Abstract
Abstract. N2 fixation fuels ∼ 50 % of new primary production in the oligotrophic South Pacific Ocean. The VAHINE experiment has been designed to track the fate of diazotroph-derived nitrogen (DDN) and carbon within a coastal lagoon ecosystem in a comprehensive way. For this, large-volume ( ∼ 50 m3) mesocosms were deployed in the New Caledonian lagoon and were intentionally fertilized with dissolved inorganic phosphorus (DIP) to stimulate N2 fixation. This study examined the temporal dynamics of the prokaryotic community together with the evolution of biogeochemical parameters for 23 consecutive days in one of these mesocosms (M1) and in the Nouméa lagoon using MiSeq 16S rRNA gene sequencing and flow cytometry. Combining these methods allowed for inference of absolute cell numbers from 16S data. We observed clear successions within M1, some of which were not mirrored in the lagoon. The dominating classes in M1 were Alpha- and Gammaproteobacteria, Cyanobacteria, eukaryotic microalgae, Marine Group II Euryarchaeota, Flavobacteriia, and Acidimicrobia. Enclosure led to significant changes in the M1 microbial community, probably initiated by the early decay of Synechococcus and diatoms. However, we did not detect a pronounced bottle effect with a copiotroph-dominated community. The fertilization with ∼ 0.8 µM DIP on day 4 did not have directly observable effects on the overall community within M1, as the data samples obtained from before and 4 days after fertilization clustered together, but likely influenced the development of individual populations later on, like Defluviicoccus-related bacteria and UCYN-C-type diazotrophic cyanobacteria (Cyanothece). Growth of UCYN-C led to among the highest N2-fixation rates ever measured in this region and enhanced growth of nearly all abundant heterotrophic groups in M1. We further show that different Rhodobacteraceae were the most efficient heterotrophs in the investigated system and we observed niche partitioning within the SAR86 clade. Whereas the location in- or outside the mesocosm had a significant effect on community composition, the temporal effect was significantly stronger and similar in both locations, suggesting that overarching abiotic factors were more influential than the enclosure. While temporal community changes were evident, prokaryotic diversity (Shannon index) only declined slightly from ∼ 6.5 to 5.7 or 6.05 in the lagoon and M1, respectively, throughout the experiment, highlighting the importance of multiple and varying sources of organic matter maintaining competition.
Highlights
The Southwest Pacific Ocean is recognized as an area with one of the highest dinitrogen (N2)-fixation rates in the global ocean (Garcia et al, 2007; Luo et al, 2012)
On the evening of day 4, M1 was fertilized with ∼ 0.8 μM dissolved inorganic phosphorus (DIP), leading to lower alkaline phosphatase activity (APA) than in the lagoon until day 19, when APA in M1 increased steeply (Fig. 1c, d) and DIP turnover times dropped below initial levels (Van Wambeke et al, 2015)
The here investigated M1 was the one with the lowest increase of chlorophyll a (Chl) a during P2, still reaching higher levels than in Nouméa lagoon waters, whereas primary production (PP) in M1 did not exceed the values measured in the lagoon (Fig. 1a, b)
Summary
The Southwest Pacific Ocean is recognized as an area with one of the highest dinitrogen (N2)-fixation rates in the global ocean (Garcia et al, 2007; Luo et al, 2012). In this region, plankton taxa capable of N2 fixation (N2-fixing or diazotrophic organisms) are very diverse (Moisander et al, 2010) and fuel up to 60 % of primary production (Bonnet et al, 2015a), yet their interactions with surrounding planktonic communities are rarely studied. No information is available on the potential effects of this N2 fixation on bacterial successions
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