In situ photosynthetic rates of tropical marine macroalgae at their lower depth limit

Most photophysiological studies of marine macroalgae have focussed on algae in waters shallower than 30 m. However some species are abundant at depths in excess of 100 m with irradiances less than 1 mol photons m-2 s-1. We examined, for the first time, the in situ efficiency of photochemical energy conversion of a variety of epilithic macroalgal species at depths from 86 to 201 m using a piloted submersible and multiple-turnover modulated chlorophyll fluorescence measurements based on the PAM technique. The irradiance at which electron transport rate reached a maximum (Ek) for green algae declined from 50 mol photons m-2 s-1 (at 90 m) to less than 10 mol photons m-2 s-1 at their lower depth limit of 140 m; photochemical quenching in response to light exposure declined markedly at depths below 100 m, while non-photochemical quenching remained low at all depths, indicating minimal photoprotective capacity in these algae. Values of Ek for encrusting Corallinales at 201 m were 4 mol photons m-2 s-1, which exceeded by 400 times the maximum ambient irradiance at that depth. In the short term, the deep-water red algae examined (in particular the encrusting species) were able to tolerate and take advantage of irradiances orders of magnitude greater than the estimated noonday surface irradiance. Non-photochemical quenching of the red algae also increased with depth, indicating these algae retain their capacity for coping with high light even when in very deep waters. Carbon stable isotope data of deep algae confirmed the diffusion of inorganic carbon with its minimal energy requirement is probably the primary means of inorganic carbon uptake. The observed lower depth limits of selected macroalgae at Penguin Bank are shallower than depth limits for comparable species reported in the literature. Occasional smothering of algae by sediment, observed at Penguin Bank, would reduce the annual photon dose, thereby reducing the depth limit.