Aqutic Life

Plants are critical to other life on this planet because they form the basis of all food webs. Most plants are autotrophic, creating their own food using water, carbon dioxide, and light through a process called photosynthesis. Some of the earliest fossils found have been aged at 3.8 billion years. These fossil deposits show evidence of photosynthesis, so plants, or the plant-like ancestors of plants, have lived on this planet longer that most other groups of organisms. At one time, anything that was green and that wasn’t an animal was considered to be a plant. Now, what were once considered “plants” are divided into several kingdoms: Protista, Fungi, and Plantae. Most aquatic plants occur in the kingdoms Plantae and Protista.

It is believed that the earth was originally an aggregation of dust and swirling gases about 4.5 billion years ago. The earliest fossil life forms are 3.8 billion years old and contain simple prokaryotic (without a membrane-bound nucleus) cells. The atmosphere at that time was mostly nitrogen gas, with large portions of carbon dioxide and water vapour. Since life evolved in this atmosphere, carbon, oxygen, hydrogen, and nitrogen (major elements of nitrogen gas, carbon dioxide and water) make up 98% of the organic materials in living organisms. There was no oxygen in the early atmosphere, so all life existed in an anaerobic environment.

Since no human was alive to document the events of the early earth, much of our information has been pieced together from studies of the fossil record. It is now believed that the earth 4.0 billion years ago was a very tumultuous place; there were violent electrical storms, radioactive substances emitting large quantities of energy, and molten rock and boiling water erupting from beneath the earth’s surface. These forces broke apart the simple gases in the atmosphere, causing them to reorganize into more complex molecules. Ultraviolet light bombarded the surface of the earth, breaking apart the complex molecules and forming new ones. These complex compounds were washed out of the atmosphere by driving rains and subsequently collected in the oceans.

Many organic molecules tend to clump together, so the early oceans probably had aggregations of organic molecules that looked like droplets of oil in water. These clusters of molecules may have been the ancestors of primitive cells. They may also have been the source of energy for early life forms; primitive cells could have used these complex compounds to satisfy their energy requirements. As these early heterotrophs increased in number, the aggregations of complex organic molecules started to become depleted. It became more and more difficult to find “food,” so competition between cells commenced. Eventually cells evolved that could make their own food from simple inorganic materials. The most successful of these early autotrophs were those that could use solar energy to create their own energy; those that could photosynthesize. They used a complex pigment system to capture and hold light energy in the form of organic molecules.

Why are plants so important? If plants hadn’t evolved to photosynthesize, life on this planet would be very different. Plants produce their food by photosynthesis, and the most important waste product of this process is oxygen - something that most of the animal life on this planet can’t live without! As the first plants began photosynthesizing, the amount of oxygen in the atmosphere increased. Some of the oxygen in the outer layer of the atmosphere was converted to ozone. Eventually there was enough ozone in the upper atmosphere to effectively filter out the harmful ultraviolet rays that are highly destructive to living organisms. This allowed organisms to survive in the surface layers of water and on land. Having an abundant supply of oxygen in the atmosphere also allowed other organisms to break down the complex energy-containing molecules formed by