The availability of iron limits primary productivity and the associated uptake of carbon over large areas of the ocean. Iron thus plays an important role in the carbon cycle, and changes in its supply to the surface ocean may have had a significant effect on atmospheric carbon dioxide concentrations over glacial-interglacial cycles. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments. It is difficult, however, to reliably assess the magnitude of carbon export to the ocean interior using such methods, and the short observational periods preclude extrapolation of the results to longer timescales. Here we report observations of a phytoplankton bloom induced by natural iron fertilization--an approach that offers the opportunity to overcome some of the limitations of short-term experiments. We found that a large phytoplankton bloom over the Kerguelen plateau in the Southern Ocean was sustained by the supply of iron and major nutrients to surface waters from iron-rich deep water below. The efficiency of fertilization, defined as the ratio of the carbon export to the amount of iron supplied, was at least ten times higher than previous estimates from short-term blooms induced by iron-addition experiments. This result sheds new light on the effect of long-term fertilization by iron and macronutrients on carbon sequestration, suggesting that changes in iron supply from below--as invoked in some palaeoclimatic and future climate change scenarios--may have a more significant effect on atmospheric carbon dioxide concentrations than previously thought.
The photo-and bioreactive components of dissolved organic matter (DOM) from three different environments were determined during long-term decomposition experiments. Terrigenous DOM was collected from a black-water system, plankton DOM was harvested from phytoplankton cultures, and lake water served as a DOM source with both terrigenous and plankton components. Photomineralization accounted for the removal of 46 and 7% of terrigenous and lake-dissolved organic carbon (DOC), respectively, while no loss in DOC was observed when plankton DOM was exposed to irradiation. Biomineralization accounted for the removal of 27% each of terrigenous and lake DOC and 74% of plankton DOC. Phototransformations of terrigenous and lake DOM resulted in 7% and 2% increases in biodegradable DOC, respectively, while no increase in biodegradable DOC was observed for irradiated plankton DOM. In two different experimental approaches, terrigenous DOM was exposed to sequential and alternating bio-and photodegradation, respectively, to determine the fractions of DOC that were bioreactive and photoreactive. About 15% of terrigenous DOC was susceptible to both biomineralization and photomineralization. These results demonstrate that biological and photochemical processes compete in the mineralization of DOC. Photomineralization of bioreactive DOC is likely an important factor determining the net effect of irradiation on the bioreactivity of DOM.
Dissolved organic matter (DOM) in the oceans is one of the largest pools of reduced carbon on Earth, comparable in size to the atmospheric CO 2 reservoir. A vast number of compounds are present in DOM, and they play important roles in all major element cycles, contribute to the storage of atmospheric CO 2 in the ocean, support marine ecosystems, and facilitate interactions between organisms. At the heart of the DOM cycle lie molecular-level relationships between the individual compounds in DOM and the members of the ocean microbiome that produce and consume them. In the past, these connections have eluded clear definition because of the sheer numerical complexity of both DOM molecules and microorganisms. Emerging tools in analytical chemistry, microbiology, and informatics are breaking down the barriers to a fuller appreciation of these connections. Here we highlight questions being addressed using recent methodological and technological developments in those fields and consider how these advances are transforming our understanding of some of the most important reactions of the marine carbon cycle.dissolved organic matter | marine microbes | cyberinfrastructure
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