BREAKING NEWS: Seagrasses a vector of coral disease under OA?

Apart from corals, seagrass meadows are a crucial component of reef ecosystems (fish nurseries, nutrient cyclers, organic carbon producers and sediment stabilisers). Similar to corals, seagrasses are colonised by microorganisms that form epiphytic biofilms on the leaves.  Therefore a seagrass plant and its epiphytic biofilm can be referred to as a seagrass holobiont. Ocean acidification (OA) on corals can create changes in the composition of the microbial biofilm associated with the coral reducing larval settlement and probably coral health. Seagrasses, on the other hand, are generally thought to benefit from OA due to the increased availability of CO2 and bicarbonate for photosynthesis. However, data on how the epiphytic biofilm on seagrass leaves might respond to OA and on the behaviour of the seagrass holobiont in future OA scenarios are still sparse.

Here Hassenrück et al, (2015) studied the epiphytic biofilm on the leaves of the seagrass Enhalus acroides at a natural CO₂ vent (pH values of 7.8) and a control site (pH values of 8.3) in Papua New Guinea. 18S ribosomal DNA sequences were used to make sure that all the seagrass shoots collected were of the same species. The leaf age was taken into consideration; carbon content of the seagrass leaves decreased with leaf age from approximately 33% to 26%. Epiphyte cover increased with leaf age. 

High taxonomic resolution provided by 16S and 18S amplicon sequencing showed that epiphyte communities seemed to be as diverse at the vent site than at the control site. Automated Ribosomal Intergenic Spacer Analysis (ARISA) identified bacterial and eukaryotic operational taxonomic units (OTU). Overall the results show that bacterial and eukaryotic epiphytes formed distinct communities at the CO₂-impacted site compared to the control site.This study went into a lot of depth on what type of bacteria was found and why, which was really interesting to read. For example OA seemed to decrease cyanobacterial abundance and diversity in microbial films. Also Reinekea, a genus of the Gammaproteobacteria, was mentioned to may play an important role in the degradation of organic matter after phytoplankton blooms. It had a reduced abundance at the vent site, which may have been caused by the decreased availability of degradable material presumably due to the lower percentage of epiphyte cover (which was also looked at). This study may of actually bought to light microbial organsisms, previously not considered in OA research.

Their biggest finding was that they detected an increased prevalence of microbial sequence types associated with coral diseases (Fusobacteria, Thalassomonas [white plague-like disease]) at the vent site under elevated pCO₂ conditions whereas eukaryotes such as certain coralline algae commonly related to healthy reefs were less diverse (which is no surprise!). This agrees with the hypothesis that coral reefs experiencing elevated pCO₂ levels will be more susceptible to diseases than reefs not yet exposed to OA. It further highlights the potential of seagrasses as vectors of coral pathogens and stresses the point that seagrasses should be viewed as a holobiont when making predictions about OA effects and ecological consequences in coral reefs.

 Reference: Hassenrück, C., Hofmann, L. C., Bischof, K., & Ramette, A. (2015). Seagrass biofilm communities at a naturally CO2‐rich vent. Environmental microbiology reports.

Can be found at: http://onlinelibrary.wiley.com/doi/10.1111/1758-2229.12282/abstract