Learn Italian: Lui è un professionista molto serio. - He is a very serious professional.

Nouns ending in -ista (like professionista, turista, dentista) are special: they use the exact same -a ending for both men and women in the singular. You can tell this one is masculine because of the pronoun Lui and the masculine article un.

If the noun ends in -a, why does the adjective serio end in -o?

Adjectives always agree with the noun's actual gender, not just its spelling. Even though professionista ends in -a, we know from Lui and un that it is a masculine noun in this sentence. Therefore, the adjective takes the normal masculine singular ending, becoming serio.

How would you say "They are very serious professionals" if referring to a group of men?

It would be Loro sono professionisti molto seri. While -ista words are identical for men and women in the singular, they have different endings in the plural. Masculine becomes -isti (professionisti) and feminine becomes -iste (professioniste).

Why doesn't molto change its ending here?