Assessing the effects of iron enrichment across holobiont compartments reveals reduced microbial nitrogen fixation in the Red Sea coral Pocillopora verrucosa

Abstract

The productivity of coral reefs in oligotrophic tropical waters is sustained by an efficient uptake and recycling of nutrients. In reef‐building corals, the engineers of these ecosystems, this nutrient recycling is facilitated by a constant exchange of nutrients between the animal host and endosymbiotic photosynthetic dinoflagellates (zooxanthellae), bacteria, and other microbes. Due to the complex interactions in this so‐called coral holobiont, it has proven difficult to understand the environmental limitations of productivity in corals. Among others, the micronutrient iron has been proposed to limit primary productivity due to its essential role in photosynthesis and bacterial processes. Here, we tested the effect of iron enrichment on the physiology of the coral Pocillopora verrucosa from the central Red Sea during a 12‐day experiment. Contrary to previous reports, we did not see an increase in zooxanthellae population density or gross photosynthesis. Conversely, respiration rates were significantly increased, and microbial nitrogen fixation was significantly decreased. Taken together, our data suggest that iron is not a limiting factor of primary productivity in Red Sea corals. Rather, increased metabolic demands in response to iron enrichment, as evidenced by increased respiration rates, may reduce carbon (i.e., energy) availability in the coral holobiont, resulting in reduced microbial nitrogen fixation. This decrease in nitrogen supply in turn may exacerbate the limitation of other nutrients, creating a negative feedback loop. Thereby, our results highlight that the effects of iron enrichment appear to be strongly dependent on local environmental conditions and ultimately may depend on the availability of other nutrients.

Short Term Exposure to Heat and Sediment Triggers Changes in Coral Gene Expression and Photo-Physiological Performance

“…It is well known that Symbiodiniaceae abundance in corals is controlled in part by the coral host, which optimizes symbiont abundance to match its environment (Cunning, 2013), thereby controlling the net benefit of their symbiosis (Hoogenboom et al, 2010). While heat is considered a strong driver of coral bleaching (Brown, 1997;Woolridge, 2013), an increase in temperature can drive nitrogen fixation which, in turn, can stimulate cell division in the endosymbionts (Rädecker et al, 2017). Therefore, it is conceivable that the active regulation of symbiont density which resulted in a significant effect in photosynthetic yield is associated with the acclimatization capacity of P. acuta under suboptimal conditions.…”

Dominance of Endozoicomonas bacteria throughout coral bleaching and mortality suggests structural inflexibility of the Pocillopora verrucosa microbiome

“…In these cases, changes in the coral‐associated bacterial community composition were interpreted as detrimental. Accordingly, changes in bacterial community structure or activity may be either beneficial or deleterious, depending on the environmental context (Ainsworth & Gates, 2016; Pogoreutz, Rädecker, Cárdenas, Gärdes, Voolstra, et al., 2017; Pogoreutz, Rädecker, Cárdenas, Gärdes, Wild, et al., 2017; Rädecker et al., 2015, 2017; Santos et al., 2014). …”

Optimization of medium using response surface methodology to enhance the growth of Effrenium voratum (Symbiodiniaceae, Dinophyceae)

“…, Rädecker et al. ). However, the literatures about the impacts of nutrients on cultured Symbiodiniaceae species have been very limited (Sakami , Karako‐Lampert et al.…”