Seasonal Shifts in Diazotrophs Players: Patterns Observed Over a Two-Year Time Series in the New Caledonian Lagoon (Western Tropical South Pacific Ocean)

Emmrick Saulia,Kendra Turk-Kubo,Sophie Bonnet,Olivier Grosso,Haley Cooperguard,Cécile Dupouy,Lasse Riemann,Martine Rodier,Britt Henke,Mar Benavides,Anne Desnues

doi:10.3389/fmars.2020.581755

Abstract

Coastal and open ocean regions of the Western Tropical South Pacific ocean have been identified as a hotspot of N2 fixation. However, the environmental factors driving the temporal variability of abundance, composition, and activity of diazotrophs are still poorly understood, especially during the winter season. To address this, we quantified N2 fixation rates and the abundance of seven diazotroph phylotypes (UCYN-A1 symbiosis, UCYN-B, UCYN-C, Trichodesmium, Het-1, Het-2 and Het-3) on a monthly basis during two full years (2012 to 2014) at four stations along a coast to open ocean transect in the New Caledonian lagoon. The total nifH gene concentration (sum of all nifH gene copies) clearly decreased from the barrier reef to the shore. Apart from UCYN-B, which peaked at very high abundances (106-108nifH gene copies L-1) at two occasions at the coastal station, the UCYN-A1 symbiosis was the most abundant group at all stations, accounting for 79 % of the total nifH gene copy counts along the transect (average abundance 4.2 ± 10.3 x 104 nifH gene copies L-1). The next most abundant groups were in order Trichodesmium (accounting for 14% of the total nifH gene copies), Het-groups (6% of the total) and UCYN-C (1% of the total). Statistical analyses reveal that the UCYN-A1 symbiosis and Het groups were associated with cold ( 25°C) waters, low NOx and PO43- concentrations, strong and (mostly) easterly winds. Average N2 fixation rates over the survey were 6.5 ± 6.7 nmol N L-1 d-1 and did not differ significantly among seasons. The interannual variability was more pronounced with average integrated rates significantly higher the second year of the survey (162 ± 122 μmol N m-2 d-1) than the first year (66 ± 91 μmol N m-2 d-1), likely due to higher seawater temperature. This dataset suggests that seasonality is less pronounced than previously thought, and that relatively high N2 fixation rates are maintained in the New Caledonian lagoon all year long, despite seasonal changes in the diazotroph community composition.

Highlights

Biological dinitrogen (N2) fixation is a main source of fixed N to the oceans and is performed by a suite of specialized cyanobacteria, bacteria and archaea called “diazotrophs” (Zehr, 2011)
Maximum temperatures were measured between December and February, while minima were recorded between August and September, corresponding to the New Caledonian warm and cool seasons
The UCYN-A symbiosis was the most abundant group, accounting for 79% of the total nifH gene copy counts along the transect

Summary

Introduction

Biological dinitrogen (N2) fixation is a main source of fixed N to the oceans and is performed by a suite of specialized cyanobacteria, bacteria and archaea called “diazotrophs” (Zehr, 2011). Fixed N input fluxes on a global basis vary between 106–120 Tg N year−1 (Galloway et al, 2004; Gruber, 2008; Jickells et al, 2017). The accuracy of those estimates is inevitably affected by the temporal and spatial resolution of measurements. Few studies have addressed how the paucity of data on spatial and temporal distribution of N2 fixation may affect global fixed N input estimates. The temporal distribution of these data points suffers from a strong seasonal bias as >65% of the N2 fixation measurements were made during the spring-summer season. Most of our recapitulative knowledge of marine N2 fixation stems from the northern hemisphere in spring-summer conditions, largely from the sunlit layer of open ocean regions of the North Atlantic and North Pacific Oceans (Benavides and Voss, 2015; Böttjer et al, 2017)

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