How microbial biofilm communities respond to ocean acidification ?

As a result of massive CO2 release due to industrial activities, pH oceans is rapidly decreasing in last decades. This process is called "ocean acidification" and is becoming a rising problem that scientific community need to investigate. Bacterial biofilm communities play a key role in understanding environmental disturbance by reduced pH in sea water. In this paper authors investigated the effects of ocean acidification on the activity ( expressed as O2 fluxes) and community composition of the tropical coral reef-associated biofilms from the Australian Great Barrier Reef.

Glass slides were immersed into a flow-through tank with natural seawater from the lagoon for 24 d. Natural biofilms grown on glass slides were subsequently exposed for 11 days to four controlled pCO2 conditions, representing four scenarios ( increasing pCO2 ) :

A) Pre-industrial: ~ 300 ppm (pH 8.1-8.2)

B) Present day: ~ 400 ppm (pH 8)

C) Projected Mid century: ~ 560 ppm (pH 7.9)

D) Projected Late century: ~ 1140 ppm (pH 7.6)

Samples collected were processed to analyze O2 fluxes, C:N ratios, bacterial phylogenetic analysis, determination of macro communities.

Results obtained show that frequency of phototrophic flora components significantly changed among the four treatments: at high pCO2 phototrophic community members exclusively comprised diatoms, green filamentous and green algae, while all red algae types were completely absent.

Most relevant bacteria found belong to Alphaproteobacteria, Bacteroidetes and Gammaproteobacteria; bacterial T-RFLP analysis shows different profiles of initial community from those at the end of the experiment: Bacteroidetes increase with rising pCO2, Alphaproteobacteria show a opposite trend (decreasing). Further, bacterial assemblages at low pCO2 had the highest variability, while high CO2 treatments showed less community variability.16S rRNA sequence analysis demonstrates that sequence affiliated with the Alphaproteobacteria were most abundant in all treatments, except in the control ( B - present day ), where Bacteroidetes affiliated sequences were most frequent. Statistically significant differences were detected between A and B compared to D scenarios, while the bacterial communities from C and D were statistically indistinguishable. Diatoms plastid affiliated sequences was the only group showing a treatment related trend and increasing with rising pCO2. Cyanobacterial sequences belonged to three different orders ( Chroococcales, Oscillatoriales and Nostocales ) in A treatment, but in D Chroococcales sequences were found exclusively.

Findings suggest that exposure during the early stages of biofilm development to short-term high CO2 levels significantly decreases algal diversity and promotes a shift towards diatom and filamentous green algae dominated biofilm communities. This confirms results of previous similar studies.
The average respiration and production rates suggests that investigated communities were net autotrophic, indicating a dominant phototrophic component in the biofilms.
Assumption of algal dominated community is supported by the fact that C:N ratios are slightly higher than the Redfield ratio. Authors hypothesize that EPS production under stress due to nutrient limitation or to CO2 bubbling treatment could elevate C and N contents, leading to exceeding the Redfield ratio, but they suggest that further research is required to investigate this hypothesis, as well as the possibility of enhanced EPS production as a protection against harmful UV- radiation, an important factor to consider in Australia.
Regarding bacterial community composition, authors suggest increasing Bacteroidetes in the biofilm may be due to the fact that many members of this group excrete exoenzymes to decompose high molecular weight organic material from detritus, which is a process accelerated by high pCO2; a potentially higher EPS production under higher pCO2 may give them a selective advantage due to the more effective acquisition of catabolic substrates. This leads to less community variability found at high pCO2.

Hence, high pCO2 affects community variability so that communities become more specialized and adapted to lower seawater pH. This could have relevant implications for ecological processes, because bacteria play a key role in biogeochemistry, especially regarding larval settlement and development.

Although many results remain not clear and further investigations are necessary to draw conclusions for future ecological scenarios, this study represents a good starting point for future research and a good multidisciplinary approach example, which lead us consider the whole issue we want to investigate, taking into account all possible related factors and so all the consequences on global scale.

Reference: Witt, V., Wild, C., Anthony, K. R. N., Diaz-Pulido, G. and Uthicke, S. (2011), Effects of ocean acidification on microbial community composition of, and oxygen fluxes through, biofilms from the Great Barrier Reef. Environmental Microbiology. doi: 10.1111/j.1462-2920.2011.02571.x