Photosynthetic characteristics of some common temperate freshwater red algal taxa (Rhodophyta)

We measured photosynthetic-irradiance curves of freshwater red algae in the Batrachospermales and Acrochaetiales from five low-order streams with one to five taxa collected from each stream in the central portion of the Ohio River basin. Four populations of Batrachospermum helminthosum, two populations of B. gelatinosum and Audouinella hermannii and one population of Batrachospermum sp. and Tuomeya americana were sampled. All five species of freshwater red algae were found within a single stream. We observed two distinct groups of algae from this stream: a low photosynthetic efficiency/high initial saturation group and a high photosynthetic efficiency/ low initial saturation group distinguished by morphology. Mucilaginous forms, such as B. helminthosum, had significantly greater maximum photosynthesis and efficiency compared to those of the more tuftlike'' or compact morphology, such as A. hermannii. Photosynthetic responses of B. helminthosum in four streams showed a positive relationship with maximum photosynthesis and efficiency with increasing canopy cover. Comparing B. gelatinosum and A. hermannii from two sites, we observed no significant impact of canopy cover. Ratios of photosynthesis to respiration were greater than one for all of the taxa studied, suggesting that freshwater red algae can be an important source of in-stream production. Our findings support previous work suggesting that both algal morphology as well as light environment alter photosynthetic responses of freshwater red algae. Understanding how species respond to varied light availabilities will increase our understanding of the ecological role, biogeography, habitat preferences and within-watershed distribution of these important benthic primary producers in streams.