This course introduces the diversity of marine ecosystems in the estuaries, open ocean and extreme marine environments. The main topics are: 1) Biodiversity of marine ecosystems; 2) Microbial driven C, N and S cycling; 3) Biomarkers and organic geochemistry; 4) Marine ecosystems under the human effect and global climate change. This is a prerequisite course for the Microbial and Chemical Oceanography.