Collaborative Research: Ecological and biogeochemical role of Rhizaria in the oligotrophic ocean

Rhizaria are large, single-celled organisms that contribute to chemical cycles in the world’s oceans. Common Rhizaria (e.g. Acantharea, Foraminifera, Polycystines, Phaeodarea) form mineral shells that are part of the ocean’s strontium, silica, and carbon budgets, and they represent at least 5.2% of the oceanic biological carbon reservoir in the surface zone. Better understanding of groups of Rhizaria that are present, their biomass, and how their populations change across different ocean depths is essential to model oceanic nutrient cycles, but this information is poorly resolved or lacking for large Rhizaria in one of the largest biomes on Earth, the central, low-nutrient ocean gyres. This collaborative project involves a team of early career and more established investigators, provides measurements needed to develop a model of Rhizaria contribution to sinking nutrients in the deep ocean, and is adding to our understanding of how nutrients move in the oceans and how changing environmental conditions affect those cycles. It is generating large datasets that can be used by oceanographic researchers and is delivering oceanographic, biological, and data-science themed experiential learning opportunities for undergraduate, graduate, and high school students. This study provides 1) greatly improved estimates of total Rhizaria biomass in the oligotrophic ocean, 2) novel measurements of vertical distribution and environmental niche characteristics of Rhizaria, 3) measurements of seasonal variation in Rhizaria biomass and community composition, 4) identification of the environmental drivers of seasonal and spatial variation in Rhizaria populations, and 5) a model-based estimate of the contributions of Rhizaria to vertical flux into the deep ocean. The investigators are using a combination of Niskin bottle sampling, particle gel traps, net tows, and cutting-edge in situ imaging (using an Underwater Vision Profiler, UVP5-DEEP) of Rhizaria to bypass limitations in other sampling methods to produce accurate estimates of Rhizaria biomass. Sampling occurs across a wide depth range (0-1200 m), at monthly intervals, and in conjunction with a wide array of other oceanographic measurements collected as part of the Bermuda Atlantic Time-series Study (BATS). UVP imagery data is integrated with molecular identification of Rhizaria using community metabarcoding, ZooSCAN imaging, and barcoding of individual Rhizaria collected from net tows and gel traps taken in conjunction with UVP casts. The investigators are using size-specific elemental analyses of Rhizaria and seasonally and vertically explicit estimates of Rhizaria biomass to model Rhizaria contributions to vertical flux. Along with data collected by the Bermuda Atlantic Time Series and Ocean Flux Program, new data on Rhizaria are being incorporated into a model of carbon and nutrient transfer into the deep ocean. The model estimates three ways that Rhizaria likely impact the carbon cycle -- by feeding on particle flux, by contributing ballast to marine snow, and by sinking directly -- and shows how these might vary across space and time. This project is jointly funded by the Biological Oceanography, Chemical Oceanography, and Established Program to Stimulate Competitive Research (EPSCoR) Programs. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.