Extracellular enzyme activity of particle-associated and free-living bacteria in the oxygen deficient zone of the Eastern Tropical North Pacific

Abstract

The downward export of particulate organic matter (POM) in the water column is a globally important mechanism for carbon sequestration in the deep ocean. It is well understood that Oxygen Deficient Zones (ODZs) more efficiently export POM with depth compared to oxic waters. However, the role of microbial hydrolytic activity on ODZ organic matter is unknown. To evaluate this role, I assessed extracellular enzyme activity (EEA) and bacterial abundance (BA) of microbial size fractions, free-living (0.2-3 µm) and particle-associated (>3 µm), using fluorescently tagged substrates and epifluorescent microscopy. Samples were collected from one or two oxic depths (40-55m) and one oxygen-deficient depth (70-140 m) at three stations in the Eastern Tropical North Pacific (ENTP) ODZ from December 19, 2015 to January 16, 2017. Significant positive correlations were observed between EEA and oxygen (P < 0.05), suggesting that EEA is elevated in oxygenated waters over that of anoxic waters. However, EEA also showed significant relationships with other environmental factors, such as temperature, fluorescence, and depth. Continuing to explore the role of microbes in POM flux may become more important as the size of ODZs increase with climate change.