Chapter 15: Ecological Diversity of Bacteria

Microbial diversity can be understood through the interconnected perspectives of metabolic capabilities, ecological roles, and evolutionary relationships, with ecological diversity emphasizing how microbial traits such as morphology, motility, environmental tolerance, and growth strategies shape interactions within ecosystems. A major focus of the chapter is the ecological diversity of phototrophic bacteria, which capture light energy through photosynthesis. Early phototrophs performed anoxygenic photosynthesis using electron donors such as hydrogen sulfide or ferrous iron, while oxygenic photosynthesis evolved in cyanobacteria and played a fundamental role in oxygenating Earth’s atmosphere. Distinct groups of phototrophic bacteria—including purple sulfur bacteria, purple nonsulfur bacteria, green sulfur bacteria, green nonsulfur bacteria, heliobacteria, and newly discovered phototrophic acidobacteria and gemmatimonadetes—demonstrate diverse metabolic strategies and adaptations to specific light, oxygen, and sulfur conditions. The chapter also highlights microbial diversity defined by key metabolic traits involved in global biogeochemical cycles. Nitrogen fixing microorganisms convert atmospheric nitrogen into biologically usable forms through nitrogenase activity, while nitrifying bacteria oxidize ammonia or nitrite during the nitrogen cycle. Other microbial groups carry out denitrification, sulfate reduction, sulfur oxidation, iron reduction, and iron oxidation, processes that drive nutrient cycling in both aerobic and anaerobic environments. Methanotrophs and methylotrophs specialize in the metabolism of methane and other single carbon compounds, playing essential roles in carbon cycling. In addition to metabolic specialization, the chapter discusses microorganisms distinguished by unique ecological strategies or morphological features, including bacterial predators such as Bdellovibrio and Myxococcus, spirochetes with distinctive endoflagellar motility, budding and stalked bacteria that reproduce through asymmetric cell division, sheath forming bacteria that inhabit aquatic environments, and magnetotactic bacteria that orient themselves along magnetic field lines using intracellular magnetosomes. Together these diverse physiological, metabolic, and ecological strategies illustrate the remarkable adaptability of microorganisms and their critical roles in maintaining ecosystem function across the biosphere.