic steps which are common to Dinoflagellata, Chromerids and Apicomplexa. Moreover, dinoflagellates have acquired seven additional enzymes that distinguish them from the Chromerids-Apicompexa lineages, while chromerids possess only a single enzyme not found in dinoflagellates. Such modifications are not found in other eukaryotic lineages save for those resulting directly from primary or secondary plastid endosymbiosis. The many differences affecting the starch pathway of Myzozoa can be explained by a single secondary or tertiary plastid endosymbiosis of a starch-containing alga followed by distinct compartmentalisation outcomes of starch metabolism in Dinoflagellata relative to chromerids and Apicomplexa.

The genus Phakellia comprises predominantly fan-shaped deep-sea sponges, but its species diversity and boundaries remain unresolved. Here, we use an integrative approach combining morphological and molecular data to investigate the Phakellia robusta species complex and describe two new species, Phakellia pallida sp. nov. and Phakellia pusilla sp. nov. Overlap in external and spicular morphology makes these taxa challenging to identify. Molecular analyses based on mitochondrial (cox1) and nuclear (28S) markers support the recognition of the three taxa as distinct species, and reveal their close evolutionary relationship. The two new species co-occur in the NE Atlantic Ocean and the Mediterranean Sea, with overlapping depth ranges, although P. pallida sp. nov. is generally found at greater depths. Our results also suggest that several previous records of P. robusta from the Mediterranean and southern parts of the NE Atlantic may instead correspond to either P. pallida sp. nov. or P. pusilla sp. nov., indicating that its distribution requires reassessment. In addition, a group I intron previously reported in the cox1 gene of P. robusta was polymorphically detected at the same insertion site in some, but not all, individuals of P. pusilla sp. nov., but it was absent from P. pallida sp. nov. individuals. Overall, our results reveal pseudocryptic diversity within Phakellia and highlight the value of integrative approaches for resolving species boundaries and reassessing deep-sea sponge diversity and distributions.

Recent years have witnessed the discovery of an immense diversity of protists in almost all ecosystems on Earth. However, a major challenge consists in identifying diversity patterns and the drivers that explain the distribution of this diversity. While several works have focused on local parameters, only a few studies have addressed this question at regional scales. In this study, we characterize the diversity and distribution of Arcellinida (testate amoebae) at regional scale of a climatically contrasted country, Bulgaria. We collected 75 samples geographically evenly distributed and representative of the country's natural ecosystems and used metabarcoding to determine their diversity. Communities composition differed markedly among habitats, revealing sharp beta diversity patterns. "Mesophilic" environments such as forests and ponds were most diverse, while acidic and oligotrophic Sphagnum peatlands were poorest and hosted the highest proportion of unique taxa. Our analyses of richness-environment based on a Random Forest approach applied to all soil systems indicated that stable humidity (low silt content, low precipitation seasonality) and intermediate soil carbon stocks had a positive effect on richness. The predicted increase in continentality in the next decades in Bulgaria calls for testing climatic models to evaluate the potential risk of declining Arcellinida biodiversity.

Pectobacterium comprises Gram-negative, pectinolytic plant pathogens that cause soft rot and blackleg diseases on numerous economically important crops. During sampling in 2018 in taro (Colocasia esculenta) fields in Hawaii, two bacterial strains PL152T and PL155 were isolated from taro. Peliminary molecular characterization placed both strains in the genus Pectobacterium. However, their taxanomic postions could not be resoved at the species level. A polyphasic taxonomic approach incorporating phenotypic characterization, 16S rRNA gene sequence analysis, multilocus sequence analysis (MLSA), whole genome based phylogeny, average nucleotide identity (ANI), and digital DNA-DNA hybridization (dDDH) was therefore employed to determine their taxonomic status. Phylogenetic and phylogenomic analyses consistently placed PL152 and PL155 together in a distinct and well supported lineage within the genus Pectobacterium, separate from currently recognized species. The two strains exhibited high genomic relatedness to each other, whereas the average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) values, were lower than the accepted species delineation thresholds of 95% and 70%, respectively, supported the delineation of a novel bacterial species. The genomic characteristic of PL152 included a genome size of 5.02 Mb and a DNA G+C content of 52%. Collectively, based on a polyphasic approach including phylogenetic, phlyogenomic, biochemical, and phenotypic evidences supports the classification of strains PL152 and PL155 as representatives of a novel species within the genus Pectobacterium, for which the name Pectobacterium pacificum sp. nov. is proposed. Strain PL152 (ICMP 2436274, LMG 32536) is designated as the type strain.

