Roger Kerouel

Bio: Roger Kerouel is an academic researcher from IFREMER. The author has contributed to research in topics: Seawater & Phosphorus. The author has an hindex of 22, co-authored 44 publications receiving 2772 citations.

A simple and precise method for measuring ammonium in marine and freshwater ecosystems

Abstract: The accurate measurement of ammonium concentrations is fundamental to understanding nitrogen biogeochemistry in aquatic ecosystems. Unfortunately, the commonly used indophenol blue method often yie...

Dynamics of particulate organic matter d15N and d13C during spring phytoplankton blooms in a macrotidal ecosystem (Bay of Seine, France)

Abstract: Two cruises (April and June 1997) were carried out in the Bay of Seine, a nitrate- and ammonium-enriched ecosystem of Western Europe, to identify the major mechanisms that control δ 15 N and δ 13 C in spring particulate organic matter (POM). Particulate organic nitrogen (PON) δ 15 N ranged between 0.8 and 5.2 ‰ in April and between 2.2 and 6.2 ‰ in June, while particulate organic carbon (POC) δ 13 C ranged between -24.3 and -19.7‰, and between -20.0 and -16.2‰ during the same periods. During spring 1997, POM was highly dominated by autochthonous phytoplankton. It is shown that the variation of PON δ 15 N is due to both nitrate mixing between river and marine waters and fractionation of N stable isotopes during nitrate utilization by phytoplankton. Therefore, similarly to what was previously shown for open ocean, δ 15 N can be used as a proxy of spring fractional nitrate utilization in coastal ecosystems. It is also shown that POC δ 13 C in spring is controlled by POC con- centration and C:N ratio (in addition to 'temperature effects'), which are considered here as indica- tors of primary production and phytoplankton degradation, respectively. The co-variation of δ 13 C and δ 15 N describes the spring phytoplankton dynamics: at the start of phytoplankton development, nitrate concentration is high (low δ 15 N) and phytoplankton production is low (low δ 13 C); then primary pro- duction increases (δ 13 C becomes higher) and the nitrate pool diminishes (δ 15 N becomes higher); at a later stage, the nitrate pool is depleted (high δ 15 N), part of the phytoplankton becomes degraded and production is still high (high δ 13 C).

Chemical environment of the hydrothermal mussel communities in the Lucky Strike and Menez Gwen vent fields, Mid Atlantic Ridge

Abstract: Water samples were collected around the communities of a hydrothermal mussel, Bathymodiolus sp., in the Lucky Strike (1700 m depth) and Menez Gwen (850 m depth) areas, Mid Atlantic Ridge, and analysed for chemical constituents. The environment surrounding the organisms consists in sea water (88 to 100%) mixed with hydrothermal fluid with pH between 6.2 and 8, Sigma S (H2S + HS-) concentrations from 0 to 62 mu mol l(-1). High dissolved organic carbon (DOC) concentrations suggest a highly productive ecosystem. Production of ammonium and DOG, consumption of nitrate and sulfide in the vicinity of the organisms are among the environmental changes found in relation with local biological activity. Concentrations of Cu and Pb were high, implying that the organisms need to have efficient strategies of adaptation. An empirical distinction between the extreme sizes of the mussels ( 6 cm and 3 cm) reveals that the size distribution of the communities is related to the presence of different environments: the large size classes are present closer to the fluid exits, with higher Sigma S concentrations, than the small size classes. This could indicate differences in the growth rate related to the availability of the energy sources, competition for energy and space, and/or different settlement periods.

