Microbial colonization and degradation of particulate organic matter (POM) are important processes that influence the structure and function of aquatic ecosystems. Although POM is readily used by aquatic fungi and bacteria, there is a limited understanding of POM-associated interactions between these taxa, particularly for early-diverging fungal lineages. Using a model ecological system with the chitin-degrading freshwater chytrid fungus
Rhizoclosmatium globosum
and chitin microbeads, we assessed the impacts of chytrid fungi on POM-associated bacteria. We show that the presence of chytrids on POM alters concomitant bacterial community diversity and structure, including differing responses between chytrid life stages. We propose that chytrids can act as ecosystem facilitators through saprotrophic feeding by producing ‘public goods’ from POM degradation that modify bacterial POM communities. This study suggests that chytrid fungi have complex ecological roles in aquatic POM degradation not previously considered, including the regulation of bacterial colonization, community succession and subsequent biogeochemical potential.
Plastics are pervasive in marine ecosystems including at the depths of our oceans. Microfibers, amongst other microscopic plastics, accumulate in deep sea sediments at concentrations up to four orders of magnitude higher than in surface waters. This is at odds with the fact that most microfibers are positively buoyant, and it is hypothesized that settling aggregates are vectors for downward transport of microfibers in the ocean. In laboratory incubations using roller tanks, we formed diatom aggregates with differing concentrations of microfibers and observed that microfiber addition stimulated aggregate formation, but decreased structural cohesion and caused them to break apart more readily, resulting in smaller average sizes. Incorporation of positively buoyant microfibers into settling aggregates reduced their size-specific sinking velocities proportional to the microfiber concentration. Slower sinking extends aggregate retention time in the upper ocean, thereby increasing the time available for organic matter remineralization in the upper water column. Here, we show that microfiber concentrations typical of those in the English Channel and Atlantic Ocean decrease potential export flux by 15-50%. Present day microfiber concentrations in surface waters may therefore be substantially reducing the efficiency of the biological carbon pump relative to the pre-plastic era.
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