There's a lot that could be said about the possibilities, but let's hold off for a while on looking to anything nutritional until after the first true leaves have appeared. Since the seedling is taking it's nutrients from seed tissues, it's pretty likely it's cultural, if not normal. The three things that come to mind are saturated soil/over-watering, too little light, and/or damping off disease. Severe over-fertilizing could be an issue, too. Anything sound familiar?

Al

Al,

Thanks so much for your response. The soil might be a little too wet, but I put it outside, so that should dry things out. Also, the seedpacket said that nasturtiums prefer plenty of moisture, but Ill hold off on watering for a few days. I don't think I've overfertiized the plant because I just stuck it in a 50 50 mix of vermiculite and seed starting mix. I see the first true leaves coming out, so I have my fingers crossed!