Basil -- Yellow Leaves at the Bottom and Dark Spots Appearing

I am living in Singapore and I would like some advice on growing basil here. There are a couple of questions:
1) The temperature easily rises above 30 degrees Celsius here. Can the basil handle full sunlight in this heat?
2) Also, the bottom leaves are beginning to lighten up and turn yellow... and I see the start of brown spots appearing. Could it be due to the fact that I am over fertilizing? I am using organic fertilizer.
3) The basil plants (about 10-15inches/ 25-40cm high) are planted about 4 inches/ 10cm apart from each other at the moment in the long brown plastic container. Is it too close to one another?

Basil loves heat and sun and if it is gradually exposed to it, I am sure it can take full sun even in your tropical climate. However, the plants seem to be stressed in some way as shown by the loss of bottom leaves and the other leaves getting brown spots. It could be that your containers are getting too hot and the heat is killing the roots. The plants will also need a lot of water to keep themselves cool by transpiring. You did not say how often you water, but if the soil is getting dry between watering, you may need to step up the frequency. To overcome rapid drying, use large pots and mulch any exposed soil.
Your spacing of the plants is OK for them while they are young or if they are miniature varieties. For standard varieties, you should plant them at least twice as far apart.
With tropical sun, the plants will probably feed heavily and if you fertilize every second watering at the strength recommended on the container, then you are not over-fertilizing.