Data_Sheet_1_Seasonal Shifts in Diazotrophs Players: Patterns Observed Over a Two-Year Time Series in the New Caledonian Lagoon (Western Tropical South Pacific Ocean).docx

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 × 104nifH 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 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 N2 fixation rates are maintained in the New Caledonian lagoon all year long, despite seasonal changes in the diazotroph community composition.

西太平洋热带南部的沿海及开阔海域已被认定为固氮作用的热点区域。然而，驱动固氮微生物数量、组成及活性随时间变化的生态环境因素仍不甚明了，尤其是在冬季。为此，本研究在2012年至2014年间，对位于新喀里多尼亚潟湖沿海至开阔海域的四个站点，按月度量化了七种固氮微生物类群（包括UCYN-A1共生体、UCYN-B、UCYN-C、鞘氨醇菌、Het-1、Het-2和Het-3）的固氮速率和丰度。结果显示，从珊瑚礁至海岸，总nifH基因浓度（所有nifH基因复制的总和）明显降低。除了UCYN-B在沿海站点两次出现丰度极高的现象（nifH基因拷贝数达到106-108拷贝/L），UCYN-A1共生体在所有站点均为最丰富的类群，占总nifH基因拷贝数的79%（平均丰度为4.2 ± 10.3 × 104拷贝/L）。其次是鞘氨醇菌（占总nifH基因拷贝数的14%）、Het类群（6%）和UCYN-C（占总nifH基因拷贝数的1%）。统计分析表明，UCYN-A1共生体和Het类群与低温（25°C）、低NOx和PO43–浓度、强烈且（主要是）东风相关。调查期间的平均固氮速率为6.5 ± 6.7 nmol N L–1 d–1，且在季节间差异不显著。年际间的变异性更为明显，调查第二年的平均综合固氮速率显著高于第一年（分别为162 ± 122 μmol N m–2 d–1和66 ± 91 μmol N m–2 d–1）。该数据集表明，季节性影响较先前所认为的要弱，且在新喀里多尼亚潟湖全年均维持相对较高的固氮速率，尽管固氮微生物类群的组成随季节变化。