A new vector of a changing marine environment: the seagrass holobiont

As marine environments are changing under the pressure of ocean acidification (OA), it is important for better understanding of this process to study the effect of decreased pH on crucial ecosystem components like seagrasses. Seagrass meadows act as nurseries for marine fauna, as sediment stabilizers and as primary producers for example. The plants colonize large areas of our tropical marine reefs and its leaves are covered with a biofilm community of epiphytes that further extend the ecological role of the plants. As seagrasses are photosynthetic oganisms, it was generally thought that an increasing CO₂ availability would positively affect the plants. However, we know that for corals – in addition to a weakened skeleton - OA causes a change in the coral mucus, which in turn results in coral diseases for example. It is thus interesting to look at the holobiont of seagrasses to understand the OA impact. Especially since data on this topic is scarce and the literature available describes research that did not address the total epiphyte community accurately. Hassenrück and collegues (2015) investigated how the epiphytic biofilm of Enhalus aroides was affected by increased CO₂ pressure, using molecular tools to avoid the limitations of earlier research.

The effect of OA was simulated with a comparison between seagrass biofilm communities at a natural CO₂ vent in Papua New Guinea (pH 7.8) and a control site (pH 8.3). At both sites, the taxonomic composition of the epiphytes at both sites was determined by 16S and 18S rRNA gene sequencing, leaf age was taken into account and data on carbon and nitrogen content of the leaves was acquired. The latter is additional information needed to distinguish life stage development from the effect of OA on the epiphytes. As E. acroides produces new leaves on a monthly basis and the seagrass shoots used represented the first four life ages, which comes down to a plant settlement period of 4-5 month that is covered in this study. However, growth rates might have increased due to low pH conditions, in which case the shoots represented older life stages as well.

Both carbon and nitrogen content decreased with leaf age, from 33% to 26% dry weight and 2% to 1% respectively. Epiphyte cover increased however, albeit with values three times lower at the vent site than at the control site. It is suggested that this difference is caused by the supression of pH-sensitive organisms at the vent site. Although the epiphyte cover increased less at the vent site, the biofilm communities of E. acroides showed the same richness at both sites, which could imply an increase in scarce epiphytes at the vent site and thus less increase in epiphyte abundance. The results however, show no significant difference in the amount of rare bacterial species per site. The authors don’t say anything about this with regards to eukaryote composition at both sites. Furthermore, three other patterns were found in the epiphyte composition, which differed a lot between the two sites. Only around 30% of the epiphytic OTUs were shared between any two of the samples. This variation was dominantly explained by sampling site, but also by leaf age, albeit four times less. Besides, the composition shifted succesionally with leaf age, which could be explained by the changing organic matter transfer from the leaves to the biofilm. Lastly, also a trend in pairwise similarity was shown between bacteria and eukaryotes. The authors suggested this as a sign of shaping and impacting responses of both communities on one and another, since the correlation found was stronger than could possibly be explained by solely abiotic factors.

The last part of the research done focuses on the taxonomic details of the community structure of the epiphytic biofilms. Most findings are supported by earlier studies and the extensive comparison with other literature characterizes this part of the research paper. This enhances the strength of the article in particular, since the topic of OA impact on seagrass epiphytes is so understudied and thus needs more context. The article covered several research questions and therefore provides the field of environmental microbiology with a lot of new insights. I was especially suprised by the way the seagrass holobiont might be a source of information to indicate the occurance of coral diseases due to OA, but there are many more clues for further research that can be derived from this study.

Article reviewed:
Hassenrück, C., Hofmann, L. C., Bischof, K., & Ramette, A. (2015). Seagrass biofilm communities at a naturally CO2‐rich vent. Environmental microbiology reports, 7(3), 516-525.