Ammonia oxidation kinetics determine niche separation of nitrifying Archaea and Bacteria

Abstract: The discovery of ammonia oxidation by mesophilic and thermophilic Crenarchaeota and the widespread distribution of these organisms in marine and terrestrial environments indicated an important role for them in the global nitrogen cycle. However, very little is known about their physiology or their contribution to nitrification. Here we report oligotrophic ammonia oxidation kinetics and cellular characteristics of the mesophilic crenarchaeon 'Candidatus Nitrosopumilus maritimus' strain SCM1. Unlike characterized ammonia-oxidizing bacteria, SCM1 is adapted to life under extreme nutrient limitation, sustaining high specific oxidation rates at ammonium concentrations found in open oceans. Its half-saturation constant (K(m) = 133 nM total ammonium) and substrate threshold (

A simple and precise method for measuring ammonium in marine and freshwater ecosystems

Abstract: The accurate measurement of ammonium concentrations is fundamental to understanding nitrogen biogeochemistry in aquatic ecosystems. Unfortunately, the commonly used indophenol blue method often yie...

Abundance and diversity of denitrifying and anammox bacteria in seasonally hypoxic and sulfidic sediments of the saline lake Grevelingen

Abstract: Denitrifying and anammox bacteria are involved in the nitrogen cycling in marine sediments but the environmental factors that regulate the relative importance of these processes are not well constrained. Here, we evaluated the abundance, diversity, and potential activity of denitrifying, anammox, and sulfide-dependent denitrifying bacteria in the sediments of the seasonally hypoxic saline Lake Grevelingen, known to harbor an active microbial community involved in sulfur oxidation pathways. Depth distributions of 16S rRNA gene, nirS gene of denitrifying and anammox bacteria, aprA gene of sulfur-oxidizing and sulfate-reducing bacteria, and ladderane lipids of anammox bacteria were studied in sediments impacted by seasonally hypoxic bottom waters. Samples were collected down to 5 cm depth (1 cm resolution) at three different locations before (March) and during summer hypoxia (August). The abundance of denitrifying bacteria did not vary despite of differences in oxygen and sulfide availability in the sediments, whereas anammox bacteria were more abundant in the summer hypoxia but in those sediments with lower sulfide concentrations. The potential activity of denitrifying and anammox bacteria as well as of sulfur-oxidizing, including sulfide-dependent denitrifiers and sulfate-reducing bacteria, was potentially inhibited by the competition for nitrate and nitrite with cable and/or Beggiatoa-like bacteria in March and by the accumulation of sulfide in the summer hypoxia. The simultaneous presence and activity of organoheterotrophic denitrifying bacteria, sulfide-dependent denitrifiers, and anammox bacteria suggests a tight network of bacteria coupling carbon-, nitrogen-, and sulfur cycling in Lake Grevelingen sediments.

Enhanced biological carbon consumption in a high CO2 ocean

Abstract: The oceans have absorbed nearly half of the fossil-fuel carbon dioxide (CO2) emitted into the atmosphere since pre-industrial times1, causing a measurable reduction in seawater pH and carbonate saturation2. If CO2 emissions continue to rise at current rates, upper-ocean pH will decrease to levels lower than have existed for tens of millions of years and, critically, at a rate of change 100 times greater than at any time over this period3. Recent studies have shown effects of ocean acidification on a variety of marine life forms, in particular calcifying organisms4, 5, 6. Consequences at the community to ecosystem level, in contrast, are largely unknown. Here we show that dissolved inorganic carbon consumption of a natural plankton community maintained in mesocosm enclosures at initial CO2 partial pressures of 350, 700 and 1,050 μatm increases with rising CO2. The community consumed up to 39% more dissolved inorganic carbon at increased CO2 partial pressures compared to present levels, whereas nutrient uptake remained the same. The stoichiometry of carbon to nitrogen drawdown increased from 6.0 at low CO2 to 8.0 at high CO2, thus exceeding the Redfield carbon:nitrogen ratio of 6.6 in today’s ocean7. This excess carbon consumption was associated with higher loss of organic carbon from the upper layer of the stratified mesocosms. If applicable to the natural environment, the observed responses have implications for a variety of marine biological and biogeochemical processes, and underscore the importance of biologically driven feedbacks in the ocean to global change.