Phytoplankton Responses to Bacterially Regenerated Iron in a Southern Ocean Eddy.

In the Subantarctic sector of the Southern Ocean, vertical entrainment of iron (Fe) triggers the seasonal productivity cycle but diminishing physical supply during the spring to summer transition forces microbial assemblages to rapidly acclimate. Here, we tested how phytoplankton and bacteria within an isolated eddy respond to different dissolved Fe (DFe)/ligand inputs. We used three treatments: one that mimicked the entrainment of new DFe (Fe-NEW), another in which DFe was supplied from bacterial regeneration of particles (Fe-REG), and a control with no addition of DFe (Fe-NO). After 6 days, 3.5 (Fe-NO, Fe-NEW) to 5-fold (Fe-REG) increases in Chlorophyll a were observed. These responses of the phytoplankton community were best explained by the differences between the treatments in the amount of DFe recycled during the incubation (Fe-REG, 15% recycled c.f. 40% Fe-NEW, 60% Fe-NO). This additional recycling was more likely mediated by bacteria. By day 6, bacterial production was comparable between Fe-NO and Fe-NEW but was approximately two-fold higher in Fe-REG. A preferential response of phytoplankton (haptophyte-dominated) relative to high nucleic acid (HNA) bacteria was also found in the Fe-REG treatment while the relative proportion of diatoms increased faster in the Fe-NEW and Fe-NO treatments. Comparisons between light and dark incubations further confirmed the competition between picophytoplankton and HNA for DFe. Overall, our results demonstrate great versatility by microorganisms to use different Fe sources that results in highly efficient Fe recycling within surface waters. This study also encourages future research to further investigate the interactions between functional groups of microbes (e.g. HNA and cyanobacteria) to better constraint modeling in Fe and carbon biogeochemical cycles.

Biogeography of Southern Ocean prokaryotes: a comparison of the Indian and Pacific sectors.

We investigated the Southern Ocean (SO) prokaryote community structure via zero-radius operational taxonomic unit (zOTU) libraries generated from 16S rRNA gene sequencing of 223 full water column profiles. Samples reveal the prokaryote diversity trend between discrete water masses across multiple depths and latitudes in Indian (71-99°E, summer) and Pacific (170-174°W, autumn-winter) sectors of the SO. At higher taxonomic levels (phylum-family) we observed water masses to harbour distinct communities across both sectors, but observed sectorial variations at lower taxonomic levels (genus-zOTU) and relative abundance shifts for key taxa such as Flavobacteria, SAR324/Marinimicrobia, Nitrosopumilus and Nitrosopelagicus at both epi- and bathy-abyssopelagic water masses. Common surface bacteria were abundant in several deep-water masses and vice-versa suggesting connectivity between surface and deep-water microbial assemblages. Bacteria from same-sector Antarctic Bottom Water samples showed patchy, high beta-diversity which did not correlate well with measured environmental parameters or geographical distance. Unconventional depth distribution patterns were observed for key archaeal groups: Crenarchaeota was found across all depths in the water column and persistent high relative abundances of common epipelagic archaeon Nitrosopelagicus was observed in deep-water masses. Our findings reveal substantial regional variability of SO prokaryote assemblages that we argue should be considered in wide-scale SO ecosystem microbial modelling.

Coccolith volume of the Southern Ocean coccolithophore Emiliania huxleyi as a possible indicator for palaeo-cell volume.

Coccolithophores are a key functional phytoplankton group and produce minute calcite plates (coccoliths) in the sunlit layer of the pelagic ocean. Coccoliths significantly contribute to the sediment record since the Triassic and their geometry have been subject to palaeoceanographic and biological studies to retrieve information on past environmental conditions. Here, we present a comprehensive analysis of coccolith, coccosphere and cell volume data of the Southern Ocean Emiliania huxleyi ecotype A, subject to gradients of temperature, irradiance, carbonate chemistry and macronutrient limitation. All tested environmental drivers significantly affect coccosphere, coccolith and cell volume with driver-specific sensitivities. However, a highly significant correlation emerged between cell and coccolith volume with Vcoccolith  = 0.012 ± 0.001 * Vcell  + 0.234 ± 0.066 (n = 23, r2  = .85, p < .0001, σest  = 0.127), indicating a primary control of coccolith volume by physiological modulated changes in cell volume. We discuss the possible application of fossil coccolith volume as an indicator for cell volume/size and growth rate and, additionally, illustrate that macronutrient limitation of phosphorus and nitrogen has the predominant influence on coccolith volume in respect to other environmental drivers. Our results provide a solid basis for the application of coccolith volume and geometry as a palaeo-proxy and shed light on the underlying physiological reasons, offering a valuable tool to investigate the fossil record of the coccolithophore E. huxleyi.

