Highly Variable and Non-complex Diazotroph Communities in Corals From Ambient and High CO2 Environments

Abstract

The ecological success of corals depends on their association with microalgae and a diverse bacterial assemblage. Ocean acidification (OA), among other stressors, threatens to impair host-microbial metabolic interactions that underlie coral holobiont functioning. Volcanic CO2 seeps offer a unique opportunity to study the effects of OA in natural reef settings and provide insight into the long-term adaptations under a low pH environment. Here we compared nitrogen-fixing bacteria (diazotrophs) associated with four coral species (Pocillopora damicornis, Galaxea fascicularis, Acropora secale, and Porites rus) collected from CO2 seeps at Tutum Bay (Papua New Guinea) with those from a nearby ambient CO2 site using nifH amplicon sequencing to characterize the effects of seawater pH on bacterial communities and nitrogen cycling. Diazotroph communities were of generally low diversity across all coral species and for both sampling sites. Out of a total of 25 identified diazotroph taxa, 14 were associated with P. damicornis, of which 9 were shared across coral species. None of the diazotroph taxa, however, were consistently found across all coral species or across all samples within a species pointing to a high degree of diazotroph community variability. Rather, the majority of sampled colonies were dominated by one or two diazotroph taxa of high relative abundance. Pocillopora damicornis and Galaxea fascicularis that were sampled in both environments showed contrasting community assemblages between sites. In P. damicornis, Gammaproteobacteria and Cyanobacteria were prevalent under ambient pCO2, while a single member of the family Rhodobacteraceae was present at high relative abundance at the high pCO2 site. Conversely, in G. fascicularis diazotroph communities were indifferent between both sites. Diazotroph community changes in response to OA seem thus variable within as well as between host species, potentially arguing for haphazard diazotroph community assembly. This warrants further research into the underlying factors structuring diazotroph community assemblages and their functional role in the coral holobiont.

Antibiotics Alter Pocillopora Coral-Symbiodiniaceae-Bacteria Interactions and Cause Microbial Dysbiosis During Heat Stress

“…Our experiment reveals that antibiotic suppression of coral-associated bacteria communities produces dramatic changes in Pocillopora coral and Cladocopium symbiont gene expression and bacteria community composition, resulting in microbial dysbiosis and diminished holobiont heat tolerance. These findings corroborate results from previous studies that documented negative effects of antibiotics on coral health (Gilbert et al, 2012;Glasl et al, 2016), and provide new insights into the complex multi-partner interactions that occur within Pocillopora coral holobionts (Brener-Raffalli et al, 2018;Pogoreutz et al, 2018;Geissler et al, 2021;Haydon et al, 2021;Li et al, 2021).…”

“…This second hypothesis is consistent with observations of decreased chlorophyll a in nitrogen-starved C. goreauii (Zhou et al, 2021) and stable photochemical efficiency and dramatically increased growth rates in Breviolium minutum cultures supplemented with casein amino acids (Kirk et al, 2020). Although the Pocillopora and Cladocopium genomes possess pathways for synthesizing most amino acids and heterotrophic feeding may account for substantial input of essential amino acids to the coral holobiont (Fox et al, 2019;Ferrier-Pagès et al, 2021), these observations suggest that certain coral-associated and/or Symbiodiniaceae-associated bacteria may provide in hospite Symbiodiniaceae with additional nitrogenous compounds that benefit holobiont metabolism (Benavides et al, 2017;Geissler et al, 2021;Glaze et al, 2021;Rädecker et al, 2021). While more experimental evidence is required to validate this conclusion, Cladocopium symbionts were recently shown to associate with diverse bacteria located within algae cells and on external cell surfaces, which supports the idea that Symbiodiniaceae-bacteria metabolic exchanges may be more common than previously recognized (Camp et al, 2020;Matthews et al, 2020;Maire et al, 2021).…”

Diversity and composition of bacterial communities associated with healthy and bleached coral Fungia sp. in Nha Trang bay, Vietnam

“…At order level (Fig. 1B), Rhospirillales, Sphingomonasdales, Rhodobacerales, and Rhizobiales are orders related to nitrogen fixing in coral (Geissler et al, 2021). They are found to have higher abundance over total in healthy SML than that of the bleached.…”

Excess labile carbon promotes diazotroph abundance in heat-stressed octocorals

“…prokaryotes capable of converting atmospheric dinitrogen (N 2 ) into ammonia, can provide an alternative nitrogen source for the holobiont, sustaining the productivity of corals when environmental nutrient availability is low [4,5]. Diazotrophs are ubiquitous members of reef-building coral and octocoral holobionts with pronounced host-dependent differences in their abundance, activity, and community structure [5][6][7][8][9].…”

“…Global and local environmental change threaten to destabilize the functioning of coral holobionts and the reefs they support [10][11][12][13]. Importantly, diazotrophs in reef-building corals are susceptible to changing environmental conditions [3,8,9,[13][14][15][16][17][18]. Previous studies have reported that coral bleaching, the stressinduced breakdown of stable holobiont functioning, not only results in the loss of Symbiodiniaceae but also coincides with the opportunistic proliferation of diazotrophs [13,19,20].…”