Bacteria are microscopic, single-celled organisms found in nearly every environment on Earth, from the deepest oceans to the highest mountains. They inhabit soil, water, air, and even the bodies of other living organisms, exhibiting immense diversity in forms and functions.
Not All Bacteria Photosynthesize
While some bacterial groups convert light energy into chemical energy, many others rely on different strategies for obtaining energy and nutrients. The bacterial kingdom encompasses a wide array of metabolic capabilities, reflecting their adaptation to diverse ecological niches. This metabolic versatility means that photosynthesis is just one of many ways bacteria sustain themselves.
Bacteria That Utilize Photosynthesis
Photosynthetic bacteria are categorized by whether they produce oxygen. Cyanobacteria are a prominent group that perform oxygenic photosynthesis, similar to plants and algae. They use chlorophyll a and accessory pigments like phycobilins to capture light energy. This process occurs on internal membrane structures called thylakoids, where water serves as the electron donor, releasing oxygen as a byproduct. Cyanobacteria significantly contributed to Earth’s atmospheric oxygenation.
Other phototrophic bacteria engage in anoxygenic photosynthesis, meaning they do not produce oxygen. These include purple bacteria and green sulfur bacteria. Instead of water, they utilize alternative electron donors such as hydrogen sulfide, elemental sulfur, or organic molecules. Purple bacteria contain bacteriochlorophylls and carotenoids, giving them characteristic colors. They thrive in anaerobic or low-oxygen environments, often found in illuminated anoxic zones of lakes or hot springs where hydrogen sulfide accumulates.
Green sulfur bacteria, including genera like Chlorobium, are obligate anaerobes that also perform anoxygenic photosynthesis. They use bacteriochlorophylls c, d, a, or e and are efficient at capturing light using specialized antenna complexes called chlorosomes, allowing them to photosynthesize in very low light conditions.
Bacteria That Obtain Energy Differently
Most bacteria acquire energy through diverse chemical processes rather than photosynthesis. Many are chemoheterotrophs, obtaining both energy and carbon from organic compounds. This group includes decomposers that break down dead organic matter, recycling nutrients into ecosystems. Pathogenic bacteria, which cause diseases, also derive energy from host organisms.
Chemolithotrophs obtain energy by oxidizing inorganic compounds. These bacteria can fix carbon dioxide to synthesize their own organic molecules, using chemical energy rather than light. For instance, nitrifying bacteria play a role in the nitrogen cycle. They oxidize ammonia to nitrite, then nitrite to nitrate, making nitrogen available for plants. Other examples include sulfur-oxidizing bacteria, which gain energy by oxidizing sulfur compounds, often found in environments rich in volcanic activity or deep-sea hydrothermal vents.
The Ecological Significance of Bacterial Photosynthesis
Photosynthetic bacteria play a foundational role in global ecosystems, particularly in biogeochemical cycles. Cyanobacteria, through oxygenic photosynthesis, are major contributors to Earth’s atmospheric oxygen, essential for aerobic life forms. They also contribute to the carbon cycle by fixing carbon dioxide into organic compounds, forming the base of many aquatic food webs as primary producers.
Anoxygenic photosynthetic bacteria contribute to carbon cycling in anaerobic environments, utilizing various carbon fixation pathways to convert carbon dioxide into biomass. For example, purple sulfur bacteria contribute to the sulfur cycle by oxidizing hydrogen sulfide, transforming it into elemental sulfur or sulfate. These phototrophic bacteria are important in nutrient cycling and energy flow within diverse habitats, from surface waters to deep-sea anoxic zones.