How is Zakat calculated on money invested in a Mudarabah company: Is it on the profit only, or on the principal and profit together?

Zakat is obligatory on money invested through Mudarabah (profit-sharing) if it reaches the Nisab (minimum taxable amount) and a full year (Hawl) has passed on it. It is to be paid at a rate of 2.5% of the total capital and profit.