Fate of bacteria transiting the gut of the deposit feeder Abarenicola pacifica: Influence of temperature and sediment food concentration

Abstract

Deposit-feeding macrofauna can substantially impact sedimentary bacteria, both by bioturbative activities and removal during gut passage. Laboratory feeding experiments were conducted with the deposit feeder Abarenicola pacifica (Polychaeta: Arenicolidae; Healy and Wells) to test for separate and interactive effects of sediment food concentration and temperature on bacterial gut passage. Food concentration was varied via dilution of ambient sediments and measured as: (1) enzyme hydrolyzable amino acids (EHAA), (2) chlorophyll a (chl a), and (3) bacterial abundance. Gut bacteria were quantified via direct counts of sediments from 3 morphologically distinct gut regions (foregut [FG], midgut [MG], hindgut [HG]), and compared with ambient sediments (SD). Changes in bacterial abundance among these 'locations' were converted to percentage data (selection, digestion and growth). Generally, FG bacterial abundances were significantly greater than SD abundances, suggesting selection for bacteria-rich fractions of sediment. The magnitude of selection was significantly greater in sediments with low food concentrations. Significant digestion occurred in the MG, although efficiencies were lower than previously reported for similar deposit feeders. Contrary to earlier findings with other arenicolids, bacterial growth in the HG was slight, and significant only at high food concentrations. Temperature did not significantly affect removal of bacteria in the gut. Feeding rates (i.e., egestion rates) co-varied with selection, and temperature-specific patterns were observed. Our results agree with earlier studies in that the bacteria-rich fractions of sediments may be preferentially selected by deposit feeders, particularly under conditions of low food concentration. This has important implications for sediment bacteria, especially when removal efficiencies remain constant, and HG growth is low. Additionally, we expect that increased feeding rates, digestive efficiencies and HG growth under high food conditions will lead to greater qualitative impacts on sedimentary bacteria.