Add aditional field with title to every node
As far as I know there's no generic #title property (it's certainly not mentioned in the render array docs); forms use them, but I don't think extends to any render array.

#prefix would be the way to go:

$content['my_additional_field'] = array(
  '#markup' =&gt; "additional field",
  '#prefix' =&gt; "&lt;h3&gt;Title&lt;/h3&gt;",
  '#weight' =&gt; 1000
);