Yes, but you took an absolutely painful way of getting there, and some of it didn't make a lot of sense.
The metric system beats the pants off the imperial system in matters like this. Rather than bothing with lbs to g to lbs to g, simply remember that the .3% means that for every 1000g or something, there are 3 g of something else. Since 1L = 1 kg, this becomes incredibly simple:
45L = 45 kg
45.4kg * .003 (3% in decimal)= .1362kg = 136.2g
Algae creates 75% of the world's oxygen, so really we're all just doing our parts...