Poo, J. S. T. | bioRxiv Science

bioRxiv2026

Distribution and dynamics of Vibrio spp. and pathogen sequences among microhabitats and monsoon seasons in Singapore's coastal waters

Marine infections are rising in temperate regions due in part to a warming climate. However, comparatively little is known about drivers of marine pathogen dynamics in the tropics where temperatures are high year-round. Vibrio bacteria are ubiquitous in both temperate and tropical marine environments, and many are important pathogens of marine organisms and humans. Here, we sampled an equatorial monsoonal site, the Singapore Strait, over 18 months and applied amplicon sequencing (16s rRNA V6-8 region) to three particle size fractions, qPCR quantitation of Vibrio spp., and generalized linear modelling with environmental variables to understand the factors that structure the Vibrio assemblage. We found that Vibrios represented the highest proportion of pathogen-related DNA sequences, with an assemblage composition influenced by microhabitats (size fractions) and monsoon seasons. Vibrio sequences were dominated by ASVs with highest identity to V. xuii, V. rotiferianus and V. caribbeanicus, and including ASVs matching agents of human disease in Singapore - V. parahaemolyticus, V. alginolyticus, and V. furnissii. Vibrio qPCR abundance was lowest in the largest size fraction (10-150 m), following planktonic DNA trends, even though pathogen-related sequences, including Vibrio, were enriched in this fraction. Temperature was a significant predictor of overall Vibrio abundance, but its effect was no longer significant after adjustment for monsoons, suggesting that the temperature association may reflect broader seasonal dynamics. Instead, effects of chlorophyll, ammonia, nitrate and suspended solids were more consistent within seasons. Furthermore, free-living and particle-associated Vibrios responded differently to environmental changes, suggesting that partitioning into microspatial habitats may lead to differences in their environmental sensitivity.

microbiology↗

bioRxiv2026

Particle association and monsoon seasonality influence microbial communities in marine coastal waters of Singapore

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.

microbiology↗