Coral mucus effects on bacterial growth, respiration, and grazing mortality in reef systems

Abstract

Coral releases mucus into the surrounding seawater, providing an important organic and nutrient source for bacteria in coral reef systems. Despite thorough investigation in previous studies, bacteria respiration and grazing mortality by nanoplankton in coral reef systems remain poorly understood. To understand organic matter cycle in reef systems, it is necessary to reveal how coral mucus influences the energy and material transfer efficiency of the microbial loop. Here, we examined the production and grazing mortality of bacteria using a dilution method and the respiration of bacteria by directly measuring oxygen consumption in Acropora mucus-supplemented seawater (MuSW) multiple times over several years. The mucus significantly enhanced the bacterial production in MuSW compared with that in seawater. Bacterial respiration in MuSW was also significantly higher than that in seawater (SW). Bacterial carbon demand in MuSW was also higher than that in SW; however, bacterial growth efficiency did not change significantly. These results suggest that coral mucus benefits both bacterial growth and respiration. On the other hand, bacterial grazing mortality was not significantly different between MuSW and SW, suggesting that coral mucus did not directly enhance the activity and/or growth of bacterial grazers. According to previous reports, nanoflagellate number increases in response to an increase in bacteria. This suggests that coral mucus would have an indirect and delayed impact on bacterial grazers, whereas it would have a direct and immediate impact on bacteria. This study highlights that coral mucus raises the level of bacterial activity and could drive matter cycles through the microbial loop in reef systems.