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

Abstract

Coastal and open ocean regions of the Western Tropical South Pacific ocean have been identified as a hotspot of N 2 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 N 2 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 (10 6-10 8 nifH 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 × 10 4 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, high NOx and PO 4 3− , weak winds from the south (occasionally southwest), while Trichodesmium and UCYN-C were associated with warmer (>25 • C) waters, low NOx and PO 4 3− concentrations, strong and (mostly) easterly winds. Average N 2 fixation rates over the survey were 6.5 ± 6.7 nmol N L −1 d −1 and did not differ significantly among seasons. The year to year variability was more pronounced with average integrated rates significantly higher in 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). This dataset suggests that seasonality is less pronounced than previously thought, and that relatively high N 2 fixation rates are maintained in the New Caledonian lagoon all year long, despite seasonal changes in the diazotroph community composition.

Subtropical coastal microbiome variations due to massive river runoff after a cyclonic event

“…The lagoon of New Caledonia presents a high rate of N 2 fixation [ 123 ], and cyanobacteria are considered important diazotrophic taxa. We found some diazotrophic cyanobacteria (UCYN-A, Trichodesmium sp., Calothrix group) in accordance with other studies [ 124 , 125 ]. In addition, we identified Proteobacteria or Planctomycetes lineages that have the nifH gene, coding for the nitrogenase reductase needed to fix atmospheric nitrogen [ 126 ].…”

Microevolutionary patterns in ecotypes of the symbiotic cyanobacterium UCYN-A revealed from a Northwest Atlantic coastal time series

“…Although prior time series analyses have helped to resolve the general seasonal patterns of UCYN-A, data presented by such studies have focused on time scales that are different from what we have presented herein (generally lower resolution for a longer period or higher resolution for a shorter period). For instance, UCYN-A has been tracked via long-term monthly sampling for upward of 8 years (32,39,(47)(48)(49) and via short-term hourly and daily sampling from several days to upward of 3 months (24,50). Given the demonstrated importance of UCYN-A both globally and in the coastal NWA (52,53), we investigated UCYN-A occurrence patterns in the microbial community at a temperate multiyear time series site located on the eastern coast of North America (Bedford Basin, N.S., Canada).…”

Diazotroph-Derived Nitrogen Assimilation Strategies Differ by Scleractinian Coral Species

“…Pico-and nanophytoplankton supplies a substantial proportion of the metabolic requirements of most scleractinian corals (Goldberg, 2018;Sangmanee et al, 2020) and represents a continuous available source of food in the reefs (Tremblay et al, 2012). Among plankton, diazotrophs (dinitrogen (N 2 )fixing prokaryotes), are particularly abundant in coral lagoon waters (Turk-Kubo et al, 2015;Messer et al, 2017;Saulia et al, 2020), and heterotrophic nutrition on diazotrophs represent a potential source of N for corals (Benavides et al, 2016;Meunier et al, 2019). This plankton fixes N 2 (Tilstra et al, 2018) transform it into a bioavailable N form (NH 4 + ), and releases part of the recently fixed N (Diazotroph-Derived N, DDN) in seawater, providing available N for the development of the planktonic food web (Berthelot et al, 2016;Bonnet et al, 2016).…”