Biologist know that some marine algae can create clouds by producing the gas dimethyl sulphide (DMS), which reacts with oxygen in air above the sea to form solid particles. These particles provide a surface on which water vapor can condense to form clouds. Lovelock contends that this process is part of global climatic-control system. According to Lovelock, Earth acts like a super organism, with all its biological and physical systems cooperating to keep it healthy. He hypothesized that warmer conditions increase algal activity and DMS output, seeding more clouds, which cool the planet by blocking out the Sun. Then, as the climate cools, algal activity and DMS level decrease and the cycle continues. In response to biologists who question how organisms presumably working for their own selfish ends could have evolved to behave in a way that benefits not only the planet but the organisms as well, Lovelock points out that cooling benefits the algae, which remain at the ocean surface, because it allows the cooled upper layers of the ocean to sink, and then the circulating water carries nutrients upward from the depths below. Algae may also benefit from nitrogen raining down from clouds they have helped to form.