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Global ocean biogeochemistry models currently employed in climate change projections use highly simplified representations of pelagic food webs. These food webs do not necessarily include critical pathways by which ecosystems interact with ocean biogeochemistry and climate. Here we present a global biogeochemical model which incorporates ecosystem dynamics based on the representation of ten plankton functional types (PFTs); six types of phytoplankton, three types of zooplankton, and heterotrophic bacteria. We improved the representation of zooplankton dynamics in our model through (a) the explicit inclusion of large, slow-growing zooplankton, and (b) the introduction of trophic cascades among the three zooplankton types. We use the model to quantitatively assess the relative roles of iron vs. grazing in determining phytoplankton biomass in the Southern Ocean High Nutrient Low Chlorophyll (HNLC) region during summer. When model simulations do not represent crustacean macrozooplankton grazing, they systematically overestimate Southern Ocean chlorophyll biomass during the summer, even when there was no iron deposition from dust. When model simulations included the developments of the zooplankton component, the simulation of phytoplankton biomass improved and the high chlorophyll summer bias in the Southern Ocean HNLC region largely disappeared. Our model results suggest that the observed low phytoplankton biomass in the Southern Ocean during summer is primarily explained by the dynamics of the Southern Ocean zooplankton community rather than iron limitation. This result has implications for the representation of global biogeochemical cycles in models as zooplankton faecal pellets sink rapidly and partly control the carbon export to the intermediate and deep ocean. [ABSTRACT FROM AUTHOR]

Fibrin microbeads (FMBs) made using thermal treatment of fibrin drops in oil can efficiently isolate mesenchymal stem cells (MSCs) from bone marrow (BM) and other similar sources and culture them continuously in suspension culture. The pure mesenchymal profile of MSCs isolated using FMBs and their differentiation potency to different mesenchymal lineages were previously described in detail. In the current study, MSCs were isolated from the BM of (GFP+) C57/bl mice using FMBs. Addition of pro-osteogenic medium with 10 mM of ss-glycerolphosphate, 50 microg/mL of ascorbic acid, and 10(-8) M of dexamethasone for 1 month resulted in ossified bone-like solid cellular structures, as seen using fluorescence and scanning electron microscopy (SEM). Such spontaneously formed structures were implanted in full-depth approximately 5-mm-diameter drilled defects in the skulls of wild-type c57/bl mice. Two months later, the excised upper parts of the skulls with the defects were viewed using fluorescence microscopy for green fluorescence protein of the cells in the defect and using SEM. They were also scanned using micro-computed tomography to visualize the formation of new hard tissue. Then the samples were processed and sectioned for hematoxylin and eosin staining and immunohistochemistry. Implanted FMBs loaded with (GFP+) MSCs formed partially mature, dense bone-like tissue using a residual moderate inflammatory process containing remnants of FMBs and neo-angiogenesis. The filled defect with bone-like tissue had a Ca/P ratio similar to that of native bone. Limited merging of the implant with the skull indicated that the induced bone regeneration derived from the MSCs that were delivered with the implant. No repair was seen in the control animals without implants or where the defect was filled with FMBs only. Repair scoring (on a 0-5 scale) was found to be 3.38+/-0.35 in the experimental arm, relative to 0 in the controls (p < 0.001).

Transplantation of adult mesenchymal stem cells (MSCs) could provide a basis for tissue regeneration. MSCs are typically isolated from bone marrow (BM) based on their preferential adherence to plastic, although with low efficiency in terms of yield and purity. Extensive expansion is needed to reach a significant number of MSCs for any application. Fibrin microbeads (FMB) were designed to attach mesenchymal cells and to provide a matrix for their expansion. The current study was aimed at isolating a high yield of purified BM-derived mouse MSCs based on their preferential adherence and proliferation on FMB in suspension cultures. MSCs could be downloaded to plastics or further expanded on FMB. The yield of MSCs obtained by the FMB isolation technique was about one order of magnitude higher than that achieved by plastic adherence, suggesting that these cells are more abundant than previously reported. FMB-isolated cells were classified as MSCs by their fibroblastic morphology, self-renewal ability, and expression profile of their surface antigens, as examined by flow cytometry and immunostaining. In cell culture, the isolated MSCs could be induced to differentiate into three different mesodermal lineages, as demonstrated by histochemical stains and by RT-PCR analyses of tissue-specific genes. MSCs were also able to differentiate into osteocytes while still cultured on FMB. Our results suggest that FMB might serve as an efficient platform for the isolation, expansion, and differentiation of mouse BM-derived MSCs to be subsequently implanted for tissue regeneration.

