## 4 Discussion
### Interactions of Temperature and CO\({}_{2}\) Concentrations on Seedling's Trait Responses  
In our study, enriched CO\({}_{2}\) concentrations alone did not significantly affect performance of _E. acoroides_ seedlings in terms of its photo-physiology or growth, nor did they affect the carbon balance of the seedlings, suggesting that they are still dependent on their internal carbon reserves. Previous studies have shown that, in many cases, seagrass are C\({}_{1}\)-limited at the current oceanic concentrations. Therefore, an increasing CO\({}_{2}\) concentration has often been related to an overall better performance, in terms of growth or photosynthetic rates, in both temperate and tropical species, including _E. acoroides_[60; 61; 31; 62; 63]. For instance, _Thalassia hemorrhagic_, which is another persistent tropical seagrass species, can increase its growth rate under CO\({}_{2}\) enrichment (\(\sim\)1000 \(\mu\)mol L\({}^{-1}\)) [64] as well as _Z. marina_, which showed an increase in growth in CO\({}_{2}\) cultures enriched two-fold when compared to the control [65]. In terms of the biochemical traits, the absence of effects of CO\({}_{2}\) on leaf nutrient contents suggests, as previously observed with increasing nitrogen concentrations in seawater, that seedlings are still sustained by internal nutrient reserves [18]. Carbon balance in the body of a plant is a major factor determining the rate of growth, which also occurs in seagrass [66]. Changes in carbon balance increasing the storage in below-ground tissues have been observed in other seagrass species under enriched CO\({}_{2}\) concentrations for longer periods than in this study [20; 67], suggesting it may be a long-term strategy of plant survival. Therefore, contrary to the beneficial effects shown in other previous studies in marine primary producers [13], our results did not show any clear positive effect of CO\({}_{2}\) enrichment. We did, however, observe interactions with temperature that could benefit seagrass development in this initial seedling phase. This interaction of increasing CO\({}_{2}\) concentrations on thermal acclimation, even within sublethal temperatures, is still not well understood in seagrass. The study results showed enhanced photo-physiological activity in terms of higher \(rETR_{\text{max}}\) and Ek values in the 27 \({}^{\circ}\)C-enriched CO\({}_{2}\) treatment, but not under the regimes of 31 \({}^{\circ}\)C, showing a weak interactive effect of both factors (\(p\)-value = 0.078). Even though CO\({}_{2}\) concentrations in the 31 \({}^{\circ}\)C-enriched treatment were greater (\(\sim\)600 \(\upmu\)atm) than in the 27 \({}^{\circ}\)C-enriched CO\({}_{2}\) treatment (\(\sim\)465 \(\upmu\)atm) due to increased CO\({}_{2}\) solubility with temperature, we did not observe an enhanced performance in the 31 \({}^{\circ}\)C-enriched CO\({}_{2}\) treatment. This higher \(rETR_{\text{max}}\) (photosynthetic capacity) was not related to the biggest leaf morphological traits, which were observed under the 31 \({}^{\circ}\)C-enriched CO\({}_{2}\) treatment nor with higher internal nutrient contents. However, a trend of increasing concentrations was observed in enriched CO\({}_{2}\) treatment. This interaction could be more important in a tropical environmental as, even though some sites where _E. acoroides_ grows are highly eutrophic [68; 69], these systems are typically oligotrophic and dissolved nutrients are usually the limiting factor [34]. A better understanding of responses at a biochemical or morphological level has shown that the high photosynthetic performance observed in some studies is more a short-term response to C\({}_{1}\) limitation that is difficult to maintain over time due to the potential limitation of other factors [22; 70; 71]. This was also observed with _P. oceanica_ seedlings, which showed higher photosynthetic performance after two months of study than at the end of the study period [20].