Final answer:
The early microbial systematics was based on d) phenotypic characteristics, which include observable traits and metabolic capabilities, as detailed in resources like Bergey's Manual.
Step-by-step explanation:
The primary basis for early microbial systematics was d) phenotypic characteristics.
Early microbiologists relied on observable traits, including morphology, staining properties, and the ability to grow in various conditions, to classify and identify bacteria.
Methods like capturing, growing, and studying the biochemical properties of microbes were essential due to the lack of distinct macroscopic features.
Over time, this expanded to include metabolic pathways and biochemical reactions, such as carbon utilization, which are detailed in resources like Bergey's Manual of Determinative Bacteriology.
While more modern methods may include genomic analysis, these phenotypic and metabolic observations laid the groundwork for early classification schemes.