Experimental investigation of the control of bacterial community composition in macrofaunal burrows

The burrows and tubes of infauna are complex and dynamic systems that promote significant heterogeneity in the ecological and geochemical properties of benthic systems. We experimentally evaluated the extent to which behaviors of macrofauna, including ventilation activity, the residence times of tubes in sediments and tube composition, influence the development and succession of microbial communities which line macrofaunal burrows (i.e. the burrow microbial biofilm), Our experiments used a burrow mimic system, which allowed manipulation of some biological and environmental factors while holding other factors constant, The biofilm microbial community was assessed using phospholipid fatty acid (PLFA) analysis, a sensitive measure of microbial biomass and a significant tool in the identification of some taxonomic and functional groups of microorganisms in marine systems, Results suggest that the burrow mimics supported the development of microbial assemblages with phospholipid signatures that were both similar to those found in natural burrows and responsive to experimental manipulation. Burrow mimic composition significantly affected PLFA signatures, within the range of compounds and concentrations observed in natural systems. Tubes that were coated with an organic polymer were characterized by lower microbial biomass than uncoated tubes and also exhibited a substantially different PLFA signature. The residence time of tube mimics in sediments also affected the PLFA signature. Tubes with a longer residence time were characterized by higher microbial biomass, and a distinct anaerobe signature, relative to tube mimics having shorter residence times, Thus, a distinct successional sequence was observed. Surprisingly, irrigation frequency, i.e. the rate at which burrows are renewed with fresh, oxic overlying water and rid of metabolic byproducts and other solutes from the host organism and surrounding sediments, had minor effects on PLFA signature within the burrow biofilm. Our results suggest that the properties of the burrow micro environment exert fundamental controls on microbial community composition in burrow biofilms and are likely to influence biogeochemical processes over short length scales in nearshore systems.