Serratia spp. (including marcescens, the "red" colored bacterium...the other 13 described species are non-pigmented) are neither photoautotrophic or chemoautotrophic. They are all chemoheterotrophic. What does this mean?
Let's break these complicated words down. Each of these words is made up of three separate words: photo/chemo, auto/hetero, trophic. Trophic refers to the manner in which an organism acquires material from the environment to grow and reproduce. An organism has a "trophic position" in a food chain...predator and prey are both trophic relationships. For bacteria, we often classify their metabolism based on how they obtain the two things all organisms need to live: energy and chemical building blocks. This brings us to the other words: Photo or chemo refers to the source of an organism's energy. Plants obtain their energy from sunlight via photosynthesis. This is called PHOTOtrophy (photo means light). You and I obtain our energy from digesting previously made organic material. The chemical bonds in the food we eat store energy, and we harvest this energy when we break the bonds during digestion. This is called CHEMOtrophy (chemo means having to do with chemistry). Chemotrophy can also be done by breaking chemical bonds in non-organic material, such as nitrates or sulfur compounds.
The last term is a bit more complicated, but it is the most important (especially because it was the source of your confusion). Auto/Hetero refer to where organisms get the chemical building blocks which they use to build their proteins, membranes, or tissues. All organisms on Earth are primarily composed of Carbon, and this term specifically indicates where the organisms obtains its carbon building-blocks. AUTOtrophs collect these building blocks from non-organic materials such as rock, air, water, etc. As an example, plants make sugar out of carbon dioxide, which is a non-organic form of carbon. This makes them PHOTO-AUTO-TROPHIC. HETEROtrophs obtain their carbon from previously-made organic compounds, like sugars and proteins and fats.
We are heterotrophs, and we can't use carbon dioxide to make our tissues the way that plants can. Hetero means mixed or different...we use this prefix because there are a huge variety of organic molecules from which we can obtain our carbon.
Bacteria are the most metabolically diverse organisms in the world. They have found ways to live off of just about anything. Serratia marcescens likes to live off of sugars, using these organic compounds to provide energy (chemo rather than photo) and to provide carbon (hetero rather than auto) in much the same way that we do. Both Serratia and Homo sapiens are classified as CHEMO-HETERO-TROPHIC. This is one reason why people rarely get infections from this bacterium: it is trying to eat us! This bacterium has been found to be a problem with people's breast implants...they get infected around the implant because the implant is full of liquid sugar, which these bacteria love!
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