The pelagic ocean harbors one of the largest ecosystems on Earth. It is responsible for approximately half of global primary production, sustains worldwide fisheries, and plays an important role in the global carbon cycle. Ocean warming caused by anthropogenic climate change is already starting to impact the marine biota, with possible consequences for ocean productivity and ecosystem services. Because temperature sensitivities of marine autotrophic and heterotrophic processes differ greatly, ocean warming is expected to cause major shifts in the flow of carbon and energy through the pelagic system. Attempts to integrate such biological responses into marine ecosystem and biogeochemical models suffer from a lack of empirical data. Here, we show, using an indoor-mesocosm approach, that rising temperature accelerates respiratory consumption of organic carbon relative to autotrophic production in a natural plankton community. Increasing temperature by 2-6°C hence decreased the biological drawdown of dissolved inorganic carbon in the surface layer by up to 31%. Moreover, warming shifted the partitioning between particulate and dissolved organic carbon toward an enhanced accumulation of dissolved compounds. In line with these findings, the loss of organic carbon through sinking was significantly reduced at elevated temperatures. The observed changes in biogenic carbon flow have the potential to reduce the transfer of primary produced organic matter to higher trophic levels, weaken the ocean's biological carbon pump, and hence provide a positive feedback to rising atmospheric CO 2.biological feedbacks ͉ carbon cycle ͉ climate change ͉ global warming ͉ marine T he ocean plays a dominant role in the climate system through storage and transport of heat (1) and by mitigating global warming through the uptake and sequestration of anthropogenic carbon dioxide (CO 2 ) (2). Over the past 40 years, Ϸ84% of the increase in the Earth's heat budget has been absorbed by the surface oceans (3), thereby increasing the average temperature of the upper 700 m by 0.1°C (4). This process is likely to accelerate in the next decades with a predicted increase in global mean surface temperature between 1.1°C (low CO 2 emission scenario B1) and 6.4°C (high CO 2 emission scenario A1FI) until the end of the 21st century (5).Sea surface warming will affect the pelagic ecosystem in 2 ways: directly through its effect on the rates of biological processes, and indirectly through decreased surface layer mixing, causing decreased nutrient supply and increased light availability for photosynthetic organisms suspended in the upper mixed layer. It is expected that these changes in the physical and chemical environment will have drastic effects on the marine biota. The sensitivity of biological processes to temperature is commonly described by the Q 10 factor, the factorial increase in the process rate for a 10°C increase in temperature. While phytoplankton growth and photosynthesis show only a moderate temperature-response (1 Ͻ Q 10 Ͻ 2) and are primari...

[1] Methyl iodide concentrations of up to 45 pmol L À1 , which flux into the marine boundary layer, have been found in low latitude waters of the Atlantic and Indian oceans. These high concentrations correlate well with the abundance of Prochlorococcus, and we have confirmed the release of methyl iodide by this species in laboratory culture experiments. Extrapolating, we estimate the global ocean flux of iodine to the marine boundary layer from this single source to be 5.3 Â 10 11 g I yr À1 , which is a large fraction of the previously estimated total global flux and the implications are far reaching. Climate prediction models suggest increases in sea surface temperature and changes in biogeographical provenances in response to global warming. Such changes are likely to increase the abundance of Prochlorococcus, and we estimate a concomitant $15% increase in the release of iodine species to the atmosphere. Potentially, this could help mitigate global warming.

An indoor mesocosm used to determine the effect of warming on microbial communities. Inset: Recycling of organic matter inside the mesocosm is mediated by the interaction of phytoplankton and bacteria.

Clonal evolution of the leukemogenic compartment may contribute to alter the therapeutic response in acute lymphoblastic leukemia (ALL). Using xenotransplantation of primary leukemia cells, we evaluated the phenotypic and genetic composition of de novo resistant very high risk precursor B-cell ALL, a subgroup defined by the persistence of minimal residual disease despite intensive chemotherapy. Analysis of copy number alterations (CNAs) showed that the xenografted leukemia, even when reconstituted from 100 cells, remained highly related to the diagnostic sample, with minor changes in CNAs, mostly deletions, emerging in most cases in the first passage into mice. At the single-cell level, the pattern of monoallelic and biallelic deletions of the CDKN2A locus revealed distinct leukemia subpopulations, which were reproducibly tracked in xenografts. In most very high risk ALL cases, the predominant diagnostic clones were reconstituted in xenografts, as shown by multiplex polymerase chain reaction analysis of immunoglobulin and T-cell receptor loci. In other cases, the pattern in CNAs and immunoglobulin and T-cell receptor rearrangement was less concordant in xenografts, suggesting the outgrowth of subclones. These results unequivocally demonstrate the existence of clonally closely related but distinct subsets of leukemia initiating cells in ALL, which has important implications for drug development and preclinical disease modeling. (Blood. 2011;118(7): 1854-1864)

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