Recolonization and cues for bacterial migration into 'mock' deposit-feeder fecal casts

Abstract

Despite the importance of sedimentary bacteria to the cycling of organic matter in the marine environment, fundamental knowledge concerning the determinants of community structure is lacking. Roles of equilibrium processes, such as predation and competition, have generally been found to be of little importance, whereas the role of disturbance in structuring microbial communities has been less explored. Disturbance associated with invertebrate ingestion can reduce bacterial biomass and alter metabolic activities and compositions of bacterial assemblages in marine sediments. The first objective of the research presented here was to determine the mechanisms by which bacterial assemblages recover following their ingestion by deposit feeders. In exposed intertidal sediments, microbial recovery could occur due to regrowth of bacterial populations surviving gut passage, or to migration from adjacent sediments. We used field manipulative experiments to exclude either regrowth, with the use of bacteria-free 'mock' fecal coils, or migration, by isolating coils from surrounding sediments on polystyrene 'latrines.' A second goal was to test whether components of deposit-feeder egesta served as cues for bacterial colonization. Supernatant fluids from the egesta of 4 deposit-feeding species were added to the artificial fecal coils. We then followed the numerical recovery of active and total bacteria in coils through time using CTC (5-cyano-2, 3-ditoyl tetrazolium chloride) and DAPI stains coupled with epifluorescence microscopy. Our findings indicate that (1) recovery was dominated by immigration, as no recolonization was observed in coils isolated from adjacent sediments, (2) components of 2 of the 4 deposit-feeding species' egesta inhibited bacterial recolonization, and (3) although recovery was observed in coils with added fecal components of the remaining 2 species, in no case was recolonization stimulated relative to seawater addition. The last finding suggests that nutritional provision is not a significant impetus for recolonization of sediment patches disturbed by deposit feeding. Competitor-free space and vertical migrations of diatoms in intertidal sediments are discussed as alternative cues for bacterial recolonization of these disturbed patches.