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<p>Global atmospheric carbon dioxide (CO₂) concentrations have been increasing at unprecedented rates since the industrial revolution. The ocean has been acting as a buffer, absorbing CO₂, resulting in rising sea temperature (ocean warming; OW) and lowering its pH (ocean acidification; OA). OA is known to cause reductions in the calcification rates of marine calcifiers, resulting in dissolution of calcium carbonate shells and skeletons. Sponges have important functional and structural roles in marine ecosystems and there is some evidence to suggest that sponges may be “winners” under future ocean climate conditions due to their high level of resilience to OA and OW and the increased availability of space as a result of reductions in more sensitive calcifying species. Although this may be the case for those sponges with skeletons made up of siliceous spicules, little is known about how calcareous sponges, with calcite spicules, may react to OA conditions. This thesis addresses a knowledge gap on how temperate calcareous sponges may respond to OA using Grantia sp. as a model species. A twenty-eight-day experiment with three control (pH 8.0) tanks and three OA (based on IPCC (RCP8.5); pH 7.6) tanks was used to measure changes in sponge size, spicule size, spicule deformation, respiration rate and microbial community structure. I found no signs of corrosion or significant change in area of sponges, however, there was a significant 25% reduction in the spicule size under the projected climate change OA “worst case scenario” conditions, a sign that sponge growth was impacted under stressful external pressure. How this reduction is spicule size will impact the sponge is still unknown. Respiration rates of sponges were not significantly different between the control or treatment sponges, and the microbiome of control and OA sponges did not significantly differ, but they did change significantly over time (T0 compared to T28 (final day of the experiment). The microbiome over time changed with increasing abundance of microbes known to have a role in nutrient cycling and assisting in marine host’s acclimation to new niches with varied environmental conditions. My results suggest that while the physiology of Grantia sp. is not significantly affected by low pH conditions consistent with those predicted for 2100 under worst case climate scenarios, spicules were impacted with treatment sponges having smaller spicules. The consequences and mechanisms resulting in smaller spicules need further investigation. Overall, this study provides evidence that like many demosponges, calcareous sponges may have some resilience to OA impacts.</p>