Description: Abstract: We investigated the bacterial community structure in surface waters along a 2500 km transect in the eastern Indian Ocean. Using high throughput sequencing of the 16S rRNA gene we measured a significant latitudinal increase in bacterial richness from 800 to 1400 OTUs (42% increase; r2=0.65; p〈0.001) from the tropical Timor Sea to the colder temperate waters. Total dissolved inorganic nitrogen, chl a, phytoplankton community structure and primary productivity strongly correlated with bacterial richness (all p〈0.01). Our data suggest that primary productivity drives greater bacterial richness. Because, N2-fixation accounts for up to 50% of new production in this region we tested whether higher N2-fixation rates are linked to a greater nifH diversity. The nifH diversity was dominated by heterotrophic Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria. We did not found any mechanistic links between nifH amplicon data, bacterial richness and primary productivity due to the overall low nifH evenness in this region. Scientific statement: Geographic gradients of marine microbial diversity is currently thought to be explained by two mechanisms, 1) diversity increases with increased productivity, and 2) it increases with increasing temperature. However, conclusive evidence for these mechanisms has been lacking from studies that span gradients in both, and it is unclear which organisms are responsible for the changes in diversity along these gradients. Here we present the first analysis of bacterial richness along the West Australian boundary current, the Leeuwin Current. Our analysis of bacterial richness along a latitudinal gradient in the eastern Indian Ocean shows support for the productivity mechanism rather than the temperature mechanism. Further, we show that bacterial richness increases towards the productive temperate waters are driven by productive eukaryotes (NO3- based) and heterotrophic N2-fixers.

Description: Marine microbes along with micro eukaryotes are key regulators of oceanic biogeochemical pathways. Here we present a high-resolution (every 0.5° latitude) dataset describing microbial pro- and eukaryotic diversity, in the surface and just below the thermocline, along a 7000km transect from 66° S at the Antarctic ice edge to the equator in the South Pacific Ocean. The transect, conducted in Austral winter, covered key oceanographic features including crossing of the polar front (PF), the subtropical front (STF) and the equatorial upwelling region. Our data indicate that temperature does not determine patterns of marine microbial richness, complementing the global model data from Ladau, et al. (2013). Rather, NH4⁺ nanoplankton and primary productivity were the main drivers for archaeal and bacterial richness. Eukaryote richness was highest in the least productive ocean region, the tropical oligotrophic province. We also observed a novel diversity pattern in the South Pacific Ocean; a regional increase in archaeal and bacterial diversity between 10° S and the equator. Our data showed that the mean latitudinal ranges of archaea and bacteria decreased with latitude, thereby not confirming the Rapoport's rule. We show that permanent oceanographic features, such as the STF and the equatorial upwelling can have a significant influence on pro- and eukaryotic richness.

Description: The distribution of diazotrophs and the magnitude of N2 fixation along with the input of new N through this process remains poorly constrained globally, but particularly in the Southern Pacific Ocean. Here we present a high-resolution dataset (every 0.5° latitude) describing the different N-cycling pathways which control the fixation and sequestration of carbon in the surface waters along a 7000 km transect in the South Pacific Ocean. Key oceanographic features along the P15S GO-SHIP transect from the Antarctic ice edge to the equator, included crossing of the subtropical front (STF), from the sub-Antarctic waters towards the oligotrophic tropics, and the equatorial upwelling region. We show how the natural isotopic abundance of particulate organic matter relate to different biogeochemical transformations in the N-cycle across four oceanic provinces. At all stations we measured N2 fixation rates. In the cold and nutrient rich waters of the Southern Ocean we found measurable N2 fixation rates (〉0.2 nmol L-1.d-1), which increased after the subtropical front, and remained at about 15 nmol L-1.d-1 until the equator. In the subtropical and tropical waters, the input of new nitrogen through N2 fixation could fuel on average 13±19% of the net primary productivity. Our data analysis showed that nitrifying organisms, from bacterial and archaeal genera such as Nitrospina, Nitrospinaceae, Nitrosopumilus, Nitrosopelagicus and Nitrosoarchaeum, prevailed in the Southern Ocean and within the STF. Nitrification rates ranged up to 60 nmol L-1.d-1 within the mixed layer depth in the Southern Ocean. Our results suggest that nitrification above the pycnoclines is an important component of the N cycle in the Southern Ocean. Our data has given us a better understanding of how the different N-cycling pathways relate to primary productivity in the South Pacific Ocean, as well as insights into the potential geographical N pathway shifts in light of the rapidly changing climate.

Description: The international GO-SHIP repeat global survey network is a programme designed to monitor ocean change and variability. In 2016, we followed the line P15S from the ice edge to the equator along 170°W in the South Pacific Ocean. Key oceanographic features included crossing of the subtropical front (STF), from the subantarctic waters towards the oligotrophic tropics, and the equatorial upwelling region. We analysed patterns in phytoplankton community composition as indicated by the pigment composition, carbon fixation efficiency, and assessed their relationship to temperature, salinity and dissolved oxygen, as well as the dissolved nutrients nitrate, nitrite, phosphate and silicate. For pigment analysis 4L water was filtered through 25mm Whatman GF/F filters using gentle vacuum filtration with a pressure drop of less than 10 kPa. Samples were snap frozen in liquid nitrogen and stored at -80°C prior to analysis on land. Pigments were analysed using High Performance Liquid Chromatography (HPLC) according to the CSIRO method see Hooker et al. (2012) at the CSIRO laboratories in Hobart. For chlorophyll a analysis, 0.525 L of sample water was gently filtered via vacuum filtration (pressure drop 〈 10 kPa) on 25 mm Whatman GF/F filters. Chlorophyll a extractions were carried out following acidification according to Parsons et al. (2013). Samples were measured on a Turner Trilogy laboratory fluorometer. At approximate every degree, water samples to measure C assimilation rates were taken from the clean underway flow through system (intake at 6m). Triplicate incubation bottles were inoculated with 20 µmol/L of NaH13CO3. All polycarbonate incubation bottles were acid rinsed three times, rinsed two times with deionized water, and rinsed three times with seawater directly from the sample point prior to incubation. Samples were incubated for 24 hours. C assimilation rates experiments were terminated by filtering each bottle (pressure drop 〈10 kPa) through a 25 mm precombusted GF/F filter. Natural abundance samples for particulate organic carbon, used as t-zero values, were obtained by filtering 4 L water samples onto pre-combusted GF/F filters. All filters were snap frozen in liquid N and stored at -80 °C. Back on land, the filters were acidified and dried overnight at 60 °C. Samples were analysed at the Isotopic Laboratory at UC Davis, California, with an Elementar Vario EL Cube or Micro Cube elemental analyser (Elementar Analysensysteme GmbH, Hanau, Germany), interfaced to a PDZ Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK). The external error of analyses was 0.2 per mil for d13C. C fixat assimilation ion rates (rho in nmol/L/h) were calculated following Dugdale and Goering (1967) and Knap et al. (1996).