Marine phototrophs are vertically structured across the water column and benthic habitats in response to pronounced gradients in light intensity and spectral composition. As light propagates through seawater, selective attenuation progressively modifies the underwater light field, generally favoring blue to blue-green wavelengths in clear waters, while dissolved and particulate matter can shift the available spectrum toward greener conditions. These optical gradients generate distinct spectral niches that influence the distribution and performance of cyanobacteria, phytoplankton, macroalgae, and photosynthetic corals. However, vertical organization cannot be explained by wavelength alone. Pigment composition, chromatic acclimation, photoacclimation, morphology, internal optics, and nutrient availability interact to determine the realized depth distribution of marine phototrophs. This mini-review synthesizes evidence across pelagic, coastal, benthic, and sedimentary environments to examine how spectral quality and irradiance jointly shape vertical ecological organization. Particular attention is given to complementary pigment absorption in macroalgae and cyanobacteria, chromatic acclimation in Synechococcus, photoacclimation within deep chlorophyll maxima, and whole-organism optical adaptations in deep-water corals and seaweeds. Available evidence indicates that spectral specialization is most ecologically consequential under conditions of strong light limitation, whereas nutrient–light trade-offs and morphological adaptations can substantially modify or override simple wavelength-based predictions. Vertical organization should therefore be understood as a dynamic outcome of interactions among underwater optical properties, organismal light-harvesting strategies, and resource availability rather than as a fixed sequence of pigment-defined depth zones. This framework also highlights the potential sensitivity of spectral niches to future changes in ocean optical conditions and variability.