Oceanographic Fronts Shape Phaeocystis Assemblages: A High-Resolution 18S rRNA Gene Survey From the Ice-Edge to the Equator of the South Pacific.

The cosmopolitan haptophyte Phaeocystis is recognized as a key contributor to marine biogeochemical cycling and important primary producer within polar marine environments. Yet, little is known about its solitary, non-colonial cell stages or its distribution during the colder, low-productivity seasons. We examined the biogeography of Phaeocystis along a high-resolution (0.5-degree latitudinal interval) transect from the Antarctic ice-edge to the equator of the South Pacific, in the austral autumn-winter. Using high-throughput 18S rRNA gene sequences with single nucleotide variable (zero-radius) operational taxonomic units (zOTUs) allowed us to explore the possibility of strain-level variation. From water samples within the upper water column, we show the presence of an abundant Phaeocystis assemblage that persisted during the colder months, contributing up to 9% of the microbial eukaryote community at high latitudes. The biogeography of Phaeocystis was strongly shaped by oceanographic boundaries, most prominently the polar and subantarctic fronts. Marked changes in dominant Phaeocystis antarctica zOTUs between different frontal zones support the concept that ecotypes may exist within the Phaeocystis assemblage. Our findings also show that the Phaeocystis assemblage did not abide by the classical latitudinal diversity gradient of increasing richness from the poles to the tropics; richness peaked at 30°S and declined to a minimum at 5°S. Another surprise was that P. globosa and P. cordata, previously thought to be restricted to the northern hemisphere, were detected at moderate abundances within the Southern Ocean. Our results emphasize the importance of oceanographic processes in shaping the biogeography of Phaeocystis and highlights the importance of genomics-based exploration of Phaeocystis, which have found the assemblage to be more complex than previously understood. The high winter relative abundance of the Phaeocystis assemblage suggests it could be involved in more complex ecological interactions during the less productive seasons, which should be considered in future studies to better understand the ecological role and strategies of this keystone species.

Distinct iron cycling in a Southern Ocean eddy.

Mesoscale eddies are ubiquitous in the iron-limited Southern Ocean, controlling ocean-atmosphere exchange processes, however their influence on phytoplankton productivity remains unknown. Here we probed the biogeochemical cycling of iron (Fe) in a cold-core eddy. In-eddy surface dissolved Fe (dFe) concentrations and phytoplankton productivity were exceedingly low relative to external waters. In-eddy phytoplankton Fe-to-carbon uptake ratios were elevated 2-6 fold, indicating upregulated intracellular Fe acquisition resulting in a dFe residence time of ~1 day. Heavy dFe isotope values were measured for in-eddy surface waters highlighting extensive trafficking of dFe by cells. Below the euphotic zone, dFe isotope values were lighter and coincident with peaks in recycled nutrients and cell abundance, indicating enhanced microbially-mediated Fe recycling. Our measurements show that the isolated nature of Southern Ocean eddies can produce distinctly different Fe biogeochemistry compared to surrounding waters with cells upregulating iron uptake and using recycling processes to sustain themselves.

Surface ocean carbon dioxide variability in South Pacific boundary currents and Subantarctic waters.

To improve estimates of the long-term response of the marine carbon system to climate change a better understanding of the seasonal and interannual variability is needed. We use high-frequency multi-year data at three locations identified as climate change hotspots: two sites located close to South Pacific boundary currents and one in the Subantarctic Zone (SAZ). We investigate and identify the main drivers involved in the seasonal an interannual (2012-2016) variability of the carbon system. The seasonal variability at boundary current sites is temporally different and highly controlled by sea surface temperature. Advection processes also play a significant role on the monthly changes of the carbon system at the western boundary current site. The interannual variability at these sites most likely responds to long-term variability in oceanic circulation ultimately related to climatic indices such as the El Niño Southern Oscillation, the Pacific Decadal Oscillation and the Southern Annular Mode (SAM). In the SAZ, advection and entrainment processes drive most of the seasonality, augmented by the action of biological processes in spring. Given the relevance of advection and entrainment processes at SAZ, the interannual variability is most probably modulated by changes in the regional winds linked to the variability of the SAM.

