Photosynthesis in an extreme shade environment: Benthic microbial mats from Lake Hoare, a permanently ice-covered Antarctic lake

We investigated the composition of benthic microbial mats in permanently ic e-covered Lake Hoare, Antarctica, and their irradiance vs. photosynthetic o xygen exchange relationships. Mats could be subdivided into three distinct depth zones: a seasonally ice-free "moat" zone and two under-ice zones. The upper under-ice zone extended from below the 3.5 m thick ice to approximat ely 13 m and the lower from below 13 m to 22 m. Moat mats were acclimated t o the high irradiance they experienced during summer. They contained photop rotective pigments, predominantly those characteristic of cyanobacteria, an d had high compensation and saturating irradiances (E-c and E-k) of 75 and 130 mu mol photons.m(-2).s(-1), respectively. The moat mats used light inef ficiently. The upper under-ice community contained both cyanobacteria and d iatoms. Within this zone, biomass (as pigments) increased with increasing d epth, reaching a maximum at 10 m. Phycoerythrin was abundant in this zone, with shade acclimation and efficiency of utilization of incident Light incr easing with depth to a maximum of 0.06 mel C fixed.mol(-1) incident photons under light-limiting conditions. Precipitation of inorganic carbon as calc ite was associated with this community, representing up to 50% of the carbo n sequestered into the sediment. The lower under-ice zone was characterized by a decline in pigment concentrations with depth and an increasing preval ence of diatoms. Photosynthesis in this community was highly shade acclimat ed and efficient, with E-c and E-k below 0.5 mu mol.m(-2).s(-1) and 2 mu mo l.m(-2).s(-1), respectively, and maximum yields of 0.04 mel C fixed.mol(-1) incident quanta. Carbon uptake in situ by both under-ice and moat mats was estimated at up to 100 and 140 mg.m(-2).day(-1), based on the photosynthes is-irradiance curves, incident irradiance, and light attenuation by ice and the water column.