As a foundational benthic primary producer in aquatic ecosystems, the complex interactions between Cladophora and associated bacteria not only influence the equilibrium of these ecosystems but also drive the propagation and evolution of antibiotic resistance genes (ARGs). Although the previous research has confirmed that the phycosphere represents a critical niche for the occurrence and dissemination of ARGs in aquatic environments, a systematic understanding of how the stage-dependent release of dissolved organic matter (DOM) by Cladophora under natural conditions regulates the assembly processes of epiphytic phytoplankton and bacterial communities, the patterns of algae-bacteria interactions, and the consequent dynamics of the associated resistome remains poorly understood.

Recently, the research team led by Prof. WU Chenxi from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences have provided new insights into how life-stage-dependent processes in the macroalgal phycosphere govern epiphytic community assembly and resistome dissemination in an oligotrophic lake. Their findings underscore that Cladophora blooms act not only as modifiers of littoral biogeochemistry but also as significant sources and amplifiers of antibiotic resistance risk. The study was published in Water Research.

In this study, the researchers focused on the Cladophora bloom in Qinghai Lake by integrating field investigations with laboratory experiments. They investigated how DOM derived from different algal morphotypes regulates the succession of epiphytic communities, with a particular emphasis on how shifts in community structure influence the dissemination of ARGs in the littoral zone.

Field investigations revealed that Cladophora blooms significantly alter littoral ecosystems by releasing DOM and nutrients, thereby shaping distinctive surrounding epiphytic communities. The phycosphere hosts distinct bacterial and phytoplankton assemblages characterized by lower diversity but higher functional specialization, dominated by taxa such as Acinetobacter and Exiguobacterium. These microbes drive nutrient cycling (e.g., nitrogen metabolism) and serve as hotspots for resistomes. Contrary to the expectation that high cell density promotes horizontal gene transfer (HGT), these findings indicate that vertical gene transfer (VGT) is the dominant pathway for ARG proliferation within the phycosphere.

The researchers further investigated changes in bacterial community structure along a DOM gradient within the phycospheres of different Cladophora morphotypes. They found that during the low-oxygen stage, DOM composition shifted from labile, protein-like substances to recalcitrant, humic-like substances. This shift drove the enrichment of specific epiphytic communities characterized by low diversity but high functionality. Additionally, along the DOM gradient, bacterial diversity increased as DOM concentration decreased. These findings underscore the selective effect of algae on phycospheric bacteria.

This study establishes a mechanistic link between algal life-stage-dependent DOM chemistry, selective microbial enrichment, and divergent pathways of resistome evolution. It highlights the ecological risks of Cladophora blooms in promoting resistance gene propagation and altering biogeochemical cycling in the littoral zone.

Effects of Cladophora blooms on nutrient cycling and antibiotic resistome evolution in the littoral zone of Qinghai Lake. (Image by IHB)

(Editor: MA Yun)

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