PHOTOSYNTHESIS IN AN EXTREME SHADE ENVIRONMENT: BENTHIC MICROBIAL MATS FROM LAKE HOARE, A PERMANENTLY ICE‐COVERED ANTARCTIC LAKE

• Published in: Journal of Phycology
• Main Authors: Hawes, Ian, Schwarz, Anne‐Maree
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1999
• Subjects: Antarctic; Hoare; Lake Hoare; Antarc*; Antarctica
• Online Access: http://dx.doi.org/10.1046/j.1529-8817.1999.3530448.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1046%2Fj.1529-8817.1999.3530448.x; https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1529-8817.1999.3530448.x

We investigated the composition of benthic microbial mats in permanently ice‐covered Lake Hoare, Antarctica, and their irradiance vs. photosynthetic oxygen 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 approximately 13 m and the lower from below 13 m to 22 m. Moat mats were acclimated to the high irradiance they experienced during summer. They contained photoprotective pigments, predominantly those characteristic of cyanobacteria, and had high compensation and saturating irradiances ( E c and E k ) of 75 and 130 μmol photons·m −2 ·s −1 , respectively. The moat mats used light inefficiently. The upper under‐ice community contained both cyanobacteria and diatoms. Within this zone, biomass (as pigments) increased with increasing depth, reaching a maximum at 10 m. Phycoerythrin was abundant in this zone, with shade acclimation and efficiency of utilization of incident light increasing with depth to a maximum of 0.06 mol C fixed·mol −1 incident photons under light‐limiting conditions. Precipitation of inorganic carbon as calcite was associated with this community, representing up to 50% of the carbon sequestered into the sediment. The lower under‐ice zone was characterized by a decline in pigment concentrations with depth and an increasing prevalence of diatoms. Photosynthesis in this community was highly shade acclimated and efficient, with E c and E k below 0.5 μmol·m −2 ·s −1 and 2 μmol·m −2 ·s −1 , respectively, and maximum yields of 0.04 mol 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 photosynthesis–irradiance curves, incident irradiance, and light attenuation by ice and the water column.