Final answer:
Ocean acidity and coral reef destruction are major threats to marine ecosystems, primarily caused by increasing CO₂ levels and rising temperatures that lead to coral bleaching and biodiversity loss.
Step-by-step explanation:
The major threats to marine ecosystems include ocean acidity and coral reef destruction. Rising atmospheric CO₂ levels due to burning fossil fuels increase the acidity of oceans, which is detrimental to marine life, especially coral reefs.
As the oceans' pH lowers, the process of calcification is hindered, affecting the structural integrity of corals. This phenomenon, alongside increased ocean temperatures, leads to coral bleaching, the loss of symbiotic algae, and ultimately reef destruction. Both factors result in a decline in marine biodiversity and have economic impacts due to the loss of ecosystems that are vital for coastal economies and various marine species.