Seaweeds - or marine algae - are one of the most diverse and dazzling organism groups on the planet. They have existed for over 1.6 billion years and survived every extinction event on the planet. Capable of thriving in some of the most extreme environments on earth, marine algae are found within in sea ice of the Arctic Ocean, as well as extreme acid, alkali and salt environments, and have been recorded at ocean depths of over 200m. Professor Juliet Brodie, internationally renowned expert on marine algae, reveals their beauty and complexity - showing how they are vital not only to our global ecosystems but to the future of the planet. Professor Brodie explains the evolutionary significance, symbiotic relationships and strange sex lives of these curious organisms, and profiles 45 different species from around the world - each chosen to reflect a unique or fascinating aspect of these extraordinary organisms.