THE VAHINE PROJECT

Biogeochemical and biological response to a diazotroph bloom in a low-nutrient, low-chlorophyll ecosystem: results from the VAHINE mesocosms experiment

PIs: Sophie Bonnet

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In marine ecosystems, biological N2 fixation provides the predominant external source of nitrogen (N; 140 ± 50 Tg N yr−1), contributing more than atmospheric and riverine inputs to the N supply. Yet the fate and magnitude of the newly fixed N in marine ecosystems is poorly understood. Moreover, whether it is preferentially and directly exported out of the photic zone, recycled by the microbial loop and/or transferred into larger organisms remains unclear. These questions were investigated in the framework of the VAHINE (VAriability of vertical and tropHIc transfer of diazotroph derived N in the south wEst Pacific) project, which specific objectives were:

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• to quantify the diazotroph-derived nitrogen which enters the planktonic food web, 

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• to investigate how the development of diazotrophs influences the subsequent diversity, gene expression and production of primary producers, heterotrophic bacterioplankton and subsequently zooplankton abundance, 

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• to examine whether different functional types of diazotrophs significantly modify the stocks and fluxes of the major biogenic elements (C, N, P), 

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• to elucidate whether the efficiency of particulate matter export depends on the development of different functional types of diazotrophs. 

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To achieve this goal, we deployed triplicate large volume (∼ 50 m3) mesocosms for 3 weeks in the tropical south-west Pacific coastal ocean (New Caledonia). The mesocosms were intentionally fertilized with  ∼ 0.8 µM dissolved inorganic phosphorus (DIP) at the start of the experiment to stimulate diazotrophy. A total of 47 stocks, fluxes, enzymatic activities and diversity parameters were measured daily inside and outside the mesocosms by the 45 scientists involved in the project. The experiment lasted for 23 days and was characterized by two distinct and successive diazotroph blooms: a dominance of diatom-diazotroph associations (DDAs) during the first half of the experiment followed by a bloom of unicellular cyanobacterial lineage C (UCYN-C during the second half of the experiment. These conditions provided a unique opportunity to compare the DDN transfer and export efficiency associated with different diazotrophs. 

The major experimental and modelling results obtained during the project and described in the VAHINE SPECIAL ISSUE and summarized in the SYNTHESIS ARTICLE.