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1. Aphotic N₂ fixation along an oligotrophic to ultraoligotrophic transect in the western tropical South Pacific Ocean

   https://doi.org/10.5194/bg-15-3107-2018  

   Benavides, Mar ; Shoemaker, Katyanne M. ; Moisander, Pia H. ; Niggemann, Jutta ; Dittmar, Thorsten ; Duhamel, Solange ; Grosso, Olivier ; Pujo-Pay, Mireille ; Hélias-Nunige, Sandra ; Fumenia, Alain ; et al ( January 2018 , Biogeosciences)

   Abstract. The western tropical South Pacific (WTSP) Ocean has been recognized as a global hot spot of dinitrogen (N₂) fixation. Here, as in other marine environments across the oceans, N₂ fixation studies have focused on the sunlit layer. However, studies have confirmed the importance of aphotic N₂ fixation activity, although until now only one had been performed in the WTSP. In order to increase our knowledge of aphotic N₂ fixation in the WTSP, we measured N₂ fixation rates and identified diazotrophic phylotypes in the mesopelagic layer along a transect spanning from New Caledonia to French Polynesia. Because non-cyanobacterial diazotrophs presumably need external dissolved organic matter (DOM) sources for their nutrition, we also identified DOM compounds using Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) with the aim of searching for relationships between the composition of DOM and non-cyanobacterial N₂ fixation in the aphotic ocean. N₂ fixation rates were low (average 0.63±0.07nmolNL⁻¹d⁻¹) but consistently detected across all depths and stations, representing ∼ 6–88% of photic N₂ fixation. N₂ fixation rates were not significantly correlated with DOM compounds. The analysis of nifH gene amplicons revealed a wide diversity of non-cyanobacterial diazotrophs, mostly matching clusters 1 and 3. Interestingly, a distinct phylotype from the major nifH subclustermore »1G dominated at 650dbar, coinciding with the oxygenated Subantarctic Mode Water (SAMW). This consistent pattern suggests that the distribution of aphotic diazotroph communities is to some extent controlled by water mass structure. While the data available are still too scarce to elucidate the distribution and controls of mesopelagic non-cyanobacterial diazotrophs in the WTSP, their prevalence in the mesopelagic layer and the consistent detection of active N₂ fixation activity at all depths sampled during our study suggest that aphotic N₂ fixation may contribute significantly to fixed nitrogen inputs in this area and/or areas downstream of water mass circulation.

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2. Modeling the physiology of the aquatic temnospondyl <i>Archegosaurusdecheni</i> from the early Permian of Germany

   https://doi.org/10.5194/fr-20-105-2017  

   Witzmann, Florian ; Brainerd, Elizabeth ( January 2017 , Fossil Record)

   Abstract. Physiological aspects like heat balance, gas exchange, osmoregulation, and digestion of the early Permian aquatic temnospondyl Archegosaurus decheni, which lived in a tropical freshwater lake, are assessed based on osteological correlates of physiologically relevant soft-tissue organs and by physiological estimations analogous to air-breathing fishes. Body mass (M) of an adult Archegosaurus with an overall body length of more than 1m is estimated as 7kg using graphic double integration. Standard metabolic rate (SMR) at 20°C (12kJh⁻¹) and active metabolic rate (AMR) at 25°C (47kJh⁻¹) were estimated according to the interspecific allometry of metabolic rate (measured as oxygen consumption) of all fish (V_O2 = 4. 8M^0. 88) and form the basis for most of the subsequent estimations. Archegosaurus is interpreted as a facultative air breather that got O₂ from the internal gills at rest in well-aerated water but relied on its lungs for O₂ uptake in times of activity and hypoxia. The bulk of CO₂ was always eliminated via the gills. Our estimations suggest that if Archegosaurus did not have gills and released 100% CO₂ from its lungs, it would have to breathe much more frequently to release enough CO₂ relative to the lung ventilation required formore »just O₂ uptake. Estimations of absorption and assimilation in the digestive tract of Archegosaurus suggest that an adult had to eat about six middle-sized specimens of the acanthodian fish Acanthodes (ca. 8cm body length) per day to meet its energy demands. Archegosaurus is regarded as an ammonotelic animal that excreted ammonia (NH₃) directly to the water through the gills and the skin, and these diffusional routes dominated nitrogen excretion by the kidneys as urine. Osmotic influx of water through the gills had to be compensated for by production of dilute, hypoosmotic urine by the kidneys. Whereas Archegosaurus has long been regarded as a salamander-like animal, there is evidence that its physiology was more fish- than tetrapod-like in many respects.

