The emergence of ocean acidification as a significant threat to calcifying organisms in marine ecosystems creates a pressing need to understand the physiological and molecular mechanisms by which calcification is affected by environmental parameters. We report here, for the first time, changes in gene expression induced by variations in pH/pCO2 in the widespread and abundant coccolithophore Emiliania huxleyi. Batch cultures were subjected to increased partial pressure of CO2 (pCO2; i.e. decreased pH), and the changes in expression of four functional gene classes directly or indirectly related to calcification were investigated. Increased pCO2 did not affect the calcification rate and only carbonic anhydrase transcripts exhibited a significant down-regulation. Our observation that elevated pCO2 induces only limited changes in the transcription of several transporters of calcium and bicarbonate gives new significant elements to understand cellular mechanisms underlying the early response of E. huxleyi to CO2-driven ocean acidification.

Molecular lipid biomarkers (hydrocarbons, alcohols, sterols and fatty acids) and compound-specific isotope analysis of suspended particulate organic matter (SPM) and surface sediments of the Mackenzie Shelf and slope (southeast Beaufort Sea, Arctic Ocean) were studied in summer 2009. The concentrations of the molecular lipid markers, characteristic of known organic matter sources, were grouped and used as proxies to evaluate the relative importance of fresh algal, detrital algal, fossil, C₃ terrestrial plants, bacterial and zooplankton material in the organic matter (OM) of this area. Fossil and detrital algal contributions were the major fractions of the freshwater SPM from the Mackenzie River with ~34% each of the total molecular biomarkers. Fresh algal, C₃ terrestrial, bacterial and zooplanktonic components represented much lower percentages, 17, 10, 4 and <1%, respectively. In marine SPM from the Mackenzie slope, the major contributions were fresh and detrital algal components (>80%), with a minor contribution of fossil and C₃ terrestrial biomarkers. Characterization of the sediments revealed a major sink of refractory algal material mixed with some fresh algal material, fossil hydrocarbons and a small input of C₃ terrestrial sources. In particular, the sediments from the shelf and at the mouth of the Amundsen Gulf presented the highest contribution of detrital algal material (60–75%), whereas those from the slope contained the highest proportion of fossil (40%) and C₃ terrestrial plant material (10%). Overall, considering that the detrital algal material is marine derived, autochthonous sources contributed more than allochthonous sources to the OM lipid pool. Using the ratio of an allochthonous biomarker (normalized to total organic carbon, TOC) found in the sediments to those measured at the river mouth water, we estimated that the fraction of terrestrial material preserved in the sediments accounted for 30–40% of the total carbon in the inner shelf sediments, 17% in the outer shelf and Amundsen Gulf and up to 25% in the slope sediments. These estimates are low compared to other studies conducted 5–20 yr earlier, and they support the increase in primary production during the last decade mainly because of the increase in the number of ice-free days and due to the strength and persistence of winds favouring upwelling

The response of Emiliania huxleyi (Lohmann) W. W. Hay et H. Mohler, Calcidiscus leptoporus (G. Murray et V. H. Blackman) J. Schiller, and Syracosphaera pulchra Lohmann to elevated partial pressure of carbon dioxide (pCO2 ) was investigated in batch cultures. We reported on the response of both haploid and diploid life stages of these three species. Growth rate, cell size, particulate inorganic carbon (PIC), and particulate organic carbon (POC) of both life stages were measured at two different pCO2 (400 and 760 parts per million [ppm]), and their organic and inorganic carbon production were calculated. The two life stages within the same species generally exhibited a similar response to elevated pCO2 , the response of the haploid stage being often more pronounced than that of the diploid stage. The growth rate was consistently higher at elevated pCO2 , but the response of other processes varied among species. Calcification rate of C. leptoporus and of S. pulchra did not change at elevated pCO2 , whereas it increased in E. huxleyi. POC production and cell size of both life stages of S. pulchra and of the haploid stage of E. huxleyi markedly decreased at elevated pCO2 . It remained unaltered in the diploid stage of E. huxleyi and C. leptoporus and increased in the haploid stage of the latter. The PIC:POC ratio increased in E. huxleyi and was constant in C. leptoporus and S. pulchra. Elevated pCO2 has a significant effect on these three coccolithophore species, the haploid stage being more sensitive. This effect must be taken into account when predicting the fate of coccolithophores in the future ocean.

