Microbial influences on deep-sea deposit feeders and detrital food sources

Abstract

Deep-sea communities are food limited, relying on detritus exported from surface waters for energy. Microbes produce, consume, and transform detrital particles, affecting their availability to deep-sea metazoans. I examined the microbial communities associated with particulate organic matter as it sinks from the euphotic zone to the abyssal plain more than four kilometers below. Compound-specific stable isotope analysis allowed me to trace both metazoan and microbial trophic processes. I found that most microbial alteration occurs in the top 500 m of the water column, the products of which support nitrifying archaeal communities. I also characterized microbial communities in sediments and in the digestive tracts of deposit-feeding echinoderms (sea cucumbers and sea urchins) to understand how microbial and metazoan food webs interact. Abyssal deposit feeders showed niche partitioning at two sites in the North Pacific, with interspecific differences in their consumption and utilization of fresh phytodetritus versus more heavily reworked small particles. More mobile species, especially holothurians capable of swimming, were able to consume fresher material, while the slower-moving species appear to rely more on microbially reworked detritus. Both small, heavily reworked particles and large, fresh particles are important to abyssal fauna, but the smaller particles are not well represented in sediment traps. The use of sediment traps to measure flux has therefore led to an underestimation of the importance of small particles to the deep benthos. Finally, I looked at changes in deposit feeder trophic ecology and gut microbiota throughout a depth gradient from shallow reefs to the abyssal plain and found that depth was the major driver of both. Deposit feeder trophic position increased from two to three (primary to secondary consumers) within the first 500 m of depth, which corresponded to the decline in particulate flux across the same depth range. Deeper-living deposit feeders are more reliant on microbial trophic intermediates than their shallow counterparts, and as depth increases the dominant gut taxa change. Detritus-based food webs in the deep ocean are influenced at multiple levels by microbial reworking, including in the often-overlooked gut microbiome.