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3. Diazotrophic <i>Trichodesmium</i> impact on UV–Vis radiance and pigment composition in the western tropical South Pacific

   https://doi.org/10.5194/bg-15-5249-2018  

   Dupouy, Cécile ; Frouin, Robert ; Tedetti, Marc ; Maillard, Morgane ; Rodier, Martine ; Lombard, Fabien ; Guidi, Lionel ; Picheral, Marc ; Neveux, Jacques ; Duhamel, Solange ; et al ( January 2018 , Biogeosciences)

   Abstract. We assessed the influence of the marine diazotrophic cyanobacterium Trichodesmium on the bio-optical properties of western tropical South Pacific (WTSP) waters (18–22°S, 160°E–160°W) during the February–March 2015 OUTPACE cruise. We performed measurements of backscattering and absorption coefficients, irradiance, and radiance in the euphotic zone with a Satlantic MicroPro free-fall profiler and took Underwater Vision Profiler 5 (UPV5) pictures for counting the largest Trichodesmium spp. colonies. Pigment concentrations were determined by fluorimetry and high-performance liquid chromatography and picoplankton abundance by flow cytometry. Trichome concentration was estimated from pigment algorithms and validated by surface visual counts. The abundance of large colonies counted by the UVP5 (maximum 7093coloniesm⁻³) was well correlated to the trichome concentrations (maximum 2093trichomesL⁻¹) with an aggregation factor of 600. In the Melanesian archipelago, a maximum of 4715trichomesL⁻¹ was enumerated in pump samples (3.2m) at 20°S,16730°E. High Trichodesmium abundance was always associated with absorption peaks of mycosporine-like amino acids (330, 360nm) and high particulate backscattering, but not with high Chl a fluorescence or blue particulate absorption (440nm). Along the west-to-east transect, Trichodesmium together with Prochlorococcus represented the major part of total chlorophyll concentration; themore »contribution of other groups were relatively small or negligible. The Trichodesmium contribution to total chlorophyll concentration was the highest in the Melanesian archipelago around New Caledonia and Vanuatu (60%), progressively decreased to the vicinity of the islands of Fiji (30%), and reached a minimum in the South Pacific Gyre where Prochlorococcus dominated chlorophyll concentration. The contribution of Trichodesmium to zeaxanthin was respectively 50, 40 and 20% for these regions. During the OUTPACE cruise, the relationship between normalized water-leaving radiance (nL_w) in the ultraviolet and visible and chlorophyll concentration was similar to that found during the BIOSOPE cruise in the eastern tropical Pacific. Principal component analysis (PCA) of OUTPACE data showed that nL_w at 305, 325, 340, 380, 412 and 440nm was strongly correlated to chlorophyll and zeaxanthin, while nL_w at 490 and 565nm exhibited lower correlations. These results, as well as differences in the PCA of BIOSOPE data, indicated that nL_w variability in the greenish blue and yellowish green during OUTPACE was influenced by other variables associated with Trichodesmium presence, such as backscattering coefficient, phycoerythrin fluorescence and/or zeaxanthin absorption, suggesting that Trichodesmium detection should involve examination of nL_w in this spectral domain.

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4. Colony formation in <i>Phaeocystis antarctica</i>: connecting molecular mechanisms with iron biogeochemistry

   https://doi.org/10.5194/bg-15-4923-2018  

   Bender, Sara J. ; Moran, Dawn M. ; McIlvin, Matthew R. ; Zheng, Hong ; McCrow, John P. ; Badger, Jonathan ; DiTullio, Giacomo R. ; Allen, Andrew E. ; Saito, Mak A. ( January 2018 , Biogeosciences)

