Diazotroph community succession during the VAHINE mesocosm experiment (New Caledonia lagoon)

Abstract

Abstract. The VAHINE mesocosm experiment, conducted in the low-nutrient low-chlorophyll waters of the Noumea lagoon (coastal New Caledonia) was designed to trace the incorporation of nitrogen (N) fixed by diazotrophs into the food web, using large volume (50 m3) mesocosms. This experiment provided a unique opportunity to study the succession of different N2-fixing microorganisms (diazotrophs) and calculate in situ net growth and mortality rates in response to fertilization with dissolved inorganic phosphate (DIP) over a 23-day period, using quantitative polymerase chain reaction (qPCR) assays targeting widely distributed marine diazotroph lineages. Inside the mesocosms, the most abundant diazotroph was the heterocyst-forming Richelia associated with Rhizosolenia (Het-1) in the first half of the experiment, while unicellular cyanobacterial Group C (UCYN-C) became abundant during the second half of the experiment. Decreasing DIP concentrations following the fertilization event and increasing temperatures were significantly correlated with increasing abundances of UCYN-C. Maximum net growth rates for UCYN-C were calculated to range between 1.23 ± 0.07 and 2.16 ± 0.07 d−1 in the mesocosms, which are among the highest growth rates reported for diazotrophs. Outside the mesocosms in the New Caledonia lagoon, UCYN-C abundances remained low, despite increasing temperatures, suggesting that the microbial community response to the DIP fertilization created conditions favorable for UCYN-C growth inside the mesocosms. Diazotroph community composition analysis using PCR targeting a component of the nitrogenase gene (nifH) verified that diazotrophs targeted in qPCR assays were collectively among the major lineages in the lagoon and mesocosm samples, with the exception of Crocosphaera-like phylotypes, where sequence types not typically seen in the oligotrophic ocean grew in the mesocosms. Maximum net growth and mortality rates for nine diazotroph phylotypes throughout the 23-day experiment were variable between mesocosms, and repeated fluctuations between periods of net growth and mortality were commonly observed. The field population of diazotrophs in the New Caledonian lagoon waters appeared to be dominated by Het-1 over the course of the study period. However, results from both qPCR and PCR analysis indicated a diverse field population of diazotrophs was present in the lagoon at the time of sampling. Two ecotypes of the Braarudosphaera bigelowii symbiont unicellular group A (UCYN-A) were present simultaneously in the lagoon, with the recently described B. bigelowii/UCYN-A2 association present at higher abundances than the B. bigelowii/UCYN-A1 association.

Contribution and pathways of diazotroph-derived nitrogen to zooplankton during the VAHINE mesocosm experiment in the oligotrophic New Caledonia lagoon

“…After DIP fertilization, from day 5 to day 14 (hereafter called P1), the abundance of het-1 increased. Following day 15, the community shifted towards dominance of UCYN-C, the abundance of which increased substantially during P2 (Turk-Kubo et al, 2015). Het-1 was the dominant diazotroph in the lagoon waters where a Trichodesmium spp.…”

“…Het-1 was the dominant diazotroph in the lagoon waters where a Trichodesmium spp. bloom began to develop during P2, after day 20 (Turk-Kubo et al, 2015). Chlorophyll a (Chl a) biomass was < 0.3 µg L −1 in all three mesocosms during P0 and P1 (Leblanc et al, 2016).…”

“…The diazotroph assemblage in the lagoon on the day that the mesocosm experiment was initiated was composed primarily of DDAs (het-1: Richelia associated with Rhizosolenia; and het-2: Richelia associated with Hemiaulus) and the symbiotic UCYN-A2 and A1 (Turk-Kubo et al, 2015). Trichodesmium spp.…”

“…het-1); from M2 (1 m depth) on day 17 for experiment E2, during a UCYN-C bloom (comprising > 80 % of diazotroph community); and from lagoon waters (1 m depth) on day 23 for E4 during a Trichodesmium spp. bloom (comprising > 80 % of diazotroph community; Turk-Kubo et al, 2015). Although each experiment was > 80 % dominated by a single diazotroph species, it must be noted that each contained other diazotroph species.…”

Diazotrophs: Overlooked Key Players within the Coral Symbiosis and Tropical Reef Ecosystems?

“…diazotrophs have colonized a wide range of habitats, such as the water column (Riemann et al, 2010;Turk-Kubo et al, 2015), the sediments and dead coral surfaces (Davey et al, 2008;Bednarz et al, 2015b), the algal turfs (Rohwer et al, 2001;Haan et al, 2014;Rix et al, 2015), microbial mats and seagrass meadows (Rohwer et al, 2001(Rohwer et al, , 2002Charpy et al, 2012;Cardini et al, 2014), as well as sponge and coral tissue (Rohwer et al, 2002;Lema et al, 2014a,b). Still, their contribution to nitrogen cycling in the coral reef ecosystem as a whole, their spatial-temporal variability, level of activity and dependence on environmental factors, are poorly understood.…”