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

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.

Effects of Ocean Acidification on Carbon and Nitrogen Fixation in the Hermatypic Coral Galaxea fascicularis

The supply of metabolites from symbionts to scleractinian corals is crucial to coral health. Members of the Symbiodiniaceae can enhance coral calcification by providing photosynthetically fixed carbon (PFC) and energy, whereas dinitrogen (N2)-fixing bacteria can provide additional nutrients such as diazotrophically-derived nitrogen (DDN) that sustain coral productivity especially when alternative external nitrogen sources are scarce. How these mutualistic associations benefit corals in the future acidifying ocean is not well understood. In this study, we investigated the possible effects of ocean acidification (OA; pHs 7.7 and 7.4 vs. 8.1) on calcification in the hermatypic coral Galaxea fascicularis with respect to PFC and DDN assimilation. Our measurements based on isotopic tracing showed no significant differences in the assimilation of PFC among different pH treatments, but the assimilation of DDN decreased significantly after 28 days of stress at pH 7.4. The decreased DDN assimilation suggests a nitrogenous nutrient deficiency in the coral holotiont, potentially leading to reduced coral calcification and resilience to bleaching and other stressful events. This contrasting impact of OA on carbon and N flux demonstrates the flexibility of G. fascicularis in coping with OA, apparently by sustaining a largely undamaged photosystem at the expense of N2 fixation machinery, which competes with coral calcification for energy from photosynthesis. These findings shed new light on the critically important but more vulnerable N cycling in hospite, and on the trade-off between coral hosts and symbionts in response to future climate change.

Monoclonal Culture and Characterization of Symbiodiniaceae C1 Strain From the Scleractinian Coral Galaxea fascicularis

The symbiosis between cnidarian hosts and photosynthetic dinoflagellates of the family Symbiodiniaceae (i.e., zooxanthellae) provides the energy foundation of coral reef ecosystems in oligotrophic waters. The structure of symbiont biota and the dominant species of algal symbiont partly shape the environmental adaptability of coral symbiotes. In this study, the algal symbiont cells were isolated from the tentacles of Galaxea fascicularis, a hermatypic coral with obvious differentiation in heat resistance, and were cultured in vitro with an improved L1 medium. An algal monoclonal cell line was established using separated algal culture drops and soft agar plating method, and named by GF19C1 as it was identified as Cladocopium sp. C1 (Symbiodiniaceae) based on its ITS1, ITS2, and the non-coding region of the plastid psbA minicircle (psbAncr) sequences. Most GF19C1 cells were at the coccoid stage of the gymnodinioid, their markedly thickened (ca. two times) cell wall suggests that they developed into vegetative cysts and have sexual and asexual reproductive potential. The average diameter of GF19C1 cells decreased significantly, probably due to the increasing mitotic rate. The chloroplasts volume density of GF19C1 was significantly lower than that of their symbiotic congeners, while the surface area density of thylakoids relative to volumes of chloroplasts was not significantly changed. The volume fraction of vacuoles increased by nearly fivefold, but there was no significant change in mitochondria and accumulation bodies. Light-temperature orthogonal experiments showed that, GF19C1 growth preferred the temperature 25 ± 1°C (at which it is maintained post-isolation) rather than 28 ± 1°C under the light intensity of 42 ± 2 or 62 ± 2 μmol photons m–2 s–1, indicating an inertia for temperature adaptation. The optimum salinity for GF19C1 growth ranged between 28–32 ppt. The monoclonal culture techniques established in this study were critical to clarify the physiological and ecological characteristics of various algal symbiont species, and will be instrumental to further reveal the roles of algal symbionts in the adaptive differentiation of coral-zooxanthellae holobionts in future studies.