Cockle as Second Intermediate Host of Trematode Parasites: Consequences for Sediment Bioturbation and Nutrient Fluxes across the Benthic Interface

Trematode parasites are distributed worldwide and can severely impact host populations. However, their influence on ecosystem functioning through the alteration of host engineering behaviours remains largely unexplored. This study focuses on a common host parasite system in marine coastal environments, i.e., the trematode Himasthla elongata, infecting the edible cockle Cerastoderma edule as second intermediate host. A laboratory experiment was conducted to investigate the indirect effects of metacercarial infection on sediment bioturbation and biogeochemical fluxes at the sediment water interface. Our results revealed that, despite high parasite intensity, the sediment reworking and bioirrigation rates, as well as nutrient fluxes, were not impacted. This finding was unexpected since previous studies showed that metacercarial infection impairs the physiological condition of cockles and induces a mechanical obstruction of their feet, thus altering their burrowing capacity. There are several explanations for such contrasting results. Firstly, the alteration of cockle behavior could arise over a longer time period following parasite infection. Secondly, the modulation of cockle bioturbation by parasites could be more pronounced in older specimens burying deeper. Thirdly, the intensity of the deleterious impacts of metacercariae could strongly vary across parasite species. Lastly, metacercarial infection alters cockle fitness through an interaction with other biotic and abiotic environmental stressors.

Electron & Biomass Dynamics of Cyanothece Under Interacting Nitrogen & Carbon Limitations

Marine diazotrophs are a diverse group with key roles in biogeochemical fluxes linked to primary productivity. The unicellular, diazotrophic cyanobacterium Cyanothece is widely found in coastal, subtropical oceans. We analyze the consequences of diazotrophy on growth efficiency, compared to NO3–-supported growth in Cyanothece, to understand how cells cope with N2-fixation when they also have to face carbon limitation, which may transiently affect populations in coastal environments or during blooms of phytoplankton communities. When grown in obligate diazotrophy, cells face the double burden of a more ATP-demanding N-acquisition mode and additional metabolic losses imposed by the transient storage of reducing potential as carbohydrate, compared to a hypothetical N2 assimilation directly driven by photosynthetic electron transport. Further, this energetic burden imposed by N2-fixation could not be alleviated, despite the high irradiance level within the cultures, because photosynthesis was limited by the availability of dissolved inorganic carbon (DIC), and possibly by a constrained capacity for carbon storage. DIC limitation exacerbates the costs on growth imposed by nitrogen fixation. Therefore, the competitive efficiency of diazotrophs could be hindered in areas with insufficient renewal of dissolved gases and/or with intense phytoplankton biomass that both decrease available light energy and draw the DIC level down.

Global synthesis of subglacial lakes and their changing role in a warming climate

<p>Subglacial lakes provide habitats for life and can modulate ice flow, basal hydrology, biogeochemical fluxes and geomorphic activity. They have been identified widely beneath the ice sheets of Antarctica and Greenland, and detected beneath the ice caps on Devon Island and Iceland, and beneath small valley glaciers. Past investigations focussed on lakes beneath individual ice masses. A scientific synthesis of different lake populations has not been made, so a unified understanding of the mechanisms controlling subglacial lake formation, dynamics, and interaction with other parts of the Earth system is lacking. Here, we integrate existing, often disparate data into a global database of subglacial lakes, enabling subglacial lake characteristics and dynamics to be classified. We use this assessment to evaluate how subglacial lakes shape microbial ecosystems and influence ice flow, subglacial drainage, sediment transport and biogeochemical fluxes. Through our global perspective, we examine how subglacial lake characteristics and function depend on the hydrologic, dynamic and mass balance regime of the ice mass beneath which they are located. By applying this synoptic understanding and perspective, we propose a conceptual model for how subglacial lakes and their impacts on the broader environment will change in a warming world. </p>

Hydromorphologic Sorting of In-Stream Nitrogen Uptake Across Spatial Scales

Nitrogen (N) uptake is a key process in stream ecosystems that is mediated mainly by benthic microorganisms (biofilms on different substrata) and has implications for the biogeochemical fluxes at catchment scale and beyond. Here, we focused on the drivers of assimilatory N uptake, especially the effects of hydromorphology and other environmental constraints, across three spatial scales: micro, meso and reach. In two seasons (summer and spring), we performed whole-reach 15N-labelled ammonium injection experiments in two montane, gravel-bed stream reaches with riffle–pool sequences. N uptake was highest in epilithic biofilms, thallophytes and roots (min–max range 0.2–545.2 mg N m−2 day−1) and lowest in leaves, wood and fine benthic organic matter (0.05–209.2 mg N m−2 day−1). At the microscale, N uptake of all primary uptake compartments except wood was higher in riffles than in pools. At the mesoscale, hydromorphology determined the distribution of primary uptake compartments, with fast-flowing riffles being dominated by biologically more active compartments and pools being dominated by biologically less active compartments. Despite a lower biomass of primary uptake compartments, mesoscale N uptake was 1.7–3.0 times higher in riffles than in pools. At reach scale, N uptake ranged from 79.6 to 334.1 mg N m−2 day−1. Highest reach-scale N uptake was caused by a bloom of thallopyhtes, mainly filamentous autotrophs, during stable low discharge and high light conditions. Our results reveal the important role of hydromorphologic sorting of primary uptake compartments at mesoscale as a controlling factor for reach-scale N uptake in streams.

The Malina oceanographic expedition: How do changes in ice cover, permafrost and UV radiation impact biodiversity and biogeochemical fluxes in the Arctic Ocean?

The MALINA oceanographic campaign was conducted during summer 2009 to investigate the carbon stocks and the processes controlling the carbon fluxes in the Mackenzie River estuary and the Beaufort Sea. During the campaign, an extensive suite of physical, chemical and biological variables was measured across seven shelf–basin transects (south-north) to capture the meridional gradient between the estuary and the open ocean. Key variables such as temperature, absolute salinity, radiance, irradiance, nutrient concentrations, chlorophyll-a concentration, bacteria, phytoplankton and zooplankton abundance and taxonomy, and carbon stocks and fluxes were routinely measured onboard the Canadian research icebreaker CCGS Amundsen and from a barge in shallow coastal areas or for sampling within broken ice fields. Here, we present the results of a joint effort to tidy and standardize the collected data sets that will facilitate their reuse in further studies of the changing Arctic Ocean. The dataset is available at https://doi.org/10.17882/75345 (Massicotte2020b).