Ambient nitrate switches the ammonium consumption pathway in the euphotic ocean.

Phytoplankton assimilation and microbial oxidation of ammonium are two critical conversion pathways in the marine nitrogen cycle. The underlying regulatory mechanisms of these two competing processes remain unclear. Here we show that ambient nitrate acts as a key variable to bifurcate ammonium flow through assimilation or oxidation, and the depth of the nitracline represents a robust spatial boundary between ammonium assimilators and oxidizers in the stratified ocean. Profiles of ammonium utilization show that phytoplankton assemblages in nitrate-depleted regimes have higher ammonium affinity than nitrifiers. In nitrate replete conditions, by contrast, phytoplankton reduce their ammonium reliance and thus enhance the success of nitrifiers. This finding helps to explain existing discrepancies in the understanding of light inhibition of surface nitrification in the global ocean, and provides further insights into the spatial linkages between oceanic nitrification and new production.

Independence of nutrient limitation and carbon dioxide impacts on the Southern Ocean coccolithophore Emiliania huxleyi.

Future oceanic conditions induced by anthropogenic greenhouse gas emissions include warming, acidification and reduced nutrient supply due to increased stratification. Some parts of the Southern Ocean are expected to show rapid changes, especially for carbonate mineral saturation. Here we compare the physiological response of the model coccolithophore Emiliania huxleyi (strain EHSO 5.14, originating from 50oS, 149oE) with pH/CO2 gradients (mimicking ocean acidification ranging from 1 to 4 × current pCO2 levels) under nutrient-limited (nitrogen and phosphorus) and -replete conditions. Both nutrient limitations decreased per cell photosynthesis (particulate organic carbon (POC) production) and calcification (particulate inorganic carbon (PIC) production) rates for all pCO2 levels, with more than 50% reductions under nitrogen limitation. These impacts, however, became indistinguishable from nutrient-replete conditions when normalized to cell volume. Calcification decreased three-fold and linearly with increasing pCO2 under all nutrient conditions, and was accompanied by a smaller ~30% nonlinear reduction in POC production, manifested mainly above 3 × current pCO2. Our results suggest that normalization to cell volume allows the major impacts of nutrient limitation (changed cell sizes and reduced PIC and POC production rates) to be treated independently of the major impacts of increasing pCO2 and, additionally, stresses the importance of including cell volume measurements to the toolbox of standard physiological analysis of coccolithophores in field and laboratory studies.

A database of marine phytoplankton abundance, biomass and species composition in Australian waters.

There have been many individual phytoplankton datasets collected across Australia since the mid 1900s, but most are unavailable to the research community. We have searched archives, contacted researchers, and scanned the primary and grey literature to collate 3,621,847 records of marine phytoplankton species from Australian waters from 1844 to the present. Many of these are small datasets collected for local questions, but combined they provide over 170 years of data on phytoplankton communities in Australian waters. Units and taxonomy have been standardised, obviously erroneous data removed, and all metadata included. We have lodged this dataset with the Australian Ocean Data Network (http://portal.aodn.org.au/) allowing public access. The Australian Phytoplankton Database will be invaluable for global change studies, as it allows analysis of ecological indicators of climate change and eutrophication (e.g., changes in distribution; diatom:dinoflagellate ratios). In addition, the standardised conversion of abundance records to biomass provides modellers with quantifiable data to initialise and validate ecosystem models of lower marine trophic levels.

Revisiting carbon flux through the ocean's twilight zone.

The oceanic biological pump drives sequestration of carbon dioxide in the deep sea via sinking particles. Rapid biological consumption and remineralization of carbon in the "twilight zone" (depths between the euphotic zone and 1000 meters) reduce the efficiency of sequestration. By using neutrally buoyant sediment traps to sample this chronically understudied realm, we measured a transfer efficiency of sinking particulate organic carbon between 150 and 500 meters of 20 and 50% at two contrasting sites. This large variability in transfer efficiency is poorly represented in biogeochemical models. If applied globally, this is equivalent to a difference in carbon sequestration of more than 3 petagrams of carbon per year.