Thats a tricky question as there is much controversy as to whether the inactive B12 analogues within spirulina will interfere with the active B12 from chlorella.

Here's what Reed Mangals Ph.D., R.D (via the Vegan Resource Group) has to say...............
“The inactive form (also called analogues) actually interferes with normal vitamin B12 absorption and metabolism”

And...................
The vegetarian society are also clear on the matter............
“Researchers have suggested that supposed B12 supplements such as spirulina may in fact increase the risk of B12 deficiency disease, as the B12 analogues can compete with B12 and inhibit metabolism. The current nutritional consensus is that no plant foods can be relied on as a safe source of vitamin B12.”

As far as I am concerned the jury is still out, and where more research is needed in this area to offer any concrete advice, but I have never been B12 deficient on a vegan diet for many many years, so that may say everything there is to say about it.

Chrisb1.