Génétique des populations de chats domestiques de l'île de la Réunion

International audienc

Seasonal Variability in Diazotroph Abundance and Gene Expression at a Coastal N\u3csub\u3e2\u3c/sub\u3e Fixation Hotspot (Outer Banks, NC)

Marine microbial dinitrogen (N2) fixation, the conversion of gaseous N2 to bioavailable species, is the primary source of new oceanic nitrogen (N). N is present in nucleic acids, amino acids, and proteins, and is essential to all life. Long considered to be a primarily oligotrophic ocean process, significant N2 fixation rates have recently been observed in coastal environments, including along the Cape Hatteras front. To see if elevated N2 fixation was a persistent feature in this region, N2 fixation rates and N2 fixer (diazotroph) abundance and gene expression were investigated through roughly monthly sampling at a field station (Jennette’s Pier) in the Outer Banks, NC, from June 2019 to August 2020, as well as a day-long cruise around the pier in August of 2019. In addition to rate and molecular samples, chlorophyll, and particulate N samples were collected and salinity/temperature profiles were measured. Using quantitative polymerase chain reaction techniques, we investigated the abundance and gene expression of diazotrophic (N2 fixing) cyanobacteria possibly responsible for these high coastal rates, and compared these results to N2 fixation rates measured using a variant of the 15N2 tracer assay. Diazotroph species investigated include Trichodesmium spp. and 5 endosymbiont cyanobacteria. Preliminary results suggest evidence of Gulf Stream intrusions within 300m from the shore, and a seasonal variability pattern of nitrogen fixation rates. This study provides useful measurements of coastal N inputs in the context of the global ocean N cycle and budget, and explores chemical and physical factors that affect these processes.https://digitalcommons.odu.edu/gradposters2021_sciences/1009/thumbnail.jp

\u3ci\u3eAlexandrium\u3c/i\u3e in the Arctic: Are Harmful Algae Spreading as the Arctic Warms?

Alexandrium tamerense is a well-studied dinoflagellate known for its ability to produce the neurotoxin that causes paralytic shellfish poisoning. Until 1970 Alexandrium tamerense was only found in Europe, North America, and Japan but has been increasingly found all over the globe. Alexandrium is characteristically found in temperate and subtropical regions and as the Arctic warms, there is considerable concern that it may be expanding into the Arctic. We found Alexandrium tamerense during a research expedition to the Alaskan Beaufort Sea shelf to study upwelling. Upwelling events are known to support seasonal blooms of phytoplankton, which are important primary producers at the base of the oceanic food web. The Beaufort Sea in the Arctic Ocean is known to experience upwelling due to storms caused by atmospheric pressure differences between air masses over Canada and Alaska. This upwelling is becoming more frequent as sea ice melts and the Arctic becomes warmer. We examined the upwelling system in the Beaufort Sea during one of these storms, by collecting surface water samples before, during, and after an upwelling event. Here we present observations of Alexandrium tamarense, found before upwelling occurred, using three different methods.https://digitalcommons.odu.edu/gradposters2021_sciences/1012/thumbnail.jp

Genetic Indicators of Iron Limitation in Wild Populations of Thalassiosira oceanica From the Northeast Pacific Ocean

Assessing the iron (Fe) nutritional status of natural diatom populations has proven challenging as physiological and molecular responses can differ in diatoms of the same genus. We evaluated expression of genes encoding flavodoxin (FLDA1) and an Fe-starvation induced protein (ISIP3) as indicators of Fe limitation in the marine diatom Thalassiosira oceanica. The specificity of the response to Fe limitation was tested in cultures grown under Fe-and macronutrient-deficient conditions, as well as throughout the diurnal light cycle. Both genes showed a robust and specific response to Fe limitation in laboratory cultures and were detected in small volume samples collected from the northeast Pacific, demonstrating the sensitivity of this method. Overall, FLDA1 and ISIP3 expression was inversely related to Fe concentrations and offered insight into the Fe nutritional health of T. oceanica in the field. As T. oceanica is a species tolerant to low Fe, indications of Fe limitation in T. oceanica populations may serve as a proxy for severe Fe stress in the overall diatom community. At two shallow coastal locations, FLD1A and ISIP3 expression revealed Fe stress in areas where dissolved Fe concentrations were high, demonstrating that this approach may be powerful for identifying regions where Fe supply may not be biologically available

Cross-Basin Comparison of Phosphorus Stress and Nitrogen Fixation in Trichodesmium

