Abstract
Abstract. 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.
Highlights
Dissolved organic carbon (DOC) in the ocean, as one of the largest carbon reservoirs on the earth, is comparable to the entire atmospheric CO2 reservoir (∼ 750 Gt; Hedges, 1992; Ogawa and Tanoue, 2003) and plays an important role in global carbon cycling and climate change
In the Glu treatment the no/slight bacterial response to enrichment with Glu, a labile DOC source, seemed to be unreasonable, but such situations do exist in oligotrophic oceans as reported in the Sargasso Sea (Carlson et al, 1996)
The bacterial specific growth rate in N + P treatments was significantly higher than that in the control (ANOVA test, p < 0.05) (Table 2), and the same result was obtained for bacterial abundance (BA) at 48 h (Fig. 1), indicating that inorganic nutrient addition could stimulate bacterial growth in the logarithmic phase
Summary
Dissolved organic carbon (DOC) in the ocean, as one of the largest carbon reservoirs on the earth, is comparable to the entire atmospheric CO2 reservoir (∼ 750 Gt; Hedges, 1992; Ogawa and Tanoue, 2003) and plays an important role in global carbon cycling and climate change. Recalcitrant dissolved organic carbon (RDOC), which comprises the largest portion of the bulk ocean DOC reservoir, can persist for thousands of years in the water column (Blitz, 1992), constituting significant carbon sequestration in the ocean. The role of microbes in formation of RDOC is proposed as the microbial carbon pump (MCP) (Jiao et al, 2010a). One of the MCP rationales lies in the constraints of microbial DOC consumption (Jiao et al, 2011)
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