Final answer:
Bacteria can act as autotrophs through photosynthesis or chemosynthesis, saprotrophs by decomposing organic matter, or parasites living at the expense of a host. Viruses do not fit these categories, as their life processes are strictly host-dependent.
Step-by-step explanation:
Organisms such as bacteria can function as autotrophs, saprotrophs, or parasites. While viruses have complex interactions with their hosts, they are not classified within these trophic categories since they are not truly 'alive' in the traditional sense and strictly require a host cell to replicate.
Autotrophs create their own food; for instance, many species of bacteria and all cyanobacteria perform photosynthesis to produce food from light. Saprotrophs decompose organic material, and parasites derive nutrients at the expense of their host. Additionally, some bacteria are chemoautotrophs, utilizing energy from chemical reactions to synthesize organic compounds.
A detailed example includes cyanobacteria, which are photoautotrophs that harness solar energy to synthesize sugars like glucose through the pathway of photosynthesis. In contrast, chemoautotrophs such as certain bacteria facilitate chemosynthesis, using energy from chemical bonds to make organic compounds. These various nutrition modes allow bacteria to play essential roles in food webs and ecosystems.