Photosynthesis of submersed macrophytes in acidified lakes II. Carbon limitation and utilization of benthic CO2 sources

Photosynthetic characteristics of carbon-dioxide limitations were analyzed for leaf tissue in a Cartesian-diver system, in which irradiance could be stringently controlled, and with whole plants in electrode macrosystems for submerged macrophytes (Juncus bulbosus L., Sphagnum auriculatum Schimp. var. inundatum (Russow) M. O. Hill) and other benthic moss and algae (Drepanocladus, Batrachospermum, and an algal mat) from acidified lakes. Light compensation points were extremely low for Juncus (1.5–6 μE m^?2 s^?1) and Sphagnum (3–10), and higher for shallow-inhabiting Batrachospermum (22–33). Leaf tissue, whole plants, and algal populations were rapidly limited by CO₂ availability under closed submersed, acidified conditions (pH 4–6).Controlled and in situ experiments were performed, in which the rooting tissue of Juncus bulbosus was partitioned from the leaves and the rates of photosynthetic carbon fixation of the foliage, utilizing dissolved inorganic ¹⁴C-carbon from the water, were analyzed under different conditions of CO₂ enhancement in the rhizosphere of the sediments. Results demonstrated that: (a) from 25 to 40% of the carbon fixed in the leaves can originate from the rhizosphere, diffuse to the leaves via internal gas lacunae, and be fixed photosynthetically; (b) photosynthetic utilization of CO₂ from the water surrounding the leaves is reduced markedly when the CO₂ concentration of the rhizosphere was increased by direct additions of CO₂, bacteria, or organic compounds (glucose, acetate) that stimulate bacterial growth. Shifts to predominance of submersed benthic primary producers with low light compensation points and adapted to acidified lakes are related in part to circumvention of carbon limitation in the water by utilization of enhanced CO₂ availability in the rhizosphere and at the sediment—water interface from bacterial degradation of organic matter, and in part to physiological mechanisms that conserve and recycle CO₂ of respiration and photorespiration.