Although irrigation water is frequently assessed for the presence of plant pathogens, large spatial and temporal surveys that provide clues on the diversity and circulation of pathogens is missing. We evaluate the diversity of soft rot Pectobacteriaceae (SRP) of the genera Dickeya and Pectobacterium over two years in a temperate, mixed use watershed. The abundance of isolated strains correlates with the agricultural gradient along the watershed with a positive correlation found with temperature, nitrate and dissolved organic carbon water concentration. We characterized 582 strains by amplification and sequencing of the gapA gene. MLSA analysis performed with 3 housekeeping genes for 99 strains and core genome analysis of 38 sequenced strains confirmed for all the strains but one the taxonomic assignation obtained with the sole gapA sequence. Pectobacterium spp. (549 isolates) were far more abundant than Dickeya spp. (33 isolates). Dickeya spp. were only observed in the lower part of the river when water temperature was above 19°C and we experimentally confirmed a decreased fitness of several Dickeya spp. at 8°C in river water. D. oryzae dominates the Dickeya spp. P. versatile and P. aquaticum dominate the Pectobacterium spp. but their repartition along the watershed was different, P. versatile being the only species regularly recovered all along the watershed. Excepting P. versatile, Dickeya and Pectobacterium spp. responsible for disease outbreak on crops were less abundant or rarely detected. This work sheds light on the various ecological behaviours of different SRP in stream water and indicates that SRP occupation is geographically structured.

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The effects of elevated partial pressure of CO 2 (pCO 2 ) and temperature on the coccolithophore Syracosphaera pulchra were investigated in isolation and in combination. Both the diploid and the haploid life stages were studied. Batch cultures were grown under 4 conditions: 400 µatm and 19°C; 400 µatm and 22°C; 740 µatm and 19°C; and 740 µatm and 22°C. The growth rate (μ) significantly increased under elevated pCO 2 only in the haploid stage and showed a different pattern with respect to temperature: it was higher at an elevated temperature in the haploid stage at 400 µatm whereas it decreased in the diploid stage at 740 µatm. Increasing both parameters together increased the growth rate by 11% in the haploid stage only. Elevated pCO 2 had a negative impact on the content of particulate organic carbon (POC), production and cell size in both life stages at 19°C, while no significant effect was observed at 22°C. Increasing temperature significantly increased the content of POC and production in the diploid stage at 740 µatm, while at 400 µatm it significantly decreased both the content of POC and production in the haploid stage. A simultaneous increase in pCO 2 and temperature had a negative effect on the content of POC and production in the haploid stage only. Neither the rate of calcification (production of particulate inorganic carbon, PIC) nor the PIC:POC ratio were significantly affected by elevated pCO 2 , temperature or their interaction. These results showed a strong interactive effect between pCO 2 and temperature in affecting the physiology of S. pulchra, an effect that was often more pronounced in the haploid life stage. Elevated pCO 2 had a stronger effect than temperature.

The influence of climate change on the ecological impacts of invasive alien species (IAS) remains understudied, with deoxygenation of aquatic environments often-overlooked as a consequence of climate change. Here, we therefore assessed how oxygen saturation affects the ecological impact of a predatory invasive fish, the Ponto-Caspian round goby (Neogobius melanostomus), relative to a co-occurring endangered European native analogue, the bullhead (Cottus gobio) experiencing decline in the presence of the IAS. In individual trials and mesocosms, we assessed the effect of high, medium and low (90%, 60% and 30%) oxygen saturation on: (1) functional responses (FRs) of the IAS and native, i.e. per capita feeding rates; (2) the impact on prey populations exerted; and (3) how combined impacts of both fishes change over invasion stages (Pre-invasion, Arrival, Replacement, Proliferation). Both species showed Type II potentially destabilising FRs, but at low oxygen saturation, the invader had a significantly higher feeding rate than the native. Relative Impact Potential, combining fish per capita effects and population abundances, revealed that low oxygen saturation exacerbates the high relative impact of the invader. The Relative Total Impact Potential (RTIP), modelling both consumer species’ impacts on prey populations in a system, was consistently higher at low oxygen saturation and especially high during invader Proliferation. In the mesocosm experiment, low oxygen lowered RTIP where both species were present, but again the IAS retained high relative impact during Replacement and Proliferation stages at low oxygen. We also found evidence of multiple predator effects, principally antagonism. We highlight the threat posed to native communities by IAS alongside climate-related stressors, but note that solutions may be available to remedy hypoxia and potentially mitigate impacts across invasion stages.

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