Table 2.
Changes in NCD abundances, activity, and putative N2 fixation in response to nutrient perturbations provide insights into their environmental drivers. NPSG—North Pacific Subtropical Gyre; WSP—Western South Pacific; ETSP—Eastern Tropical South Pacific; SCS—South China Sea; MedSea—Mediterranean Sea; RT-qPCR—reverse transcription qPCR; N—nitrogen; Fe—iron; P—phosphate; DOC—dissolved organic C; DON—dissolved organic N; DOP—dissolved organic P; DCMU—photosynthesis inhibitor (3-(3,4-dichlorophenyl)-1,1-dimethylurea); and GX—xanthan gum.
| NCD(s)a | Region (depth) | Type of analysesc | Environmental perturbation(s) | Synthesis of findings | Reference |
|---|---|---|---|---|---|
| Gamma A (AO15) | NPSG (0 – 25 m) | RT-qPCR; N2 fixation | P | No significant stimulation of N2 fixation or nifH transcripts | Zehr et al. (2007) |
| Cluster-3 | NPSG (40 m) | qPCR | DOC | Cluster-3 increased in +DOC | Bombar et al. (2013) |
| Gamma A (γ24774A11) | WSP (5 m) | qPCR | N, P, Fe, DOC, Fe/P, N/P, N/Fe, N/P/Fe/DOC | Abundances increased in response to Fe and Fe/P in westernmost stations; decreased in +N | Moisander et al. (2012) |
| Gamma A (γ24774A11) | WSP (3 m) | qPCR & RT-qPCR; N2 fixation | DOC, DON, DOP, inhibition of photosynthesis using DCMU | No nifH expression across all treatment implying they were not actively fixing N2; DCMU additions suppressed nearly all N2 fixation implying that the most active N2-fixers were phototrophs | Benavides et al. (2018b) |
| P1, P4, P7 | ETSP (95 m) | qPCR, N2 fixation | Glucose, O2 | N2 fixation rates increased in +glucose, +O2; P7 abundances increased in +glucose, +O2 | Löscher et al. (2014) |
| Unknownb | ESTP (15 m) | N2 fixation rates | Fe, N or N/Fe, P, glucose | N2 fixation rates increased in +Fe and +glucose and occasionally in +N/Fe | Dekaezemacker et al. (2013) |
| P2, P4, P6, P7 | ESTP (0 m) | nifH transcript sequencing & RT-qPCR; N2 fixation | Glucose | N2 fixation stimulated by +glucose in eddy cores; nifH transcription from P2, P4, P6, P7 in eddy samples (not measured in experiments) | Löscher et al. (2016) |
| Unknownb | WNA (coastal) | N2 fixation (ARA) | DOC, organic detritus, light and dark incubations | Water column N2 fixation stimulated in +DOC, + organic detritus treatments | Paerl and Prufert (1987) |
| Gamma A, gamma P, Clll-Church | ENA (1 – 3 m) | qPCR, N2 fixation | N, P, Fe, dust | Gamma A abundances increased most in +NFe and +dust treatments; Gamma P and CIII were undetected or not quantifiable | Langlois et al. (2012) |
| Mainly 1G | MedSea (5 m) | nifH sequencing; N2 fixation | Dust under contemporary and future temp. and pH scenarios | Increased N2 fixation rates in response to dust additions in stations dominated by NCDs (cyanobacteria also present) | Ridame et al. (2022) |
| Diverse cluster I and III NCDs | MedSea (5 m) | nifH transcript sequencing; N2 fixation | GX, N, P, DOC, NP, DOC/P, DOC/N, DOC/N/P; light and dark incubations | N2 fixation stimulated by DOC in both light and dark incubations; increased relative abundances of NCD nifH transcripts in +DOC/N/P; increased N2 fixation and NCD transcript relative abundances in +GX (cyanobacteria also present) | Rahav et al. (2016) |
| M6411A02, M6413A02, M6433A04 | WSP (aphotic) | qPCR (environmental samples, not nutrient exp.); N2 fixation | DOC, DON | N2 fixation stimulated by amino acid (+DON) additions; assumed to be NCDs, but recently Trichodesmium has been shown to fix N2 in mesopelagic waters (Benavides et al. 2022) | Benavides et al. (2015) |
| αETSP-2, cIII-ETSP | ETSP (aphotic) | nifH sequencing & qPCR, N2 fixation | Amino acids, simple sugars | Aphotic N2 fixation rates increased in +amino acids and +DOC treatments; identified NCDs did not change in abundance. | Bonnet et al. (2013) |
| Unknown (Gulf of Aqaba), 1 G (Med Sea) | MedSea,Gulf of Aqaba (aphotic) | nifH sequencing (environmental samples, not nutrient exp.; N2 fixation | GX, amino acids | Aphotic N2 fixation rates increased in +amino acids (Gulf of Aqaba) and +GX (Med Sea) | Rahav et al. (2013) |
a
NCD nifH catalog name referenced when possible.
b
NCDs suspected to be dominant N2-fixers.
c
N2 fixation measured on the whole community.