Award: OCE-1657885

The Bermuda Atlantic Time-series Study (BATS) located near the center of the North Atlantic Subtropical Gyre has been conducting monthly oceanographic observations since October 1988 with the overarching research goal of improving our understanding of the time-varying components of the ocean carbon cycle, related biogenic elements(e.g., nitrogen, phosphorus, silica), and identifying the relevant physical, chemical and ecosystem properties responsible for this variability. Specifically, this BATS research is tasked with quantifying the role of ocean-atmosphere coupling and climate variability on air-sea exchange of CO2, and in particular carbon export to the ocean interior for which a key question is determining nutrient regulation on upper ocean phytoplankton primary production rates. Historically, field experiments and modelling have suggested nitrogen and iron are the limiting elements for primary productivity though there is growing recognition that other macro- and micronutrients such as phosphate, silicic acid, zinc, cobalt, and vitamin B12 could also constrain the growth of phytoplankton, through co-limitations and secondary limitations. For this collaborative research at the BATS site, we employed two newly developed oceanographic capabilities for advancing the investigation of nutrient-phytoplankton interactions which are: (1) the detection of protein biomarkers by targeted metaproteomics and (2) large volume in situ sampling by a new vertical autonomous underwater vehicle Clio. Targeted metaproteomics is a new environmental method developed by the Saito laboratory (Woods Hole Institute of Oceanography, WHOI) that enables proteomic techniques to detect and quantitate protein biomarkers in complex environmental samples. The new vehicle Clio (Figure 1 from Breier et al., 2020, and Figure 2), also developed at WHOI represents a fully automated and high-throughput capability for collection of multiple large volume (100-300l) in-situ particulate samples during a single 12-14 hour deployment allowing for increased sampling resolution. For this project we have collected monthly proteomic samples from the upper 200m of the ocean at the BATS site since 2015 using conventional in-situ pumps attached to a cable and winch system, and successfully conducted the scientific validation of the AUV -Clio on three BATS expeditions in 2018 to investigate seasonal variability in phytoplankton nutrient stresses. Additionally, a transect from Bermuda - Woods Hole in June of 2019 was conducted to assess basin scale spatial distributions of protein biomarkers. For each BATS cruise Clio made 3 dives down to ~ 1000m while on the transect cruise Clio conducted 9 dives, completing two major objectives of its first ocean section of 1km depth across this transect, and successfully diving to a new depth maximum of 4100m. Synthesis efforts are ongoing though results so far from the proteomic samples have already given important insights into the biogeochemical processes occurring in the North Atlantic Subtropical oligotrophic gyre. In the euphotic zone (approximately 0-100m), a layering of nutrient stressors is evident, and that the seasonal deepening of the upper ocean thermocline is represented within the nitrogen and phosphorus transporters and element sparing systems, becoming more prevalent through the summer period. In contrast, within the chlorophyll maximum (typically at the base of the euphotic zone) iron stress proteins become increasingly pronounced, which is surprising given the relatively high iron input from dust deposition in this region of the Atlantic Ocean, but indicates isolation of the chlorophyll maximum from surface waters, particularly in the summer months. In addition, transporters for organic forms of nutrients (dissolved organic nitrogen and phosphorous) are seen within all of these profiles, implying these organic sources are key to the functioning of the upper ocean ecosystem at the BATS site. Together these results clearly imply a layering of multiple nutrient stressors, rather than model results that imply single nutrient limitation. Finally, Clio demonstrated its valuable ability to conduct high resolution sampling of the chlorophyll maximum at every 5m while holding depth to less than 10cm whereas a co-occurring wire-based pump deployment varied in depth range by more than 15m (Figure 3). The resulting metaproteomes from these high-resolution Clio samples were distinct, and able to clearly show dynamic transitions in the iron, nitrogen and phosphorous stress proteins at this important biological region, which is not possible with conventional methods. This collaborative research has combined new chemical laboratory methods and novel oceanographic sampling platforms to make advances in chemical oceanography, such as, the discovery of the fine structure of adaptive phytoplankton responses to chemical gradients in the chlorophyll maxima, while leveraging the long-term BATS research efforts. The addition of these proteomic data to the BATS site, offers significant potential for specifically quantifying functional processes that regulate phytoplankton growth and ultimately upper ocean carbon export. Last Modified: 07/01/2021 Submitted by: Rodney Johnson