23 Aug 2021

23 Aug 2021

Review status: this preprint is currently under review for the journal BG.

N2 fixation in the Mediterranean Sea related to the composition of the diazotrophic community, and impact of dust under present and future environmental conditions

Céline Ridame1, Julie Dinasquet2,3, Søren Hallstrøm4, Estelle Bigeard5, Lasse Riemann4, France Van Wambeke6, Matthieu Bressac7, Elvira Pulido-Villena6, Vincent Taillandier7, Frederic Gazeau7, Antonio Tover-Sanchez8, Anne-Claire Baudoux5, and Cécile Guieu7 Céline Ridame et al.
  • 1Sorbonne University, CNRS, IRD, LOCEAN: Laboratoire d’Océanographie et du Climat: Expérimenta tion et Approches Numériques, UMR 7159, 75252 Paris Cedex 05, France
  • 2Scripps Institution of Oceanography, University of California San Diego, USA
  • 3Sorbonne University, CNRS, Laboratoire d'Océanographie Microbienne, LOMIC, 66650 Banyuls-sur-Mer, France
  • 4Marine Biology Section, Department of Biology, University of Copenhagen, 3000 Helsingør, Denmark
  • 5Sorbonne University, CNRS, Station Biologique de Roscoff, UMR 7144 Adaptation et Diversité en Milieu Marin, France
  • 6Aix-Marseille Université, Université de Toulon, CNRS/INSU, IRD, Mediterranean Institute of Oceanography (MIO), UM 110, 13288, Marseille, France
  • 7Sorbonne Université, CNRS, Laboratoire d'Océanographie de Villefranche, LOV, 06230 Villefranche-sur-Mer, France
  • 8Department of Ecology and Coastal Management, Institute of Marine Sciences of Andalusia (CSIC), 11510 Puerto Real, Cádiz, Spain

Abstract. N2 fixation rates were measured in the 0–1000 m layer at 13 stations located in the open western and central Mediterranean Sea (MS) during the PEACETIME cruise (late spring 2017). While the spatial variability of N2 fixation was not related to Fe, P nor N stocks, the surface composition of the diazotrophic community indicated a strong eastward increasing longitudinal gradient for the relative abundance of non-cyanobacterial diazotrophs (NCD) (mainly γ-Proteobacteria) and conversely eastward decreasing for UCYN-A (mainly -A1 and -A3) as did N2 fixation rates. UCYN-A4 and A3 were identified for the first time in the MS. The westernmost station influenced by Atlantic waters, and characterized by highest stocks of N and P, displayed a patchy distribution of diazotrophic activity with an exceptionally high rate in the euphotic layer of 72.1 nmol N L−1 d−1, which could support up to 19 % of primary production. At this station at 1 %PAR depth, UCYN-A4 represented up to 94 % of the diazotrophic community. These in situ observations of higher UCYN-A relative abundance in nutrient rich stations while NCD increased in the more oligotrophic stations, suggest that the nutrient conditions could determine the composition of the diazotrophic communities and in turn the N2 fixation rates.

The impact of Saharan dust deposition on N2 fixation and diazotrophic communities was also investigated, under present and future projected conditions of temperature and pH during short term (3–4 days) experiments at three stations. New nutrients from simulated dust deposition triggered a significant stimulation of N2 fixation (from 41 % to 565 %). The strongest increase in N2 fixation was observed at the stations dominated by NCD and did not lead on this short time scale to change in the diazotrophic community composition. Under projected future conditions, N2 fixation was either exacerbated or unchanged, in that later case this was probably due to a too low nutrient bioavailability or an increased grazing pressure. The future warming and acidification likely benefited NCD (Pseudomonas) and UCYN-A2 while disadvantaged UCYN-A3 without knowing which effect (alone or in combination) is the driver, especially since we do not know the temperature optima of these species not yet cultivated as well as the effect of acidification.

Céline Ridame et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-190', Anonymous Referee #1, 12 Sep 2021
  • RC2: 'Comment on bg-2021-190', Anonymous Referee #2, 20 Sep 2021

Céline Ridame et al.

Céline Ridame et al.


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Short summary
We show that in the Mediterranean Sea spatial variability in N2 fixation is related to the diazotrophic community composition reflecting different nutrient requirements among species. Nutrient supply by Saharan dust is of great importance to diazotrophs as shown by the strong stimulation of N2 fixation after a simulated dust event under present and future climate conditions; the magnitude of stimulation depends on the degree of limitation related to the diazotrophic community composition.