Global relationship between phytoplankton diversity and productivity in the ocean

The shape of the productivity–diversity relationship (PDR) for marine phytoplankton has been suggested to be unimodal, that is, diversity peaking at intermediate levels of productivity. However, there are few observations and there has been little attempt to understand the mechanisms that would lead to such a shape for planktonic organisms. Here we use a marine ecosystem model together with the community assembly theory to explain the shape of the unimodal PDR we obtain at the global scale. The positive slope from low to intermediate productivity is due to grazer control with selective feeding, which leads to the predator-mediated coexistence of prey. The negative slope at high productivity is due to seasonal blooms of opportunist species that occur before they are regulated by grazers. The negative side is only unveiled when the temporal scale of the observation captures the transient dynamics, which are especially relevant at highly seasonal latitudes. Thus selective predation explains the positive side while transient competitive exclusion explains the negative side of the unimodal PDR curve. The phytoplankton community composition of the positive and negative sides is mostly dominated by slow-growing nutrient specialists and fast-growing nutrient opportunist species, respectively.Marie Curie International Outgoing Fellowship (FP7)Gordon and Betty Moore FoundationSpain. Ministerio de Economía y Competitividad (Ramon y Cajal Contracts

Coalition unionism : exploring how and when coalitions contribute to union renewal in Sydney, Toronto and Chicago

Item does not contain fulltextWe have previously identified eight novel autoantibody targets in the cerebrospinal fluid of multiple sclerosis (MS) patients, including sperm-associated Ag 16 (SPAG16). In the current study, we further investigated the autoantibody response against SPAG16-a protein with unknown function in the CNS-and its expression in MS pathology. Using isoelectric focusing, we detected SPAG16-specific oligoclonal bands in the cerebrospinal fluid of 5 of 23 MS patients (22%). Analysis of the anti-SPAG16 Ab reactivity in the plasma of a total of 531 donors using ELISA demonstrated significantly elevated anti-SPAG16 Ab levels (p = 0.002) in 32 of 153 MS patients (21%) compared with all other control groups with 95% specificity for the disease. To investigate the pathologic relevance of anti-SPAG16 Abs in vivo, anti-SPAG16 Abs were injected in mice with experimental autoimmune encephalomyelitis, resulting in a significant disease exacerbation. Finally, we demonstrated a consistent upregulation of SPAG16 in MS brain and experimental autoimmune encephalomyelitis spinal cord lesions, more specifically in reactive astrocytes. We conclude that SPAG16 is a novel autoantibody target in a subgroup of MS patients and in combination with other diagnostic criteria, elevated levels of anti-SPAG16 Abs could be used as a biomarker for diagnosis. Furthermore, the pathologic relevance of anti-SPAG16 Abs was shown in vivo

Metabolic flexibility as a major predictor of spatial distribution in microbial communities

A better understand the ecology of microbes and their role in the global ecosystem could be achieved if traditional ecological theories can be applied to microbes. In ecology organisms are defined as specialists or generalists according to the breadth of their niche. Spatial distribution is often used as a proxy measure of niche breadth; generalists have broad niches and a wide spatial distribution and specialists a narrow niche and spatial distribution. Previous studies suggest that microbial distribution patterns are contrary to this idea; a microbial generalist genus (Desulfobulbus) has a limited spatial distribution while a specialist genus (Methanosaeta) has a cosmopolitan distribution. Therefore, we hypothesise that this counter-intuitive distribution within generalist and specialist microbial genera is a common microbial characteristic. Using molecular fingerprinting the distribution of four microbial genera, two generalists, Desulfobulbus and the methanogenic archaea Methanosarcina, and two specialists, Methanosaeta and the sulfate-reducing bacteria Desulfobacter were analysed in sediment samples from along a UK estuary. Detected genotypes of both generalist genera showed a distinct spatial distribution, significantly correlated with geographic distance between sites. Genotypes of both specialist genera showed no significant differential spatial distribution. These data support the hypothesis that the spatial distribution of specialist and generalist microbes does not match that seen with specialist and generalist large organisms. It may be that generalist microbes, while having a wider potential niche, are constrained, possibly by intrageneric competition, to exploit only a small part of that potential niche while specialists, with far fewer constraints to their niche, are more capable of filling their potential niche more effectively, perhaps by avoiding intrageneric competition. We suggest that these counter-intuitive distribution patterns may be a common feature of microbes in general and represent a distinct microbial principle in ecology, which is a real challenge if we are to develop a truly inclusive ecology

Phytoplankton dynamics in relation to seasonal variability and upwelling and relaxation patterns at the mouth of Ria de Aveiro (West Iberian Margin) over a four-year period

From June 2004 to December 2007, samples were weekly collected at a fixed station located at the mouth of Ria de Aveiro (West Iberian Margin). We examined the seasonal and inter-annual fluctuations in composition and community structure of the phytoplankton in relation to the main environmental drivers and assessed the influence of the oceano-graphic regime, namely changes in frequency and intensity of upwelling events, over the dynamics of the phytoplankton assemblage. The samples were consistently handled and a final subset of 136 OTUs (taxa with relative abundance > 0.01%) was subsequently submitted to various multivariate analyses. The phytoplankton assemblage showed significant changes at all temporal scales but with an overriding importance of seasonality over longer-(inter-annual) or shorter-term fluctuations (upwelling-related). Sea-surface temperature, salinity and maximum upwelling index were retrieved as the main driver of seasonal change. Seasonal signal was most evident in the fluctuations of chlorophyll a concentration and in the high turnover from the winter to spring phytoplankton assemblage. The seasonal cycle of production and succession was disturbed by upwelling events known to disrupt thermal stratification and induce changes in the phytoplankton assemblage. Our results indicate that both the frequency and intensity of physical forcing were important drivers of such variability, but the outcome in terms of species composition was highly dependent on the available local pool of species and the timing of those events in relation to the seasonal cycle. We conclude that duration, frequency and intensity of upwelling events, which vary seasonally and inter-annually, are paramount for maintaining long-term phytoplankton diversity likely by allowing unstable coexistence and incorporating species turnover at different scales. Our results contribute to the understanding of the complex mechanisms of coastal phytoplankton dynamics in relation to changing physical forcing which is fundamental to improve predictability of future prospects under climate change.Portuguese Foundation for Science and Technology (FCT, Portugal) [SFRH/BPD/ 94562/2013]; FEDER funds; national funds; CESAM [UID/AMB/50017]; FCT/MEC through national funds; FEDERinfo:eu-repo/semantics/publishedVersio

