Complex sentence: Karl Otto Stetter (born July 16,1941) is a German microbiologist and astrobiologist, an expert on microbial life at the upper temperature, and one of the most important scientists currently working in this field.
Split into simpler sentences: Karl Otto Stetter (born July 16,1941) is a German microbiologist and authority on astrobiology. He is an expert on microbial life at high temperatures, and one of the most important scientists currently working in this field.