The origin of multicellular animals from unicellular ancestors remains a major evolutionary question. To address this transition, a reconstruction of the nature of the last unicellular ancestor of animals (LUAA) is key, and requires broad sampling of extant unicellular relatives, which together with animals form the clade Holozoa. Environmental surveys suggest that unicellular Holozoa diversity extends beyond the five groups with characterized members (Choanoflagellatea, Ichthyosporea, Filasterea, Corallochytrea, and Atreyea). However, the extent of the environmental diversity within and outside these groups remains unclear. To fill this gap, we here systematically re-evaluated Holozoa diversity using environmental 18S rDNA sequences compiled from updated datasets, including long-read metabarcoding surveys. We report nine uncharacterized lineages inside Holozoa (seven newly identified here), provisionally placed outside the described groups. We also uncovered a previously unreported diverse lineage deeper inside Opisthokonta, separate from Holozoa and Fungi. In addition, we identified 21 uncharacterized lineages within Choanoflagellatea and Ichthyosporea (eleven newly reported here). We used short-read metabarcoding datasets to characterize the environmental and geographical distributions of these lineages. Many are globally distributed and some can reach relative abundances comparable to those of the described groups, while others are restricted to specific environments. Our findings reveal substantial uncharacterized diversity missing from the cultured taxa used in comparative studies of animal origins. By placing this diversity in a phylogenetic and environmental context, our study will guide the targeted isolation efforts needed to broaden the taxon sampling available to reconstruct LUAA and unravel the unicellular-to-multicellular transition that gave rise to animals.

Four rhizobial strains isolated from root nodules of tropical leguminous plants in Costa Rica were characterized. Strain LEGMi-135b was isolated from Lonchocarpus felipei root nodules, LEGMi-12c from Cojoba arborea, LEGMi-198b from Calliandra haematocephala and Rhizobium altiplani LEGMi-166a from Mimosa pigra. Phylogenetic analyses based on the 16S rRNA gene and four concatenated housekeeping genes (recA, atpD, GATase and gyrB) placed these strains within the genera Martinezella (LEGMi-12c, LEGMi-135b, and LEGMi-198b) and Rhizobium (LEGMi-166a). Genome-based analyses showed that strain LEGMi-135b exhibited an average nucleotide identity (ANI) value of 94% and a digital DNA-DNA hybridization (dDDH) value of 53.2% compared with its closest related species, Martinezella hainanensis CCBAU 57015T, supporting its classification as a distinct species. LEGMI-12c exhibited ANI and dDDH values of 98% and 87%, respectively, compared with Martinezella hainanensis CCBAU 57015T. LEGMi-198b showed the highest similarity to Rhizobium cerradonense CNPSo 3464, with ANI and dDDH values of 98% and 81.4%, respectively; however, this taxon has not been validly published. LEGMi-166a exhibited ANI and dDDH values of 100% and 99.3%, respectively, compared with Rhizobium altiplani BR 10423, confirming its assignment to this species. Additionally, differentiation of strains LEGMi-135b and LEGMi-198b from their closest phylogenetic relatives was supported by phenotypic, physiological, and biochemical analyses. Based on the combined genomic, phenotypic, and biochemical evidence, we propose the establishment of the novel rhizobial species Martinezella legmii sp. nov., with strain LEGMi-135b designated as the type strain (LEGMi-135b), and Martinezella ivanii sp. nov., with strain LEGMi-198b designated as the type strain (LEGMi-198b). This study expands our understanding of rhizobial diversity associated with nitrogen-fixing symbioses, particularly among tropical leguminous plants from Costa Rica

Background: Cyanobacteria are the major phototrophs in many endolithic ecosystems, yet their genomic diversity and metabolic adaptations remain poorly characterized. Here, we investigated cyanobacterial diversity in endolithic dolomite rocks from the Piora Valley in the southern Swiss Alps using genome-resolved metagenomics. Results: We reconstructed 138 medium-quality metagenome-assembled genomes (MAGs) from four neighboring endolithic rock communities, including 15 cyanobacterial MAGs distributed across six order-level lineages. The cyanobacterial communities showed limited taxonomic overlap among rocks, and most recovered MAGs represented potentially novel species. Only one Gloeobacter lineage occurred in all sampled rocks, which we propose as Gloeobacter dolomitensis sp. nov., representing the first Gloeobacter genome recovered from an endolithic environment. Comparative genomic analyses identified a cynS gene encoding cyanase unique to G. dolomitensis among the Gloeobacter. Phylogenetic analyses support its horizontal acquisition from Pseudomonadota. Interestingly, the genome of G. dolomitensis lacks a canonical CynABD cyanate transporter and instead contains a distinct ATP-synthase binding cassette (ABC) transporter adjacent to cynS. Although the available genomic and structural evidence does not establish cyanate transport, its genomic association with cynS suggests a potentially non-canonical mechanism. We further show that horizontal gene transfer was unevenly distributed among cyanobacterial lineages, with Acaryochloridales acting predominantly as donors, Chroococcidiopsidales as recipients, and Gloeobacterales showing a comparatively high frequency of interphylum gene exchange. Conclusions: The Piora Valley dolomite environment represents an underexplored reservoir of cyanobacterial diversity, including basal and previously undescribed lineages. The discovery of G. dolomitensis and its unusual cyanate-associated genomic architecture expands our understanding of Gloeobacterales diversity and highlights the potential importance of horizontal gene acquisition and metabolic innovation in endolithic microbial communities.

