Background/Question/Methods The maintenance of diversity remains a major question in ecology and evolution that will affect the abilities of communities to function and adapt under ongoing climate changes. Biological diversity can promote community resilience to stress or disturbance only if species respond to disturbances in different ways, or in other words, if they are ecophysiologically divergent in some way. Reduced water flow along the sea floor (benthos) and competition for nutrients due to uptake by other benthic organisms creates a concentration boundary layer that reduces access to nutrients and chemical exchange along the benthos, which has been shown to reduce algal productivity and intensify resource competition. Crustose coralline algae inhabit intertidal and subtidal zones, often living below a canopy of large macroalgae that compete for nutrients and filter light. Here, we focused on the activity of carbon concentrating mechanisms together with photosynthetic performance to determine functional divergence in two focal cryptic species complexes, Bossiella (2 spp.) and Crusticorallina (5 spp.), within the crustose coralline algae.
Results/Conclusions Photophysiological and photosynthetic measurements varied by species and by season. Effective quantum yield, Y, differed between cryptic coralline algae only later in the growing season during peak kelp canopy cover, indicating that cryptic species inhabit different light environments that are driven by kelp cover. Cryptic species also differed in NPQmax, the maximum value of non-photosynthetic quenching, which is a mechanism that regulates and protects photosynthetic machinery from high light irradiance. As expected, closely related species exhibiting an upright rather than crustose morphology had higher photosynthetic rates under light limiting conditions. These photophysiological data suggest different recent environmental histories based on fine-scale niche partitioning of the benthos, leading to differences in light availability and productivity. Finally, higher Y was correlated by species with lower reliance on carbon concentrating mechanisms (CCMs), enzymatic pathways that increase dissolved inorganic carbon availability. Together, these physiological data suggest that ecophysiological niche partitioning is an important process that interacts with competition for space that has been extensively studied in this community, and contributes to coexistence of closely related species.