   Abstract. Phaeocystis antarctica is an important phytoplankter of the Ross Sea where it dominates the early season bloom after sea ice retreat and is a major contributor to carbon export. The factors that influence Phaeocystis colony formation and the resultant Ross Sea bloom initiation have been of great scientific interest, yet there is little known about the underlying mechanisms responsible for these phenomena. Here, we present laboratory and field studies on Phaeocystis antarctica grown under multiple iron conditions using a coupled proteomic and transcriptomic approach. P. antarctica had a lower iron limitation threshold than a Ross Sea diatom Chaetoceros sp., and at increased iron nutrition (&gt;120pM Fe') a shift from flagellate cells to a majority of colonial cells in P. antarctica was observed, implying a role for iron as a trigger for colony formation. Proteome analysis revealed an extensive and coordinated shift in proteome structure linked to iron availability and life cycle transitions with 327 and 436 proteins measured as significantly different between low and high iron in strains 1871 and 1374, respectively. The enzymes flavodoxin and plastocyanin that can functionally replace iron metalloenzymes were observed at low iron treatments consistent with cellular iron-sparing strategies, with plastocyanin havingmore »a larger dynamic range. The numerous isoforms of the putative iron-starvation-induced protein (ISIP) group (ISIP2A and ISIP3) had abundance patterns coinciding with that of either low or high iron (and coincident flagellate or the colonial cell types in strain 1871), implying that there may be specific iron acquisition systems for each life cycle type. The proteome analysis also revealed numerous structural proteins associated with each cell type: within flagellate cells actin and tubulin from flagella and haptonema structures as well as a suite of calcium-binding proteins with EF domains were observed. In the colony-dominated samples a variety of structural proteins were observed that are also often found in multicellular organisms including spondins, lectins, fibrillins, and glycoproteins with von Willebrand domains. A large number of proteins of unknown function were identified that became abundant at either high or low iron availability. These results were compared to the first metaproteomic analysis of a Ross Sea Phaeocystis bloom to connect the mechanistic information to the in situ ecology and biogeochemistry. Proteins associated with both flagellate and colonial cells were observed in the bloom sample consistent with the need for both cell types within a growing bloom. Bacterial iron storage and B₁₂ biosynthesis proteins were also observed consistent with chemical synergies within the colony microbiome to cope with the biogeochemical conditions. Together these responses reveal a complex, highly coordinated effort by P. antarctica to regulate its phenotype at the molecular level in response to iron and provide a window into the biology, ecology, and biogeochemistry of this group.

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5. Physiological and biochemical responses of <i>Emiliania huxleyi</i> to ocean acidification and warming are modulated by UV radiation

   https://doi.org/10.5194/bg-16-561-2019  

   Tong, Shanying ; Hutchins, David A. ; Gao, Kunshan ( January 2019 , Biogeosciences)

   Abstract. Marine phytoplankton such as bloom-forming, calcite-producingcoccolithophores, are naturally exposed to solar ultraviolet radiation (UVR,280–400nm) in the ocean's upper mixed layers. Nevertheless, the effects ofincreasing carbon dioxide (CO₂)-induced ocean acidification and warming have rarelybeen investigated in the presence of UVR. We examined calcification andphotosynthetic carbon fixation performance in the most cosmopolitancoccolithophorid, Emiliania huxleyi, grown under high(1000µatm, HC; pH_T: 7.70) and low (400µatm,LC; pH_T: 8.02) CO₂ levels, at 15^∘C,20^∘C and 24^∘C with or without UVR. The HCtreatment did not affect photosynthetic carbon fixation at 15^∘C,but significantly enhanced it with increasing temperature. Exposure to UVRinhibited photosynthesis, with higher inhibition by UVA (320–395nm) thanUVB (295–320nm), except in the HC and 24^∘C-grown cells, in whichUVB caused more inhibition than UVA. A reduced thickness of the coccolith layerin the HC-grown cells appeared to be responsible for the UV-inducedinhibition, and an increased repair rate of UVA-derived damage in theHC–high-temperature grown cells could be responsible for lowered UVA-induced inhibition.While calcification was reduced with elevated CO₂ concentration,exposure to UVB or UVA affected the process differentially, with the formerinhibiting it and the latter enhancing it. UVA-induced stimulationmore »of calcification washigher in the HC-grown cells at 15 and 20^∘C, whereas at24^∘C observed enhancement was not significant. The calcificationto photosynthesis ratio (Cal∕Pho ratio) was lower in the HC treatment,and increasing temperature also lowered the value. However, at 20 and24^∘C, exposure to UVR significantly increased the Cal∕Phoratio, especially in HC-grown cells, by up to 100%. This implies thatUVR can counteract the negative effects of the “greenhouse” treatment onthe Cal∕Pho ratio; hence, UVR may be a key stressor when considering theimpacts of future greenhouse conditions on E. huxleyi.

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