Prevotella melaninogenica is a Gram-negative bacterium that has been implicated in numerous disorders such as chronic obstructive pulmonary disease (COPD) and cystic fibrosis, and despite its abundance in the human microbiome, there is still limited knowledge on the fundamental physiology describing Prevotella melaninogenica. To properly advance Prevotella biology, it is critical to report and document such characterizations of Prevotella; therefore, the purpose of this study is to examine and define the phenotypic and growth characteristics of P. melaninogenica ATCC 25845. In this study, we found that Prevotella melaninogenica ATCC 25845 is a Gram-negative bacterium that is predominantly short and medium rod-shaped, with occasional filamentous rods. SEM imaging revealed that this filamentation may serve as division centers, points from which multiple daughter cells form. We also describe the antibiotic profile, which showed susceptibility to four classes and resistance to six classes of antibiotics. Carbon source growth kinetic characterization revealed a preference for carbohydrate sources.

A diverse array of photosynthetic phytoplankton drives primary production in equatorial surface waters. Among these, the cyanobacterium Prochlorococcus is an important contributor to net primary production in these typically iron-limited, high-nutrient and low-chlorophyll (HNLC) regions. Here, we explore the diversity of these organisms, in part, through targeted enrichment of Prochlorococcus cells using field-based high-speed cell sorting techniques. We demonstrate that the genomes of Prochlorococcus belonging to the low-light adapted LLI clade, and isolated from the surface of the Equatorial Pacific Ocean, are depleted in functions related to the assimilation of urea, nitrite, and amino acids. These are the first examples of LLI Prochlorococcus that have lost the ability to use nitrite, a trait considered to be a core feature of this clade. All new equatorial cultures of LLI Prochlorococcus appear to use a distinct isoform of protoporphyrinogen IX oxidase (HemG), for the biosynthesis of a chlorophyll precursor, that does not require the use of iron-containing heme. In contrast, the heme-dependent HemJ isoform is typically used by Prochlorococcus found outside equatorial HNLC waters. Together, these findings suggest that low-light adapted Prochlorococcus in the equatorial ocean possess accessory gene content that reflects adaptation to the generally iron-limited but nitrogen-replete conditions of surface waters.

Ecdysozoans is one of the most abundant and diverse groups of metazoans and grow through moulting the rigid exoskeleton that provided protection. But the moulting process is often dangerous, resulting in malformations or death. Injuries have been well documented in the calcitic exoskeleton of trilobites, some of which have been attributed to moulting complications when long spines are deformed. Injuries in trilobites are often found in the trunk rather than the cephalon, and abnormal genal spines are rarely documented. Here we examine injuries in eleven specimens of Olenoides serratus from the Burgess Shale (Cambrian, Wuluian) and demonstrate an increased frequency of genal spine malformations compared to other trilobites. The unusual pattern of malformations may be attributed to moulting injuries of the long and delicate genal spines in O. serratus. To moult, trilobites curved ventrally to press the anterior margin of their cephalon into the substrate and open the facial sutures. This process exerted force on the librigenae. To exit the exuvia, the genal spines were forced to bend as the old librigenae dipped ventrally along the anterior edge, resulting in the injured genal spines in O. serratus. A similar process of moulting resulting in injuries may be seen in other early Euarthropods.

Biflagellate zoospores are critical agents in the spread of Phytophthora spp., plant pathogens causing major agronomic and economic losses worldwide. These motile cells navigate soil water films toward plant roots using taxis triggered by environmental cues. Here, we investigate how potassium modulates zoospore motility by analyzing trajectory and flagellar beating parameters across KCl concentrations. Using high-speed imaging, we apply kymograph analysis and Fourier transform of the cell body direction over time to quantify flagellar dynamics, while low-speed imaging provides trajectory characteristics. Our results demonstrate that increasing potassium concentration significantly reduces swimming speed while enhancing directional changes. We establish a direct correlation between altered swimming speed and changes in flagellar beating parameters. Our findings indicate that potassium differentially affects anterior and posterior flagella beating. Furthermore, potassium exerts multiple effects, influencing beat amplitude, wave propagation speed and direction- change frequency. This reveals potassium as a key modulator of zoospore motility, triggering a shift from exploratory swimming to confined, reoriented movement. By uncovering the biomechanical responses of zoospores to potassium, this study advances our understanding of how surrounding signals regulate pathogen dispersal in the environment.

