A alga cresce quando o nitrato está alto.

Portuguese often uses the definite article with nouns when talking about things in a general or class-wide sense.

So:

• A alga cresce = Algae grow / The algae grows
• O nitrato está alto = Nitrate is high / The nitrate level is high

This is very natural in Portuguese, even where English would often leave the article out.

A few similar examples:

• O açúcar faz mal em excesso. = Sugar is bad in excess.
• A água está fria. = The water is cold / Water is cold.

So the articles here do not necessarily mean one specific alga and one specific nitrate sample. They can also express a general statement.

Yes. Alga is a feminine singular noun, so it takes the feminine singular article a.

• a alga = the algae / the alga
• as algas = the algae (plural)

Even though many words ending in -a are feminine, that is not always true in Portuguese. But in this case, alga really is feminine.

Yes. Nitrato is a masculine singular noun, so it takes o.

• o nitrato = the nitrate
• os nitratos = the nitrates

This also affects adjectives that describe it. That is why the sentence says alto, not alta.

Because alto agrees with o nitrato, which is masculine singular.

Agreement in Portuguese usually works like this:

• masculine singular: alto
• feminine singular: alta
• masculine plural: altos
• feminine plural: altas

So:

• o nitrato está alto
• a temperatura está alta

The adjective must match the noun it describes.

Portuguese usually uses estar for a state, condition, or current level, and ser for something more permanent, essential, or defining.

Here, high nitrate is treated as a condition or level, so estar is the natural choice:

• O nitrato está alto. = The nitrate is high.

Using ser here would sound unnatural in normal Portuguese, because nitrate being high is not viewed as an inherent identity; it is a measurement or condition.

This is very common with things like levels, temperatures, and states:

• A temperatura está alta.
• A pressão está baixa.
• O pH está estável.

In Portuguese, the present tense is often used for general truths, habits, scientific facts, and cause-and-effect statements.

So:

• A alga cresce quando o nitrato está alto.

means something like:

• Algae grow when nitrate is high
• The algae grows when nitrate is high

It expresses a regular relationship, not just one future event.

English does this too in many cases:

• Plants grow when they get sunlight.
• Water boils at 100°C.

So the present tense is exactly what you would expect here.

Quando means when, and se means if.

In this sentence, the idea is that whenever nitrate is high, algae grow. It describes a repeated or expected relationship, so quando fits well.

• quando = when
• se = if

Compare:

• A alga cresce quando o nitrato está alto.
  = Algae grow when nitrate is high.
  (general pattern)

• A alga cresce se o nitrato estiver alto.
  = Algae will grow if nitrate is high.
  (more conditional/hypothetical)

So quando sounds more like a general fact here.

Yes. Portuguese allows some flexibility here.

You can also say:

• Quando o nitrato está alto, a alga cresce.

This means the same thing. The difference is mainly focus or style:

• A alga cresce quando o nitrato está alto.
  starts with the result
• Quando o nitrato está alto, a alga cresce.
  starts with the condition

Both are natural.

The accent mark shows the stressed syllable and also helps distinguish está from other words.

• está = is
• esta = this (feminine) / this one

Examples:

• O nitrato está alto. = The nitrate is high.
• Esta alga cresce rápido. = This algae grows fast.

So the accent is important for both pronunciation and meaning.

Grammatically, alga is singular in Portuguese.

• a alga = the alga / algae in a generic sense
• as algas = the algae (plural)

In Portuguese, a singular noun can sometimes represent a whole category in a general statement, especially with the definite article.

So A alga cresce... can sound like a general biological statement, even though it is singular in form.

If you want to make it explicitly plural, you could say:

• As algas crescem quando o nitrato está alto.

That is also correct and natural.

Yes, very often that is how it is understood.

In natural usage, o nitrato está alto usually means the nitrate level is high, even if the word nível is not stated explicitly.

Portuguese often leaves that understood:

• O nitrato está alto. = The nitrate is high / The nitrate level is high.
• O fósforo está baixo. = The phosphorus level is low.

If you want to be more explicit, you could say:

• O nível de nitrato está alto.

But the shorter version is very common.

A natural Brazilian Portuguese pronunciation is approximately:

a AW-ga KREH-si KWAN-doo u nee-TRAH-tu es-TAH AW-too

A few notes:

• alga: the al often sounds like aw
• cresce: stress on cre
• quando: stress on quan
• nitrato: stress on tra
• está: stress on the last syllable
• alto: again, al sounds like aw

A rough IPA version is:

/a ˈaw.gɐ ˈkɾe.si ˈkwɐ̃.du u niˈtɾa.tu esˈta ˈaw.tu/

You usually would not use está crescendo here if you are stating a general fact.

• cresce = grows / does grow
• está crescendo = is growing

So:

• A alga cresce quando o nitrato está alto.
  = general truth / habitual relationship

• A alga está crescendo porque o nitrato está alto.
  = the algae is growing right now because the nitrate is high

The progressive form estar + gerund is for an action in progress, not for a general rule.