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      The complex effects of ocean acidification on the prominent N2-fixing cyanobacterium Trichodesmium.

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          Abstract

          Acidification of seawater caused by anthropogenic carbon dioxide (CO2) is anticipated to influence the growth of dinitrogen (N2)-fixing phytoplankton, which contribute a large fraction of primary production in the tropical and subtropical ocean. We found that growth and N2-fixation of the ubiquitous cyanobacterium Trichodesmium decreased under acidified conditions, notwithstanding a beneficial effect of high CO2 Acidification resulted in low cytosolic pH and reduced N2-fixation rates despite elevated nitrogenase concentrations. Low cytosolic pH required increased proton pumping across the thylakoid membrane and elevated adenosine triphosphate production. These requirements were not satisfied under field or experimental iron-limiting conditions, which greatly amplified the negative effect of acidification.

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          A bacterial method for the nitrogen isotopic analysis of nitrate in seawater and freshwater.

          We report a new method for measurement of the isotopic composition of nitrate (NO3-) at the natural-abundance level in both seawater and freshwater. The method is based on the isotopic analysis of nitrous oxide (N20) generated from nitrate by denitrifying bacteria that lack N2O-reductase activity. The isotopic composition of both nitrogen and oxygen from nitrate are accessible in this way. In this first of two companion manuscripts, we describe the basic protocol and results for the nitrogen isotopes. The precision of the method is better than 0.2/1000 (1 SD) at concentrations of nitrate down to 1 microM, and the nitrogen isotopic differences among various standards and samples are accurately reproduced. For samples with 1 microM nitrate or more, the blank of the method is less than 10% of the signal size, and various approaches may reduce it further.
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            Trichodesmium, a Globally Significant Marine Cyanobacterium

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              Enhanced biological carbon consumption in a high CO2 ocean.

              The oceans have absorbed nearly half of the fossil-fuel carbon dioxide (CO2) emitted into the atmosphere since pre-industrial times, causing a measurable reduction in seawater pH and carbonate saturation. If CO2 emissions continue to rise at current rates, upper-ocean pH will decrease to levels lower than have existed for tens of millions of years and, critically, at a rate of change 100 times greater than at any time over this period. Recent studies have shown effects of ocean acidification on a variety of marine life forms, in particular calcifying organisms. Consequences at the community to ecosystem level, in contrast, are largely unknown. Here we show that dissolved inorganic carbon consumption of a natural plankton community maintained in mesocosm enclosures at initial CO2 partial pressures of 350, 700 and 1,050 microatm increases with rising CO2. The community consumed up to 39% more dissolved inorganic carbon at increased CO2 partial pressures compared to present levels, whereas nutrient uptake remained the same. The stoichiometry of carbon to nitrogen drawdown increased from 6.0 at low CO2 to 8.0 at high CO2, thus exceeding the Redfield carbon:nitrogen ratio of 6.6 in today's ocean. This excess carbon consumption was associated with higher loss of organic carbon from the upper layer of the stratified mesocosms. If applicable to the natural environment, the observed responses have implications for a variety of marine biological and biogeochemical processes, and underscore the importance of biologically driven feedbacks in the ocean to global change.
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                Author and article information

                Journal
                Science
                Science (New York, N.Y.)
                American Association for the Advancement of Science (AAAS)
                1095-9203
                0036-8075
                May 05 2017
                : 356
                : 6337
                Affiliations
                [1 ] State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, Fujian, P.R. China.
                [2 ] Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL 32306, USA.
                [3 ] Department of Geosciences, Princeton University, Princeton, NJ 08544, USA.
                [4 ] State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, Fujian, P.R. China. dshi@xmu.edu.cn.
                Article
                science.aal2981
                10.1126/science.aal2981
                28450383
                f0333f20-cb4b-40ab-acb2-417619455cdf
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