Siliceous polycystines and phaeodarians are open‐ocean planktonic protists found throughout the water column and characterized by complex siliceous skeletons that are formed, at least partly, through the uptake of silicic acid. These protists contribute to the marine organic carbon (C) and biogenic silica (bSi) pools, but little is known about their contribution to the silica (Si) biogeochemical cycle. Here we report the first measurements of the Si uptake rate of polycystine and phaeodarian cells from samples collected in the Mediterranean Sea using the 32Si‐based method. The elementary composition (bSi, particulate organic carbon and nitrogen) of these organisms was also measured. Combining our results with published data on the distribution and abundance of Polycystina and Phaeodaria in the global ocean, we conclude that these organisms could contribute from 0.2 to 2.2 mmol Si m−2 of the marine standing stock of bSi and from 2 to 58 Tmol Si yr−1 (1% to 19%) of the global oceanic biogenic silica production. The implications for the global marine Si cycle are discussed.

Protists are the dominant eukaryotes in the biosphere where they play key functional roles. While protists have been studied for over a century, it is the high-throughput sequencing of molecular markers from environmental samples -the approach of metabarcoding -that has revealed just how diverse, and abundant, these small organisms are. Metabarcoding is now routine to survey environmental diversity, so data have rapidly accumulated from a multitude of environments and at different sampling scales. This mass of data has provided unprecedented opportunities to study the taxonomic and functional diversity of protists, and how this diversity is organised in space and time. Here, we use metabarcoding as a common thread to discuss the state of knowledge in protist diversity research, from technical considerations of the approach to important insights gained on diversity patterns and the processes that might have structured this diversity. In addition to these insights, we conclude that metabarcoding is on the verge of an exciting added dimension thanks to the maturation of high-throughput long-read sequencing, so that a robust eco-evolutionary framework of protist diversity is within reach. ll

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Made up of calcareous coralline algae, maerl beds play a major role as ecosystem engineers in coastal areas throughout the world. They undergo strong anthropogenic pressures, which may threaten their survival. The aim of this study was to gain insight into the future of maerl beds in the context of global and local changes. We examined the effects of rising temperatures (+3°C) and ocean acidification (−0.3 pH units) according to temperature and pH projections (i.e., the RCP 8.5 scenario), and nutrient (N and P) availability on three temperate maerl species (Lithothamnion corallioides, Phymatolithon calcareum, and Lithophyllum incrustans) in the laboratory in winter and summer conditions. Physiological rates of primary production, respiration, and calcification were measured on all three species in each treatment and season. The physiological response of maerl to global climate change was species‐specific and influenced by seawater nutrient concentrations. Future temperature–pH scenario enhanced maximal gross primary production rates in P. calcareum in winter and in L. corallioides in both seasons. Nevertheless, both species suffered an impairment of light harvesting and photoprotective mechanisms in winter. Calcification rates at ambient light intensity were negatively affected by the future temperature–pH scenario in winter, with net dissolution observed in the dark in L. corallioides and P. calcareum under low nutrient concentrations. Nutrient enrichment avoided dissolution under future scenarios in winter and had a positive effect on L. incrustans calcification rate in the dark in summer. In winter conditions, maximal calcification rates were enhanced by the future temperature–pH scenario on the three species, but P. calcareum suffered inhibition at high irradiances. In summer conditions, the maximal calcification rate dropped in L. corallioides under the future global climate change scenario, with a potential negative impact on CaCO3 budget for maerl beds in the Bay of Brest where this species is dominant. Our results highlight how local changes in nutrient availability or irradiance levels impact the response of maerl species to global climate change and thus point out how it is important to consider other abiotic parameters in order to develop management policies capable to increase the resilience of maerl beds under the future global climate change scenario.

BackgroundMany fish species have been introduced in wild ecosystems around the world to provide food or leisure, deliberately or from farm escapes. Some of those introductions have had large ecological effects. The north American native rainbow trout (Oncorhynchus mykiss Walbaum, 1792) is one of the most widely farmed fish species in the world. It was first introduced in Spain in the late 19th century for sport fishing (Elvira 1995) and nowadays is used there for both fishing and aquaculture. On the other hand, the European native brown trout (Salmo trutta L.) is catalogued as vulnerable in Spain. Detecting native and invasive fish populations in ecosystem monitoring is crucial, but it may be difficult from conventional sampling methods such as electrofishing. These techniques encompass some mortality, thus are not adequate for some ecosystems as the case of protected areas. Environmental DNA (eDNA) analysis is a sensitive and non-invasive method that can be especially useful for rare and low-density species detection and inventory in water bodies.MethodsIn this study we employed two eDNA based methods (qPCR and nested PCR-RFLP) to detect salmonid species from mountain streams within a protected area, The Biosphere Reserve and Natural Park of Redes (Upper Nalón Basin, Asturias, Northern Spain), where brown trout is the only native salmonid. We also measured some habitat variables to see how appropriate for salmonids the area is. The sampling area is located upstream impassable dams and contains one rainbow trout fish farm.ResultsEmploying qPCR methodology, brown trout eDNA was detected in all the nine sampling sites surveyed, while nested PCR-RFLP method failed to detect it in two sampling points. Rainbow trout eDNA was detected with both techniques at all sites in the Nalón River’ (n1, n2 and n3). Salmonid habitat units and water quality were high from the area studied.DiscussionIn this study, a high quantity of rainbow trout eDNA was found upstream and downstream of a fish farm located inside a Biosphere Reserve. Unreported escapes from the fish farm are a likely explanation of these results. Since salmonid habitat is abundant and the water quality high, the establishment of rainbow trout populations would be favored should escapes occur. Environmental DNA has here proved to be a valuable tool for species detection in freshwater environments, and the probe-based qPCR highly sensitive technique for detection of scarce species. We would recommend this method for routine monitoring and early detection of introduced species within natural reserves.

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Collodaria (Retaria) are important contributors to planktonic communities and biogeochemical processes (e.g., the biologic pump) in oligotrophic oceans. Similarly to corals, Collodaria live in symbiosis with dinoflagellate algae, a relationship that is thought to explain partly their ecological success. In the context of global change, the robustness of the symbiotic interaction, and potential subsequent bleaching events are of primary interest for oceanic ecosystems functioning. In the present study, we compared the ultrastructure, morphology, symbiont density, photosynthetic capacities and respiration rates of colonial Collodaria exposed to a range of temperatures corresponding to natural conditions (21 • C), moderate (25 • C), and high (28 • C) thermal stress. We showed that symbiont density immediately decreased when temperature rose to 25 • C, while the overall Collodaria holobiont metabolic activity increased. When temperature reached 28 • C, the holobiont respiration nearly stopped and the host morphological structure was largely damaged, as if the host tolerance threshold has been crossed. Over the course of the experiment, the photosynthetic capacities of remaining algal symbionts were stable, chloroplasts being the last degraded organelles in the microalgae. These results contribute to a better characterization and understanding of temperature-induced bleaching processes in planktonic photosymbioses.