Rising anthropogenic CO(2) emissions acidify the oceans, and cause changes to seawater carbon chemistry. Bacterial biofilm communities reflect environmental disturbances and may rapidly respond to ocean acidification. This study investigates community composition and activity responses to experimental ocean acidification in biofilms from the Australian Great Barrier Reef. Natural biofilms grown on glass slides were exposed for 11 d to four controlled pCO(2) concentrations representing the following scenarios: A) pre-industrial (∼300 ppm), B) present-day (∼400 ppm), C) mid century (∼560 ppm) and D) late century (∼1140 ppm). Terminal restriction fragment length polymorphism and clone library analyses of 16S rRNA genes revealed CO(2) -correlated bacterial community shifts between treatments A, B and D. Observed bacterial community shifts were driven by decreases in the relative abundance of Alphaproteobacteria and increases of Flavobacteriales (Bacteroidetes) at increased CO(2) concentrations, indicating pH sensitivity of specific bacterial groups. Elevated pCO(2) (C + D) shifted biofilm algal communities and significantly increased C and N contents, yet O(2) fluxes, measured using in light and dark incubations, remained unchanged. Our findings suggest that bacterial biofilm communities rapidly adapt and reorganize in response to high pCO(2) to maintain activity such as oxygen production.

Natural and anthropogenic impacts such as terrestrial runoff, influence the water quality along the coast of the Great Barrier Reef (GBR) and may in turn affect coral reef communities. Associated bacterial biofilms respond rapidly to environmental conditions and are potential bioindicators for changes in water quality. As a prerequisite to study the effects of water quality on biofilm communities, appropriate biofilm substrates for deployment in the field must be developed and evaluated. This study investigates the effect of different settlement substrates (i.e. glass slides, ceramic tiles, coral skeletons and reef sediments) on bacterial biofilm communities grown in situ for 48 days at two locations in the Whitsunday Island Group (Central GBR) during two sampling times. Bacterial communities associated with the biofilms were analysed using terminal restriction fragment length polymorphism (T-RFLP) and clone library analyses of 16S rRNA genes. Findings revealed that substrate type had little influence on bacterial community composition. Of particular relevance, glass slides and coral skeletons exhibited very similar communities during both sampling times, suggesting the suitability of standardized glass slides for long-term biofilm indicator studies in tropical coral reef ecosystems.

b16S rRNA gene molecular analysis elucidated the spatiotemporal distribution of bacterial biofilm communities along a water quality gradient. Multivariate statistics indicated that terrestrial runoff, in particular dissolved organic carbon and chlorophyll a concentrations, induced shifts of specific bacterial communities between locations and seasons, suggesting microbial biofilms could be suitable bioindicators for water quality.T ropical coral reefs are facing global climate change (e.g., warming sea surface temperatures), and local-scale disturbances at an increasing rate (13,21,50). However, local-scale natural (e.g., monsoonal rainfalls and cyclones) and anthropogenic (e.g., landbased activities) impacts result in terrestrial runoff and deteriorate water quality, putting coastal regions at greater risk (18). The Proserpine River catchment in the Whitsunday Island area, Great Barrier Reef (GBR), Australia, is considered a priority coastal management region due to land clearing and agriculture (i.e., sugarcane farming) (17). In particular during the summer wet season, terrestrial runoff inshore leads to higher concentrations of nutrients, chlorophyll a (Chl a), and suspended sediments, forming cross-shelf water quality gradients (5,7,11,17,44). Changes in water quality show significant alterations in coral reef dynamics (14,30,44). Alterations also include associated microorganisms that predominately exist as surface-attached biofilms (32), contributing importantly to ecosystem productivity, biogeochemical fluxes (4, 27), and invertebrate larval settlement (46, 49). Bacterial communities are highly responsive indicators of changing environmental conditions (35,40), as demonstrated in rivers (3), estuaries (33, 34), and polar (45) and temperate (8) marine waters. Biofilms may therefore also find application as a biomonitoring tool of transient spatiotemporal variability and more persistent ecosystem change due to large-scale GBR river catchment management decisions (controlling farming activities) or global climate change. Despite the suitability of bacterial biofilm communities as environmental bioindicators, biofilms in tropical coral reefs remain poorly explored.This study therefore aimed to explore the spatiotemporal variation and response of bacterial biofilm communities in coastal coral reefs to seasonal terrestrial runoff disturbances at pristine and human-impacted sites. Biofilms were established during repeated summer wet and winter dry seasons at 5 nearshore fringing reefs along a water quality gradient in the Whitsunday Islands. A molecular approach (terminal restriction fragment length polymorphism [T-RFLP] and 16S rRNA gene clone libraries) and a multivariate statistical approach (permutational multivariate analysis of variance [PERMANOVA], principal component analysis [PCA], and distance-based redundancy analysis [dbRDA]) were used to identify the effects of location and season and the most decisive water quality parameters to determine the main controls driving microbial community shifts. Finall...