The Shifts of Diazotrophic Communities in Spring and Summer Associated with Coral Galaxea astreata, Pavona decussata, and Porites lutea

Abstract

The coral holobiont often resides in oligotrophic waters; both coral cells and their symbiotic dinoflagellates possess ammonium assimilation enzymes and potentially benefit from the nitrogen fixation of coral-associated diazotrophs. However, the seasonal dynamics of coral-associated diazotrophs are not well characterized. Here, the seasonal variations of diazotrophic communities associated with three corals, Galaxea astreata, Pavona decussata, and Porites lutea, were studied using nifH gene amplicon pyrosequencing techniques. Our results revealed a great diversity of coral-associated diazotrophs. nifH sequences related to Alphaproteobacteria, Deltaproteobacteria, and Gammaproteobacteria were ubiquitous and dominant in all corals in two seasons. In contrast with the coral P. decussata, both G. astreata and P. lutea showed significant seasonal changes in the diazotrophic communities and nifH gene abundance. Variable diazotroph groups accounted for a range from 11 to 49% within individual coral samples. Most of the variable diazotrophic groups from P. decussata were species-specific, however, the majority of overlapping variable groups in G. astreata and P. lutea showed the same seasonal variation characteristics. Rhodopseudomonas palustris- and Gluconacetobacter diazotrophicus-affiliated sequences were relatively abundant in the summer, whereas a nifH sequence related to Halorhodospira halophila was relatively abundant in spring G. astreata and P. lutea. The seasonal variations of all diazotrophic communities were significantly correlated with the seasonal shifts of ammonium and nitrate, suggesting that diazotrophs play an important role in the nitrogen cycle of the coral holobiont.

Analysis of nifH DNA and RNA reveals a disproportionate contribution to nitrogenase activities by rare plankton-associated diazotrophs

“…An analysis of beta-dispersion [47] was used to quantify betweencommunity variations in the total and the active PADs communities, and a redundancy analysis (RDA) to explore the relationship between diazotrophic groups and environmental parameters. Statistical analyses in this study were performed using the R vegan package (v3.5.1) and an R-based pipeline, as previously described [46].…”

Diversity and Activity of Diazotrophs in Great Barrier Reef Surface Waters

“…Some early studies demonstrated that biological N 2 fixation might play an important role in supplying bioavailable N within benthic reef habitats (Webb et al, 1975; Wiebe et al, 1975; Larkum et al, 1988), and more recently, diazotrophic bacteria have been shown to be an important constituent of the coral holobiont, supplying N requirements for symbiotic Symbiodinium (Lesser et al, 2004, 2007; Lema et al, 2012, 2014; Olson and Lesser, 2013; Zhang et al, 2016). However, within the pelagic environment of the GBR the role of biological N 2 fixation is less well-understood (Furnas et al, 2011), despite evidence to suggest that large discrepancies exist between nutrient availability and phytoplankton growth (Furnas et al, 2005).…”

Analysis of nifH DNA and RNA reveals a disproportionate contribution to nitrogenase activities by rare plankton-associated diazotrophs

“…1:where R is the resulting ratio of nifH mRNA/ nifH DNA, E is the amplification efficiency, and ΔCt the cycle threshold difference between nifH mRNA and nifH DNA. The amplification efficiency was calculated using established standards, as described in [62]. The nifH RNA/DNA ratio of the main PADs groups was further calculated by multiplying the nifH RNA/DNA ratio obtained from the bulk community by the respective frequencies of the dominant groups (Eq.…”

“…An analysis of beta-dispersion [63] was used to quantify between-community variations in the total and the active PADs communities, and a redundancy analysis (RDA) to explore the relationship between diazotrophic groups and environmental parameters. Statistical analyses in this study were performed using the R vegan package (v3.5.1) and an R-based pipeline, as previously described [62].…”