Vibrio coralliilyticus (V. coralliilyticus) is an emerging coral pathogen associated with tissue-loss diseases in reef-building corals and diseases in other marine organisms, such as oyster larvae. Despite the great ecological and economic significance of this pathogenic bacterium, characterization of bacteriophages infecting V. coralliilyticus remains limited, particularly of temperate phages. In this study, we describe the isolation and characterization of four novel phages - Vibrio phages Satoshi, Kolob, Ushijima, and Kane - obtained from coastal waters near L[a]ie, Hawaii, using the indigenous Hawaiian V. coralliilyticus strain OCN008 as the host. All four phages produced small, turbid plaques and encoded integrases, consistent with temperate lifestyles. The genome organization and taxonomic relationships of the viruses were examined using whole-genome sequencing and comparative genomics, while representative virion morphologies were examined by transmission electron microscopy. To the best of the authors' knowledge, these are the first V. coralliilyticus phages reported from Oahu, Hawaii. Comparative genomic analyses support the placement of all four phages within the class Caudoviricetes, but outside of any previously described orders, families, or genera within the class. Satoshi, Kolob, and Ushijima, along with previously described V. coralliilyticus phage CKB-S2, cluster within one genus-level lineage, while Kane, which has no close relatives in available databases, represents a distinct, previously undescribed lineage. All four phages were also found to contain homologs to experimentally established virulence factors, which may contribute to the pathogenicity of the host bacterium during lysogeny. Together, these isolates expand the diversity of phages infecting V. coralliilyticus and support the proposal of two new genera of Caudoviricetes: the genus Mokushimavirus, containing Satoshi, Kolob, Ushijima, and CKB-S2, and the genus Laievirus, containing Kane.

Traditional veterinary pathology investigations on wild animals generally conclude a primary cause of death based on a combination of observed clinical signs, gross and microscopic lesions, microbial culture and PCR. However, these methods may fail to capture the broader microbial community of the host. We employed metatranscriptomic sequencing to characterise the potential pathobiome of necropsied marine turtles, comparing these findings with corresponding veterinary pathology reports. Tissue samples were collected from 76 individuals spanning three species of marine turtles -- the green turtle (Chelonia mydas), the hawksbill turtle (Eretmochelys imbricata), and the loggerhead turtle (Caretta caretta) -- across the coastline of New South Wales, Australia. Reported causes of mortality ranged from infectious disease to trauma. We identified a diverse range of parasites within the genera Neospirochis, Hapalotrema, Paraorgymatobothrium, Eimeria and Callitetrarhynchus, and bacteria including the genera Aeromonas, Clostridium, and Vibrio. Novel viruses of the families Astroviridae, Caliciviridae, Hepaciviridae, Hepeviridae, Nyamivirdae, Orthomyxoviridae, Picobirnaviridae and Rhabdoviridae were also identified. In cases where infectious disease was noted in the pathology report, we frequently observed high pathogen read counts consistent with the listed primary diagnoses. However, we also detected a range of incidental infections involving eukaryotic parasites, bacteria, and viruses. The diversity of infectious agents detected in marine turtles suggests that the observed disease outcomes are unlikely to be driven by a single causative agent, but rather by complex interactions between host health, the pathobiome, and environmental stressors. We discuss the potential implications for turtle health and conservation.

