'algal-supporting' is a modern English compound formed from 'algal' (relating to algae) + 'supporting' (present participle of 'support'). 'Algal' derives from 'algae' + adjectival suffix '-al'; 'algae' ultimately comes from Latin 'alga' and Greek 'álgē' (seaweed). 'Supporting' comes from Old French/Latin roots of 'supportare' (to carry underneath).

The element 'algal' developed from the noun 'algae' (Modern Latin 'alga' from Greek), with the adjectival suffix '-al' creating 'algal' in scientific usage. 'Support' comes from Latin 'supportare' (to carry under) through Old French 'suporter' and Middle English 'supporten'; the present participle 'supporting' has long been used to form compound adjectives (e.g., 'cost-supporting'). Together they formed the descriptive compound 'algal-supporting' in recent scientific and environmental contexts.

Originally, 'support' had the physical sense 'to carry or hold up'; in compounds like 'algal-supporting' the sense has broadened to 'providing conditions or resources that allow something to grow or persist' — here, allowing algal growth.