Fibrin microbeads (FMB), made of extensively cross-linked dense and partially denatured fibrin, were used as a matrix for efficient isolation of mesenchymal stem cells (MSC) from rat bone marrow (BM). After 2 days of incubation of FMB with whole BM in suspension, a high number of cells of mesenchymal origin attached to the FMB. On the 14th day after their transfer to plastic, the yield of the cells isolated via FMB was approximately 3-4 times higher than that obtained by currently used protocols based solely on plastic adhesion. This implies that the number of MSC in BM may be higher than previously reported. FACS analyses and immunostaining showed the mesenchymal characteristics of these cells by positive staining for fibronectin, vimentin, CD49E, and CD29. Immediately after isolation, less than 20% of the cells still expressed the hematopoietic markers CD11b and CD45. Most of these cells were eventually eliminated after further expansion of the isolated cells on plastic. Cells isolated via FMB were expanded in culture for more than 4 months and could be defined as MSC along this time period based on their ability to differentiate into precursors of mesenchymal tissues, such as osteogenic, adipogenic, and chondrogenic cells. Similar differentiation plasticity was observed in clones derived from single cells from whole MSC populations isolated via FMB. Based on our results we propose that FMB can serve as a 3-dimensional biodegradable matrix for isolation, differentiation, and possibly implantation of MSC for tissue regeneration.

Iron supply has a key role in stimulating phytoplankton blooms in high-nitrate low-chlorophyll oceanic waters. However, the fate of the carbon fixed by these blooms, and how efficiently it is exported into the ocean's interior, remains largely unknown. Here we report on the decline and fate of an iron-stimulated diatom bloom in the Gulf of Alaska. The bloom terminated on day 18, following the depletion of iron and then silicic acid, after which mixed-layer particulate organic carbon (POC) concentrations declined over six days. Increased particulate silica export via sinking diatoms was recorded in sediment traps at depths between 50 and 125 m from day 21, yet increased POC export was not evident until day 24. Only a small proportion of the mixed-layer POC was intercepted by the traps, with more than half of the mixed-layer POC deficit attributable to bacterial remineralization and mesozooplankton grazing. The depletion of silicic acid and the inefficient transfer of iron-increased POC below the permanent thermocline have major implications both for the biogeochemical interpretation of times of greater iron supply in the geological past, and also for proposed geo-engineering schemes to increase oceanic carbon sequestration.

Food-web processes are important controls of oceanic biogenic carbon flux and ocean-atmosphere carbon dioxide exchange. Two key controlling parameters are the growth efficiencies of the principal trophic components and the rate of carbon remineralization. We report that bacterial growth efficiency is an inverse function of temperature. This relationship permits bacterial respiration in the euphotic zone to be computed from temperature and bacterial production. Using the temperature-growth efficiency relationship, we show that bacterial respiration generally accounts for most community respiration. This implies that a larger fraction of assimilated carbon is respired at low than at high latitudes, so a greater proportion of production can be exported in polar than in tropical regions. Because bacterial production is also a function of temperature, it should be possible to compute euphotic zone heterotrophic respiration at large scales using remotely sensed information.

Red tide-forming dinoflagellates maximize cell numbers during periods of low light intensities in two ways. For short-term exposures to suboptimal light intensities such as might occur during recirculation in frontal convergences, cell division rates can be maintained at the expense of stored carbon for up to two generation times. During longer periods, corresponding to subsurface transport below a pycnocline, cell division rates eventually decrease as a portion of the fixed carbon is diverted to replenishing stored carbon. As a result, maximum rates of cell division can be resumed rapidly upon advection into surface waters where light intensities are optimal for growth.

Silicon is an essential element for diatom frustule synthesis and is usually taken up only by dividing cells. With Ge, a radioactive analog of Si, the cell cycle marker event of frustule formation was identified for individual species of diatom. The frequency of cells within a population undergoing this division event was estimated, and the cell division rate was calculated. In laboratory cultures, these rates of cell division and those calculated from changes in cell numbers were similar. By dual labeling with Ge(OH)(4) and NaHCO(3), rates of cell division and photosynthesis were coincidently measured for diatoms both in laboratory cultures and when isolated from natural populations in estuarine, offshore, and polar environments. These techniques permit the coupling between photosynthesis and cell division to be examined in situ for individual species of diatom.

Photosynthetic rates of phytoplankton were significantly lower after analysis by flow cytometry (FCM) than before. Exposure to the laser beam during the sorting process caused significant physiological damage. The cellular content of a radiolabel accumulated prior to FCM was not affected by FCM. Although it may not be possible to use FCM to preconcentrate cells for further physiological studies, samples may be incubated with stable or radioactive isotopes and then analyzed by FCM.

Planktotrophic larvae that occur beneath the annual sea ice in the Antarctic assimilate organic solutes and preferentially ingest bacteria, whereas they actively exclude phytoplankton. In regions where phytoplankton biomass is temporally limited by light or nutrient concentrations, the growth and development of planktotrophic larvae may not be directly coupled to phytoplankton production.