A study led by master’s student Pin-Yi Chen, Prof. Chih-hao Hsieh and Prof. Yi-Chun Yeh at the Institute of Oceanography, National Taiwan University, has uncovered how the assembly processes of marine heterotrophic bacterial communities change across a chlorophyll a gradient. The study found that as the regional chlorophyll a concentration increased, the relative importance of community assembly transitioned from stochastic to deterministic processes. The findings were published in ISME Communications on July 28, 2026.
Understanding how microbial communities assemble (i.e., changes in species composition across sites) is essential for explaining the biogeographic distribution of marine microorganisms and predicting biogeochemical cycling in the ocean. Nevertheless, the assembly processes along environmental gradients have remained elusive, especially in continental shelf regions shaped by dynamic ocean currents and nutrient supply. In this study, the regional chlorophyll a gradient was used as an environmental proxy. Previous studies have shown that environments with higher chlorophyll a generally support greater phytoplankton productivity and provide larger amounts of phytoplankton-derived organic matter, which can selectively promote the growth of specific heterotrophic bacteria. Consequently, deterministic assembly processes were expected to become more important with increasing chlorophyll a concentrations, whereas stochastic processes were expected to dominate under lower chlorophyll a concentrations.
To investigate microbial assembly across a chlorophyll a gradient, the research team synthesized observational data from 13 cruises conducted between 2014 and 2018. The study covered a 280-km cross-shelf transect in the southern East China Sea, including eight stations spanning nearshore to offshore waters. The results revealed a distinct regional chlorophyll a gradient across the cruises, with higher chlorophyll a concentrations closely associated with vertical mixing and nutrient inputs. Under high chlorophyll a conditions, the relative abundances of phytoplankton-associated bacterial groups, including Flavobacteriales and Rhodobacterales, together with the picophytoplankton order Synechococcales, became increasingly dominant, indicating a shift toward communities associated with higher primary productivity.
The research team further applied a Variation Partitioning Analysis (VPA) framework to quantify assembly processes shaping heterotrophic bacterial communities. The results showed that as regional chlorophyll a concentrations increased, biotic filtering of picophytoplankton became stronger, suggesting potentially important ecological interactions between picophytoplankton and heterotrophic bacteria. In contrast, dispersal limitation decreased with increasing chlorophyll a, consistent with greater water-mass exchange under high-productivity conditions. Consequently, the relative importance of deterministic processes compared to stochastic processes increased with the regional chlorophyll a concentration. Among the deterministic processes, biotic filtering explained more variation than environmental filtering, suggesting an ecological buffering effect in which stronger biotic interactions reduce the influence of environmental variability on bacterial community assembly.
This study reveals how heterotrophic bacterial community assembly shifts across a regional chlorophyll a gradient and highlights the importance of biotic interactions in bacterial communities. The findings contribute to a deeper understanding of marine microbial ecology and biogeochemical cycling and provide a new scientific basis for predicting how marine microbial ecosystems may respond to environmental change.
Schematic of heterotrophic bacterial community assembly along a regional chlorophyll a gradient. Networks represent metacommunities composed of local community patches (circles) linked by dispersal paths (lines). At low chlorophyll a concentrations (left), assembly is dominated by stochastic processes characterized by high dispersal limitation and weak biotic filtering of picophytoplankton. Conversely, at high chlorophyll a concentrations (right), assembly shifts toward deterministic processes driven by lower dispersal limitation and strong biotic filtering of picophytoplankton.
Reference: Chen, P. Y., Shiah, F. K., Gong, G. C., Hsieh, C. H., & Yeh, Y. C. (2026). Chlorophyll a gradients shift heterotrophic bacterial assembly from dispersal limitation to biotic filtering of picophytoplankton. ISME Communications, ycag215. https://doi.org/10.1093/ismeco/ycag215
