Simple clone scripting

I'm a complete noob just pootling along and trying to make a fish-tank with an ecosystem for fun. I want to start the base of the food chain with plankton but I'm having difficulty, as soon as I try to add a variable to the instantiated prefab I get a null object reference.

There are two scripts, the first is used to generate the plankton;

    var Plankton_Archae_Prefab:GameObject;
function Start () {
for (var i : int = 0;i < 100; i++) {
    var position = Vector3(Random.Range(0, 500),Random.Range(100, 300), Random.Range(0, 500));
    Instantiate(Plankton_Archae_Prefab, position, Quaternion.identity);
    }
}

The second script is attached to an individual plankton, this is the one that throws a wobbly;

var Plankton_Archae_Prefab:GameObject;

var Currentage; //Current Age of Plankton var Maxage; //Maximum Age of Plankton var hp; //Hit Points var div; //Number of cell divisions

var speed = 5; //Movement speed

var PlankEnergy; //Energy of plankton

function Update () { PlankEnergy++; rigidbody.position = Vector3.Lerp(rigidbody.position,rigidbody.position + Vector3((Random.value-0.5)*speed,(Random.value-0.5)*speed,(Random.value-0.5)*speed),Time.time); if(PlankEnergy==10){ Instantiate(Plankton_Archae_Prefab, rigidbody.position, Quaternion.identity); } }

I've tried moving the PlankEnergy variable into the update function, making it a private variable and all sorts. If I remove the PlankEnergy variable then the script works fine, problem is I need that variable :-)

Note: Although it's designed for plankton the basic script is really a study in bacteria.