The human gut is home to more than 100 trillion bacteria, where various environmental factors, such as nutrients and gases, exist and influence them. The levels of fermentation gases in the gut, including CO2, fluctuate depending on the host's diet. However, how CO2, an environmental factor, affects the physiological states of gut anaerobes remains poorly understood, but CO2 is often utilized in their cultivation. Akkermansia muciniphila (A. muciniphila) is widely studied as a beneficial human gut microbe. However, the effects of CO2 on the physiological state of A. muciniphila are not fully understood. In this study, we focused on CO2 as an environmental factor and aimed to clarify how CO2 affects the physiological state of A. muciniphila. Culture experiments showed that compared with the absence of CO2, the presence of CO2 significantly increased the growth of A. muciniphila. The results of the metabolomic analysis revealed distinct metabolic profiles in the presence or absence of CO2. The tracing analysis using stable 13C-labelled CO2 demonstrated that CO2 was incorporated into various metabolites in the TCA cycle, the de novo purine/pyrimidine synthesis pathways, and other pathways. In addition, the activity of pyruvate carboxylase, which catalyses an anaplerotic reaction involving CO2 fixation to supply oxaloacetate (OAA) to the TCA cycle, was significantly higher in A. muciniphila cells grown in the presence of CO2. Moreover, supplementation with OAA increased the growth of A. muciniphila in a dose-dependent manner, mirroring the effects of CO2 supplementation. These findings suggest that A. muciniphila utilizes CO2 for an anaplerotic reaction in the TCA cycle and other pathways involving CO2 incorporation, facilitating metabolic flux to promote its robust growth.

Bacterial endosymbionts that establish long-term associations inside of host cells often lose a huge number of genes and become dependent on their hosts for essential cellular functions. One such example is the mealybug endosymbiont Tremblaya princeps: its ~139-kb genome encodes only ~120 protein coding genes and lacks all recognizable genes for lipid and fatty acid synthesis, cell wall components, membrane transport, protein translocation, and transmembrane proteins, raising fundamental questions of how Tremblaya's cellular envelope is formed and maintained. Here, using a combination of transmission electron microscopy (TEM) and focused ion beam scanning electron microscopy (FIB-SEM) of an entire endosymbiont-containing insect cell, we identified extensive and specific contacts between host cell mitochondria and Tremblaya. These contacts occurred at close membrane appositions and contained electron-dense bridge-like structures closely resembling classical eukaryotic membrane contact sites. Whole-cell segmentation showed that approximately 53% of host mitochondria were in contact with Tremblaya, with some mitochondria extensively wrapped by Tremblaya-associated membranes. Mitochondria in contact with Tremblaya were larger and contained proportionally greater cristae volume and surface density compared to non-contact mitochondria, exhibiting differences in both external and internal mitochondrial architecture. As membrane contact sites are well known to mediate lipid and metabolite exchange between membranes, these findings suggest that direct interactions with host organelles may be a source of vital membrane and cellular components for endosymbionts that have lost the autonomous ability to produce and transport these molecules.

Marine microbial communities vary seasonally and spatially, shaped by physiochemical gradients and biotic interactions. Spatiotemporal variability of seawater microbiomes in tropical coastal regions is comparatively understudied, especially in Southeast Asia with high pressures from development and anthropogenic activities. Here, we examined the microspatial and temporal variability of bacterioplankton in the Singapore Strait, which is a tropical system in Southeast Asia influenced by monsoons. By applying total community amplicon sequencing to three particle size fractions reflecting microspatial habitats of seawater (0.22-1 {micro}m ''free-living'', 1-10 {micro}m ''large cells and small aggregates'' and 10-150 {micro}m ''particle-associated''), we show that the seawater microbial community was structured by their microspatial habitats, monsoon seasons and physiochemical variables. Monsoon-driven alternation of water masses originating from the South China Sea to the East and from off the Sumatran coast to the West resulted in a distinct community during the Southwest monsoon across all three size fractions. Total DNA (biomass) was lowest in the 10-150 {micro}m fraction, but these particle-associated sequences had the highest diversity and were surprisingly enriched with picocyanobacterial and some pathogenic taxa, potentially reflecting aggregate formation and/or host-derived particle association. This fraction also exhibited greater temporal variability than the two smaller fractions, which could be related to a more heterogenous environment in particle microhabitats. Overall, this study demonstrated compositional variability in the bacterioplankton across space and time in an equatorial region influenced by monsoons.

This study addresses the functional diversity of wild Limnospira platensis strains from two continents. Wild strains retain regulatory and metabolic flexibility often lost during domestication, making them essential for exploring natural adaptation within the species evolutionary history. We investigated the comparative growth performance of five Limnospira strains under controlled temperature and light regimes to identify phenotypic differences attributable to single environmental variables. We then evaluated their biochemical and biophysical responses under naturally fluctuating light conditions, linking energy use strategies to phylogenetic relationships. Our analyses reveal that while certain traits, such as fine-tuning of photosynthetic reactions, are shared among closely related strains, overall growth performance is highly variable even within phylogenetically similar groups. For example, photosensitive strains (e.g., UTEX 3086) differ markedly from high light tolerant strains (e.g., UTEX LB 2342) in their mechanisms of coping with light stress. Together, these comparative analyses highlight how environmental sensitivity and energy-use strategies vary across strains of the same species, reflecting broader patterns of diversification within Limnospira.