We investigated the phosphorus (P) status and N2 fixation rates of Trichodesmium populations from the North Pacific, western South Pacific, and western North Atlantic. Colonies of Trichodesmium were collected and analyzed for endogenous alkaline phosphatase (AP) activity using enzyme-labeled fluorescence ( ELF) and for nitrogenase activity using acetylene reduction. AP hydrolyzes dissolved inorganic phosphate (DIP) from dissolved organic phosphorus and is active in Trichodesmium colonies experiencing P stress. Across multiple stations in the subtropical North and South Pacific, there was low to moderate ELF labeling in Trichodesmium, although labeling was present in other taxa. In contrast, Trichodesmium ELF labeling in the North Atlantic ranged from low to high. Low ELF labeling corresponded with high DIP concentrations while high ELF labeling occurred only at North Atlantic stations with DIP concentrations \u3c = 40 nmol L-1, indicating that Trichodesmium was not experiencing dramatic P stress in the Pacific Ocean while P stress was evident in the western North Atlantic. However, nitrogenase activity was significantly higher in the P-stressed western North Atlantic than in the Pacific Ocean (0.40-1.30 compared to 0.01-0.46 nmol C2H4 h-1 colony-1. These data underscore the differential basin-level importance of P availability to Trichodesmium and suggest that factors other than P are constraining their N2 fixation rates in the Pacific

A Coastal N₂ Fixation Hotspot at the Cape Hatteras Front: Elucidating Spatial Heterogeneity in Diazotroph Activity Via Supervised Machine Learning

In the North Atlantic Ocean, dinitrogen (N2) fixation on the western continental shelf represents a significant fraction of basin‐wide nitrogen (N) inputs. However, the factors regulating coastal N2 fixation remain poorly understood, in part due to sharp physico‐chemical gradients and dynamic water mass interactions that are difficult to constrain via traditional oceanographic approaches. This study sought to characterize the spatial heterogeneity of N2 fixation on the western North Atlantic shelf, at the confluence of Mid‐ and South Atlantic Bight shelf waters and the Gulf Stream, in August 2016. Rates were quantified using the 15N2 bubble release method and used to build empirical models of regional N2 fixation via a random forest machine learning approach. N2 fixation rates were then predicted from high‐resolution CTD and satellite data to infer the variability of its depth and surface distributions, respectively. Our findings suggest that the frontal mixing zone created conditions conducive to exceptionally high N2 fixation rates (\u3e 100 nmol N L−1 d−1), which were likely driven by the haptophyte‐symbiont UCYN‐A. Above and below this hotspot, N2 fixation rates were highest on the shelf due to the high particulate N concentrations there. Conversely, specific N2 uptake rates, a biomass‐independent metric for diazotroph activity, were enhanced in the oligotrophic slope waters. Broadly, these observations suggest that N2 fixation is favored offshore but occurs continuously across the shelf. Nevertheless, our model results indicate that there is a niche for diazotrophs along the coastline as phytoplankton populations begin to decline, likely due to exhaustion of coastal nutrients

Examining Ecological Succession of Diatoms in California Current System Cyclonic Mesoscale Eddies

The California Current System is a diatom-dominated region characterized by seasonal coastal upwelling and additional elevated mesoscale activity. Cyclonic mesoscale eddies in the region trap productive coastal waters with their planktonic communities and transport them offshore with limited interaction with surrounding waters, effectively acting as natural mesocosms, where phytoplankton populations undergo ecological succession as eddies age. This study examines diatom community composition within two mesoscale cyclonic eddies that formed in the same region of the California Current System 2 months apart and in the California Current waters surrounding them. The diatom communities were analyzed in the context of shifting environmental gradients and through a lens of community succession to expand our understanding of biophysical interactions in California Current System cyclonic eddies. Diatom communities within each eddy were different from non-eddy communities and varied in concert with salinity and dissolved iron (Fe) concentrations. The younger, nearshore eddy displayed higher macronutrient and dissolved Fe concentrations, had higher values for diatom Shannon diversity and evenness, and had nutrient ratios indicative of either eventual silicic acid (Si) or Fe limitation or possibly co-limitation. The older, offshore eddy displayed low macronutrient and dissolved Fe concentrations, was likely nitrate-limited, and had lower diatom Shannon diversity and evenness indices. Sequences from the genus Rhizosolenia, some of which form vertically migrating mats to bypass nitrate limitation, dominated in the older eddy. This is of potential significance as the prevalence of Rhizosolenia mats could impact estimates of carbon cycling and export in the wider California coastal area