An experimental study on the effects of nutrient enrichment on organic carbon persistence in the western Pacific oligotrophic gyre

Carbon sequestration in the ocean is of great concern with respect to the mitigation of global warming. How to hold the fixed organic carbon in the presence of tremendous numbers of heterotrophic microorganisms in marine environments is the central issue. We previously hypothesized that excessive nutrients would ultimately decrease the storage of organic carbon in marine environments. To test this, a series of in situ nutrient enrichment incubation experiments were conducted at a site (17.59° N, 127.00° E) within the western Pacific oligotrophic gyre. Five treatments were employed: glucose (Glu), algal exudation organic material (EOM), nitrate (N) and phosphate (P), N and P in combination with glucose and a control with no added nutrients. The results showed that the dissolved organic carbon consumption rates and bacterial community specific growth rates were enhanced by inorganic nutrient enrichment treatments during the initial 48 h incubation. At the end of 14 days of incubation, about one-third (average 3.3 μmol C kg−1) more organic carbon was respired in the glucose-enriched incubation with the addition of inorganic nutrients compared to that without. In contrast, when nutrients were limiting, glucose could not be efficiently used by the bacteria and thus it remained in the environment. These results suggest that repletion of inorganic nutrients could facilitate microbial consumption of organic carbon and thus has a significant impact on carbon cycling in the environment.