How do I open the gate?

Hint 1 of 12:

You need to get rid of the algae first.

Hint 2 of 12:

See "How do I get rid of the algae" for details.

Hint 3 of 12:

When the algae's gone, it will reveal a hole.

Hint 4 of 12:

The hole seems to be a mechanism of some sort.

Hint 5 of 12:

The mechanism opens the gate.