Ice sheets as a missing source of silica to the polar oceans

Ice sheets play a more important role in the global silicon cycle than previously appreciated. Input of dissolved and amorphous particulate silica into natural waters stimulates the growth of diatoms. Here we measure dissolved and amorphous silica in Greenland Ice Sheet meltwaters and icebergs, demonstrating the potential for high ice sheet export. Our dissolved and amorphous silica flux is 0.20 (0.06-0.79) Tmol year(-1), ∼50% of the input from Arctic rivers. Amorphous silica comprises >95% of this flux and is highly soluble in sea water, as indicated by a significant increase in dissolved silica across a fjord salinity gradient. Retreating palaeo ice sheets were therefore likely responsible for high dissolved and amorphous silica fluxes into the ocean during the last deglaciation, reaching values of ∼5.5 Tmol year(-1), similar to the estimated export from palaeo rivers. These elevated silica fluxes may explain high diatom productivity observed during the last glacial-interglacial period

Maximum photosynthetic efficiency of size-fractionated phytoplankton assessed by C-14 uptake and fast repetition rate fluorometry

Under high nutrient concentrations and sufficient light conditions, large phytoplankton may display higher photosynthetic efficiency than smaller cells. This is unexpected since smaller phytoplankton, because of their higher surface to volume ratio, possess a greater ability to take up nutrients and absorb light. In order to investigate the causes of the increased photosynthetic efficiency in larger phytoplankton, we assessed the maximum photosynthetic efficiency of coastal assemblages in three size classes ( 20 mu m) by concurrently conducting C-14- based photosynthesis-irradiance experiments and fast repetition rate fluorescence measurements. The light-saturated, chlorophyll-specific photosynthesis (P-max(b)) and the maximum photosystem II (PSII) photochemical efficiency (F-v/ F-m) of each size class were determined during winter mixing (March 2003) and summer stratification (June 2003). During winter mixing, size-fractionated P-max(b) and F-v/F-m were similar in all size classes. In contrast, during summer stratification, size-fractionated P-max(b) and F-v/F-m were significantly higher in the > 20-mu m size class. In the entire data set, size-fractionated P-max(b) and F-v/F-m were not significantly correlated. However, a significant relationship was found between size-fractionated P-max(b) and F-v/F-m for phytoplankton assemblages acclimated to low light conditions. Under high light, an excess PSII capacity may be responsible for the discrepancy between size-fractionated P-max(b) and F-v/F-m measurements, whereas under low light conditions, photosynthetic electron transport chain and components downstream of PSII become more balanced, which results in a tight covariation between both variables. Higher maximum photosynthetic efficiencies of large-sized phytoplankton might be associated with a higher PSII photochemical efficiency characteristic of certain taxonomic groups such as diatoms

Continuity in the photosynthetic production of dissolved organic carbon from eutrophic to oligotrophic waters

International audienceWe studied the photosynthetic production of dissolved organic carbon (DOCp) during summer oligotrophic conditions in the Celtic Sea. The production rate of size-fractionated, particulate organic carbon (POCp) was also determined, as well as the relationship between irradiance and DOCp and the kinetics of DOCp during a 24 h light-dark cycle. Mean (1 SE), euphotic layer-integrated POCp was 510 +/- 61 mg C m(-2) d(-1), and cells in the 15 000 mg C m(-2) d(-1), PER was relatively constant at similar to 20%, and integrated DOCp could be predicted from integrated POCp with the equation: log (DOCp) = 0.96 x log (POCp) - 0.51 (r(2) = 0.90, n = 35, p < 0.001)

Size dependence of coastal phytoplankton photosynthesis under vertical mixing conditions

We have determined the relationship between carbon-specific photosynthesis and phytoplankton cell size in a coastal ecosystem. The normalized size spectra of carbon (C) biomass and photosynthesis allow to determine both biomass and photosynthesis within any size class along the community size spectrum. By dividing the size spectra of photosynthesis and biomass, the size spectrum of C-specific photosynthesis is derived. Our results indicated a high variability in the slope of the C-specific photosynthesis size spectrum. Under favourable conditions for growth, in the upper euphotic layer, the slope was positive, indicating that larger phytoplankton attained higher C-specific photosynthesis rates than the smaller cells. This pattern represents a significant departure from the expected, literature value of -0.25 for the size-scaling of biomass-specific metabolism. We suggest that this change in the slope may be caused by the changes in the taxonomic composition along the community size spectrum. Towards the bottom of the euphotic layer, we observed a decrease in the slope of the C-specific photosynthesis size spectrum, which could be associated with an enhanced package effect in larger cells under light-limited conditions. These results question the applicability of single and overall exponents to describe the size scaling of photosynthesis in natural phytoplankton assemblages