Unifying microorganisms and macrograzers in intertidal rocky shore ecological networks.

Over the past decades, our understanding of the vital role microbes play in ecosystem processes has greatly expanded. However, we still have limited knowledge about how microbial communities interact with larger organisms. Many existing representations of microbial interactions are based on co-occurrence patterns, which do not provide clear insights into trophic or non-trophic relationships. In this study, we untangled trophic and non-trophic interactions between macroscopic and microscopic organisms on a marine rocky shore. Five abundant mollusk grazers were selected, and their consumptive (grazing) and nonconsumptive (grazer pedal mucus) interactions with bacteria in biofilms were measured using 16S rRNA-gene amplicon sequencing. While no significant effects on a commonly used measure of biofilm grazing (chlorophyll a concentration) were observed, detailed image analysis revealed that all grazers had a detrimental impact on biofilm cover. Moreover, different grazers exhibited distinct effects on various bacterial groups. Members of the Alteromonadaceae, Burkholderiaceae, Flavobacteriaceae, Halieaceae, Phycisphaeraceae, Rhodobacteraceae, Rickettsiaceae, Saprospiraceae, and Vibrionaceae families experienced positive trophic effects from specific grazers. In contrast, members of the Flavobacteriaceae, Pirellulaceae, Rhodobacteraceae, Rubritaleaceae, and Saprospiraceae families were negatively affected by trophic interactions with other grazers. Some members of the Gammaproteobacteria, Flavobacteriaceae, Ilumatobacteraceae, Pirellulaceae, Rickettsiales, Rhodobacteraceae, and Rubritaleaceae families exhibited non-trophic positive interactions with specific grazers. Meanwhile, members of the Family DEV007 (Verrucomicrobiales), Flavobacteriaceae, Ilumatobacteraceae, Legionellaceae, Rickettsiales, Rhodobacteraceae, Saprospiraceae, and Xanthobacteraceae families exhibited non-trophic negative interactions with particular grazers. Both trophic and non-trophic interactions shift the microbial community toward enhanced recycling, energy efficiency, and stress resilience. Grazer activity, through biomass removal and exudates like pedal mucus, reduces photosynthetic groups like diatoms, halting dimethylsulfoniopropionate (DMSP) production and negatively impacting sulfur-cycling bacteria and associated parasites. This research complements the ecological network of the intertidal rocky shore in central Chile and represents the first attempt to construct an interaction network between macroorganisms and bacteria. It reveals that the strength of trophic and non-trophic interactions varies depending on the grazer and bacterial group involved. While some bacterial groups responded broadly, others showed specialized responses to specific macroorganisms. Overall, this study highlights the potential for integrating microbes into ecological networks, offering valuable insights methodologies for quantifying interactions across domains.