Why is the increasing amount of carbon dioxide being taken up by the oceans a cause for concern?

It cause acidification of oceans which harms ocean biodiversity.

Explanation:

Rising of atmospheric CO2 is a cause of concern for the environmentalist because intake of this CO2 is also increasing by oceans. After all, the ocean keeps the carbon cycle in balance.

When this CO2 enters the oceans it converts into carbonic acid and due to this carbonic acid, our ocean is becoming more acidic.

This increasing acidity in the ocean reduces the ability of corals to build skeleton and marine biodiversity is very much dependent on corals. Therefore increasing the amount of CO2 is a cause of concern.