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Early cyanobacteria in stromatolites are thought to be responsible for increasing the amount of oxygen in the primeval Earth's atmosphere through their continuing photosynthesis. They were the first known organisms to photosynthesize and produce free oxygen. 
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b. ventricles
The right and left ventricles 
 
        
                    
             
        
        
        
A holdfast is a root-like structure that anchors aquatic sessile organisms, such as seaweed, other sessile algae, stalked crinoids, benthic cnidarians, and sponges, to the substrate.
        
             
        
        
        
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The carrying capacity is a measure of how many individuals can a given ecosystem provide for. An individual and its population is dependent on various components of its ecosystem for necessities such as food, habitat, etc. An ecosystem can only successfully support a given population.
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An individual organism that supplies living tissue to be used in another body, as a person who furnished blood for transfusion or an organ for transplantation in a histocompatible recipient that doesnt function.
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