Photoacclimation by Arctic cryoconite phototrophs.

Cryoconite is a matrix of sediment, biogenic polymer and a microbial community that resides on glacier surfaces. The phototrophic component of this community is well adapted to this extreme environment, including high light stress. Photoacclimation of the cryoconite phototrophic community on Longyearbreen, Svalbard, was investigated using in situ variable chlorophyll fluorescence. Rapid light curves (RLCs) and induction-recovery curves were used to analyse photosystem II quantum efficiency, relative electron transport rate and forms of downregulation including non-photochemical quenching (NPQ) and state transitions in cyanobacteria. Phototrophs used a combination of behavioural and physiological photochemical downregulation. Behavioural downregulation is hypothesised to incorporate chloroplast movement and cell or filament positioning within the sediment matrix in order to shade from high light, which resulted in a lack of saturation of RLCs and hence overestimation of productivity. Physiological downregulation likely consisted of biphasic NPQ, comprising a steadily induced light-dependent form and a light-independent NPQ that was not reversed with decreasing light intensity. State transitions by cyanobacteria were the most likely physiological downregulation employed by cyanobacteria within the mixed phototroph community. These findings demonstrate that cryoconite phototrophs combine multiple forms of physiological and behavioural downregulation to optimise light exposure and maximise photosynthetic productivity. This plasticity of photoacclimation enables them to survive productively in the high-light stress environment on the ice surface.

Anterior screw fixation for a pediatric odontoid nonunion: a case report.

STUDY DESIGN: A case of an odontoid nonunion in a child treated with anterior screw fixation. OBJECTIVES: To demonstrate that an anterior screw procedure can be performed with an odontoid nonunion with resultant fusion to maintain range of motion. METHODS: A 15-year-old boy presented with pain in his neck following a rugby football injury. Admission plain radiographs and computed tomography scan demonstrated an odontoid nonunion. Radiographs of a previous cervical spine injury 2 years before demonstrated a missed odontoid fracture. RESULTS: The child was initially treated conservatively with halo vest immobilization, which did not result in healing. Direct anterior screw fixation was performed and the fracture united 5 months following surgery. DISCUSSION: The nonunion was asymptomatic for 2 years until the second injury when it became clinically symptomatic. It did not respond to conservative treatment and was unstable on screening requiring operative intervention. CONCLUSIONS: Very few cases have been reported of pediatric odontoid nonunions. If the fracture pattern allows, then direct anterior screw fixation should be considered in order to maintain range of motion at the atlantoaxial articulation.