Photosynthetic pigments in anoxygenic bacteria: Bacteriochlorophylls used by anoxygenic phototrophs show absorbance maxima predominantly in which portion of the electromagnetic spectrum?

Introduction / Context:
Anoxygenic phototrophic bacteria use bacteriochlorophyll pigments to harvest light energy without evolving oxygen. The specific wavelengths absorbed determine ecological niches (e.g., light available in sediments or microbial mats) and dictate how these microbes compete or coexist with oxygenic phototrophs such as cyanobacteria and plants.

Given Data / Assumptions:

• Bacteriochlorophylls differ from plant chlorophylls in structure and spectral properties.
• Question asks for the portion of the spectrum where absorbance maxima typically occur.
• Focus is on predominant, characteristic maxima, not minor bands.

Concept / Approach:

Bacteriochlorophyll variants (e.g., BChl a, b, c, d, e, g) commonly exhibit strong absorbance in the near-infrared region (approximately 800–1020 nm depending on type and protein environment). This enables these organisms to use wavelengths that penetrate deeper into biofilms or sediments and are not efficiently captured by oxygenic phototrophs that primarily absorb in blue and red regions.

Step-by-Step Solution:

Verification / Alternative check:

Spectral data from photochemical reaction centers of purple and green sulfur bacteria show strong Qy bands in near-infrared, confirming the ecological partitioning of light across microbial communities.

Why Other Options Are Wrong:

Green/blue: major bands for oxygenic chlorophylls, not the signature of bacteriochlorophyll maxima.

Ultraviolet: damaging to biomolecules and not a primary absorption band for these pigments.

Common Pitfalls:

Assuming all “chlorophylls” absorb identically; confusing accessory carotenoids (which can absorb blue/green) with the core bacteriochlorophyll maxima.

Final Answer:

infrared