AME 66:117-131 (2012)  -  DOI: https://doi.org/10.3354/ame01562

ABSTRACT: Global (i.e. ocean warming) and local (i.e. land runoff) anthropogenic disturbances affect coastal coral reefs worldwide. Terrestrial runoff, leading to reduced light and increased nutrient availability, may have interactive effects with ocean warming in promoting shifts in benthic communities. Because microbial biofilms rapidly reflect environmental changes, we investigated the quantitative (C, N and chlorophyll a contents) and qualitative (microbial community composition) parameters and metabolic responses (O₂ fluxes) of biofilms established on glass slides to combinations of manipulated water temperatures (26, 29 and 31°C), nitrate (0.5, 1.0 and 1.4 µM), and light availabilities (40 and 200 µmol photons m⁻² s⁻¹) in a 28 d flow-through aquarium experiment. The findings revealed that, independent of light availability, a temperature of 31°C significantly decreased the 24 h net O₂ production and all of the quantitative parameters. Under high light, additive effects of 31°C and 1.4 µM nitrate reduced the 24 h net O₂ production. Terminal restriction fragment length polymorphisms (T-RFLP) of 16S rRNA genes showed light-independent temperature-induced microbial community shifts driven by increases in the relative abundance of Oceanospirillum (Gammaproteobacteria) and decreases of Cyanobacteria. The relative abundances of diatom plastids increased in response to elevated nitrate only in high-light-exposed biofilms. Overall, high temperatures altered microbial biofilm community composition, biomass and productivity. Under predicted near-future inner reef scenarios (low light and high nitrate availability), biofilms become light-limited through sedimentation, while outer (high light and low nitrate availability) inshore reef biofilms remain nitrate-limited. Understanding the interactive effects of environmental changes on microbial biofilm communities may contribute to bioindicator development and improved coastal management strategies for coral reefs.

KEY WORDS: Light · Nitrate · Temperature · Water quality · Terrestrial runoff · Coral reef