Learn Spanish (Latin America): Esas células minúsculas se deben investigar inmediatamente. - Those minuscule cells must be investigated immediately.

Because of the passive se structure! In this sentence, the plural noun phrase Esas células minúsculas acts as the subject. When you use a verb phrase like 'must investigate' (deber investigar) with the passive se, the conjugated modal verb (deben) has to agree with that plural subject.

Can I attach the se to the infinitive to make it deben investigarse?

Yes, absolutely! When you have a conjugated verb followed by an infinitive, you have two options for pronoun placement: right before the conjugated verb (se deben investigar) or attached directly to the end of the infinitive (deben investigarse). Both are perfectly correct and mean the exact same thing.

Would it be wrong to use the singular form and say se debe investigar esas células?

Yes, grammatically that would be incorrect, though it is a mistake even native speakers sometimes make in casual speech. Because the things receiving the action (esas células) are plural inanimate objects, Spanish grammar requires the passive se structure, meaning the verb must be plural to match (se deben). Using the singular se debe makes it an impersonal sentence, which is generally only used when the object is a person introduced by the personal a.

Why does the adjective minúsculas require a written accent mark?