What did the first algae produce? What was its significance?
Oxygen. The first algae was most likely the first plant life on Earth also, and via the oxygen they released, they made the atmosphere much more habitable for more complex life, particularly animal life.
The early atmosphere was formed mainly from gases released from the volcanic activity and the formation of the Earth's surface.
Its composition was mostly carbon dioxide, ammonia, methane and water vapour.
As the planet cooled, the water vapour began to condense and formed the oceans (note the planet didn't look at all like today's).
In the oceans, plant life had begun to evolve by some random process, as well as the ocean absorbing the CO2 in the atmosphere, the early plants did too, and produced oxygen as a by-product of this.
Over roughly the next 1 billion years, the plants produced more oxygen, building up the atmosphere, which allowed them to become more complex as well as the animal life which struggled originally in the early atmosphere.
If it was not for the early plant life, we would have never existed.
Hope this helps.
p.s. I recommend looking at this for further reading.
The Miller Urey experiment tried to produce life by mimicking the early atmosphere, however it only produced the basic building blocks of life, that being amino acids. I suggest reading more into this topic as it is interesting.