Protist communities along freshwater–marine transition zones in Hudson Bay (Canada)

Abstract

One of the most striking ecological divides on Earth is between marine and nearby freshwater environments, as relatively few taxa can move between the two. Microbial eukaryotes contribute to biogeochemical and energy cycling in both fresh and marine waters, with little species overlap between the two ecosystems. Arctic and sub-Arctic marine systems are relatively fresh compared to tropical and temperate systems, but details of microbial eukaryote communities along river-to-sea transitions are poorly known. To bridge this knowledge gap, we investigated three river-to-sea transitions (Nelson, Churchill, and Great Whale Rivers) in sub-Arctic Hudson Bay through 18S rRNA amplicon sequencing to identify microbial eukaryotes along the salinity and biogeochemical gradients. Salinity acted as the principal dispersal barrier preventing freshwater microorganisms from colonizing marine coastal waters, with microbial eukaryote communities of the three rivers clustering together. Just offshore, communities clustered by coastal regions associated with nutrient concentrations. Analysis of indicator species revealed that communities in the nitrate-depleted coastal water off the Churchill and Great Whale Rivers were dominated by heterotrophic taxa and small photosynthetic protists. In contrast, the Nelson offshore community was characterized by a high proportion of the diatom Rhizosolenia. A distinct community of heterotrophic protists was identified in the three estuarine transition zones, suggesting specialized estuarine communities. Such specialization was most marked in the Nelson River system that was sampled more intensely and showed estuarine circulation. The autochthonous community was composed of the bacterial grazers Katablepharis, Mataza, and Cryothecomonas, as well as brackish species of the diatoms Skeletonema and Thalassiosira. These findings suggest that flow regulation on the Nelson River that modifies estuarine circulation would affect estuarine community composition and distribution in the transition zone.

Size-Fractionated Microbiome Structure in Subarctic Rivers and a Coastal Plume Across DOC and Salinity Gradients

“…The mixotrophic ciliate Mesodinium rubrum also showed a relationship with salinity. This taxon was in highest abundance at salinity 10.07 and has been previously identified as the most abundant OTU in the GWR estuary at salinity 12.5 (Jacquemot et al, 2021). This species captures and retains plastids from the cryptophytes Teleaulax gracilis and Plagioselmis prolonga (Rial et al, 2015); these two cryptophytes also occurred in our sequences, mainly at moderate salinity in the GWR plume.…”

Environmental drivers of spring primary production in Hudson Bay

“…However, inshore stations 19 and 22 were characterized by a low K d0 (PAR) of 0.12 m -1 , a deep Z eu of 49 m, and low TChl a of <1 mg m -3 . Inshore station 46, which was located near the Nelson River estuary, differed from these characteristics with a K d0 (PAR) of 0.23 m -1 , a Z eu of 24 m, and TChl a of >1 mg m -3 attributed to a phytoplankton bloom in the Nelson River estuary (Jacquemot et al, 2021). NPP varied largely in the open water between 170 mg C m -2 d -1 at station 22 Sampled water depths of 2 m, depth of euphotic zone (Z eu ), 10 m above the sea floor (Z bot ), and 5-cm ice-bottom sections.…”

The Great Whale River ecosystem: ecology of a subarctic river and its receiving waters in coastal Hudson Bay, Canada

“…These have revealed a diverse bacterial assemblage dominated by taxa in the Burkholderiales, but with numerous rare taxa, and evidence that microbial community richness increases down the river (Blais et al In press). Like other river systems discharging into Hudson Bay, the estuarine transition zone of the Great Whale River contains a distinctive community of microbial eukaryotes, including the photosynthetic ciliate Mesodinium rubrum (Jacquemot et al 2021). The river contains highly variable concentrations of the greenhouse gases methane and carbon dioxide (Matveev et al 2020), implying variable rates of decomposition and methanogenesis, or variable effects of sediment and ground water exchanges as well as ventilation to the atmosphere by waterfalls and rapids, but none of these processes have been measured directly to date.…”

“…Localized studies of the river and associated estuary have been conducted out of the research station of the Centre d'études nordiques (CEN) at Whapmagoostui-Kuujjuarapik from the 1970s onwards (Bhiry et al 2011), including a project on sea ice processes (Gosselin et al 1990) and permafrost wetlands that drain into the Great Whale River ecosystem (Vincent et al 2017). There have also been recent oceanographic measurements in the region by icebreakers (Jacquemot et al 2021).…”