Abstract
Microbes are ubiquitous but our knowledge of their effects on consumers is limited in benthic marine systems. Shorelines often form hotspots of microbial and detritivore activity due to the large amounts of detrital macrophytes that are exported from other coastal ecosystems, such as kelp forests, and accumulate in these systems. Shoreline ecosystems therefore provide a useful model system to examine microbial-detritivore interactions. We experimentally test whether bacteria in the biofilm of kelp provide a bottom-up influence on growth and reproductive output of detritivores in shorelines where detrital kelp accumulates, by manipulating the bacterial abundances on kelp (Ecklonia radiata). The growth rates for both male and female amphipods (Allorchestes compressa) were greater in treatments containing bacteria than those in which bacteria were reduced through antibiotic treatment, and this effect was greater for males offered aged kelp. The proportions of ovigerous females were greater when reared on kelp with intact bacteria, indicating a more rapid reproductive development in the presence of more bacteria. Bacterial abundance had little to no influence on nutrient content and palatability of kelp, based on tissue toughness, nitrogen and carbon content and C:N ratio. Thus, the most likely pathway for a microbial effect on detritivores was through feeding on kelp-associated bacteria. Regardless of the pathway, kelp-associated microbes have a strong influence on the fitness of a highly abundant detritivore that feeds preferentially on E. radiata in shoreline systems, and therefore form a hidden trophic step in this “brown” food web and a hotspot of secondary production.
Highlights
Microbes are ubiquitous and can produce strong bottom-up and top-down controls on ecosystem functions (Crowther and Grossart, 2015)
We show that bacteria in the biofilm of the kelp E. radiata enhance both the growth rate and the reproductive output of the amphipod A. compressa fed on fresh and aged kelp in a lab experiment mimicking a detrital-based, shoreline food web
We argue that bacteria form a “hidden” trophic step in wrack along the shoreline in coastal, marine environments, and such a role could be important in other coastal, marine ecosystems
Summary
Microbes are ubiquitous and can produce strong bottom-up and top-down controls on ecosystem functions (Crowther and Grossart, 2015). Our Bacteria Facilitate Detrital Food Web understanding of the bottom-up controls of microbes on consumers is limited (except pelagic systems, see Pomeroy et al, 2007), they are known to form highly abundant and diverse communities on the surfaces of benthic marine macrophytes (Egan et al, 2013; Weigel and Pfister, 2019). Bacteria form the main component of microbial communities in benthic systems (Wahl et al, 2012), providing a range of functions that can have both positive (e.g., provision of nutrients) and negative (e.g., disease) effects on their host (Egan et al, 2013). There has been very limited research on the influence of macrophyte-associated microbes on consumers, despite the importance of grazing and detritivory in coastal marine systems (Poore et al, 2012)
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