We applied a multilocus phylogenetic approach to elucidate the origin of serradella and lupin Bradyrhizobium strains that persist in soils of Western Australia and South Africa. The selected strains belonged to different randomly amplified polymorphic DNA (RAPD)-PCR clusters that were distinct from RAPD clusters of applied inoculant strains. Phylogenetic analyses were performed with nodulation genes (nodA, nodZ, nolL, noeI), housekeeping genes (dnaK, recA, glnII, atpD), and 16S-23S rRNA intergenic transcribed spacer sequences. Housekeeping gene phylogenies revealed that all serradella and Lupinus cosentinii isolates from Western Australia and three of five South African narrow-leaf lupin strains were intermingled with the strains of Bradyrhizobium canariense, forming a well supported branch on each of the trees. All nodA gene sequences of the lupin and serradella bradyrhizobia formed a single branch, referred to as clade II, together with the sequences of other lupin and serradella strains. Similar patterns were detected in nodZ and nolL trees. In contrast, nodA sequences of the strains isolated from native Australian legumes formed either a new branch called clade IV or belonged to clade I or III, whereas their nonsymbiotic genes grouped outside the B. canariense branch. These data suggest that the lupin and serradella strains, including the strains from uncultivated L. cosentinii plants, are descendants of strains that most likely were brought from Europe accidentally with lupin and serradella seeds. The observed dominance of B. canariense strains may be related to this species' adaptation to acid soils common in Western Australia and South Africa and, presumably, to their intrinsic ability to compete for nodulation of lupins and serradella.

Nitrogen (N) availability affects phytoplankton photosynthetic performance and regulates marine primary production (MPP) across the global coast and oceans. Bio‐optical tools including Fast Repetition Rate fluorometry (FRRf) are particularly well suited to examine MPP variability in coastal regions subjected to dynamic spatio‐temporal fluctuations in nutrient availability. FRRf determines photosynthesis as an electron transport rate through Photosystem II (ETRPSII), requiring knowledge of an additional parameter, the electron requirement for carbon fixation (KC), to retrieve rates of CO2‐fixation. KC strongly depends upon environmental conditions regulating photosynthesis, yet the importance of N‐availability to this parameter has not been examined. Here, we use nutrient bioassays to isolate how N (relative to other macronutrients P, Si) regulates KC of phytoplankton communities from the Australian coast during summer, when N‐availability is often highly variable. KC consistently responded to N‐amendment, exhibiting up to a threefold reduction and hence an apparent increase in the efficiency with which electrons were used to drive C‐fixation. However, the process driving this consistent reduction was dependent upon initial conditions. When diatoms dominated assemblages and N was undetectable (e.g., post bloom), KC decreased predominantly via a physiological adjustment of the existing community to N‐amendment. Conversely, for mixed assemblages, N‐addition achieved a similar reduction in KC through a change in community structure toward diatom domination. We generate new understanding and parameterization of KC that is particularly critical to advance how FRRf can be applied to examine C‐uptake throughout the global ocean where nitrogen availability is highly variable and thus frequently limits primary productivity.

No abstract

The contribution of planktonic diazotrophs to the overall N budget is a key unknown in the eastern Indian Ocean. Here we investigated the relationships between dissolved inorganic nutrients, phytoplankton pigment composition, microbial community structure, nitrogen fixation rates and the δ 15 N of fractionated zooplankton samples along the shelf break of Western Australia (32° to 12°S) in September 2012. Bulk nitrogen fixation rates declined from 4.8 nmol l −1 h −1 in the colder and more saline sub-tropical waters at higher latitudes to 1.5 nmol l −1 h −1 in the warmer and fresher Timor Sea at lower latitudes. A regional bloom of Trichodesmium was identified between 13° and 9°S in the Timor Sea. Trichodesmium-specific N 2 fixation rates were 0.05 ± 0.01 nmol colony −1 h −1. Highest dissolved inorganic nitrogen (DIN) concentrations occurred at the highest NH 4 + :NO 3 − ratios, thereby deviating from the paradigm that greater DIN concentrations come primarily from increased NO 3 − through advection, mixing or upwelling. Both the microplankton and nanoflagellate fraction declined significantly in warmer waters, with higher DIN concentrations but decreasing % NO 3 − . A clear increase in the prokaryotic diagnostic pigment zeaxanthin was seen with increasing temperatures from higher to lower latitudes. The microbial community, measured using automated ribosomal intergenic spacer analysis (ARISA), clustered strongly according to the water mass biogeochemistry including temperature, salinity, DIN and phosphate concentrations (p < 0.001). Isotope analysis suggested that injections of low δ 15 N from N 2 fixation lowered the zooplankton δ 15 N signature of animals up to ~500 µm in size and that nearly 47% of the fixed nitrogen was used by zooplankton (≤500 µm fraction) in the Timor Sea.

How the functional traits (FTs) of phytoplankton change with temperature is important for understanding the impacts of ocean warming on phytoplankton mediated biogeochemical fluxes. This study quantifies the thermal performance curves (TPCs) of FTs in the cosmopolitan model diatom, Thalassiosira pseudonana, to advance understanding of trade-offs between physiological (photoacclimation, carbon fixation, nitrate, phosphate, and silicate uptake) and morphological traits (cell volume and frustule silicification). We show that each FT has substantial phenotypic plasticity and exhibits a unique TPC, varying in both shape and thermal optimum, and diverging from the growth response. The TPC for growth was symmetric with a thermal optimum (T opt ) of 18 • C. In comparison, the TPC for primary productivity was warm-skewed with a T opt around 21 • C, whereas frustule silicification decreased linearly with increasing temperature. Together, this suggests that the optimal temperature for overall fitness is a balance of trade-offs in the underlying functional traits. Moreover, these results demonstrate that growth is not necessarily an accurate estimate of overall biogeochemical performance and that temperature change will likely influence elemental fluxes such as carbon and silicon. Finally, we show that temperature-driven changes in individual traits e.g., photoacclimation, can mimic responses experienced under other environmental stressors (high light) and so a multi-trait assessment is essential for accurate interpretation of the cellular impact of warming. This study also reveals that multi-trait analysis, in the context of TPCs, provides insight into the cellular physiology regulating the whole cell response and has the potential to provide better estimates of how diatom-mediated biogeochemical fluxes are likely to be impacted in the context of ocean warming. Analyzing the response of multiple traits more comprehensively over other environmental gradients may therefore provide a useful framework to advance understanding of how taxon-specific functional traits will respond to multifaceted ocean change.