Seaweeds interact with their environment through the algal surface, by exchanging compounds and detecting abiotic and biotic stress. Epiphytic microbial communities modify this surface and are involved in a range of processes that influences the physiological properties of their host. Canopy-forming seaweeds in intertidal and subtidal regions are well known for their high productivity and structural diversity, providing a habitat for a wide variety of marine life. The canopy-forming seaweed Cystoseira (Fucales, Phaeophyceae) can dominate the sublittoral fringe to lower sublittoral zones and contributes to the biodiversity of Mediterranean coastal ecosystems as a canopy-forming seaweed by supporting a divers, associated marine community. Since the 1970's, retracting Cystoseira populations, including C. compressa, were reported, to the point of being locally lost. The mechanism causing this on-going and extensive loss of C. compressa in the Mediterranean Sea is not fully understood. We hypothesize that changes in environmental conditions affect biofilm composition predominantly via physiological response of the host. We expect that the negative synergystic effects on Cystoseira compressa affects the associated epiphytic microbiome which may exacerbate the stress conditions. These negative effects will decrease the stability of the epiphytic microbial community on C. compressa and as a consequence of this decreasing stability, bacterial taxa related with the natural degradation of the brown seaweed are expected to be more abundant than thalli under natural conditions. This associated microbiome of C. compressa is characterized under different treatments, by metabarcoding of the V1-V3 hypervariable regions of the 16S rRNA gene using the Illumina MiSeq platform. These treatments simulate environmental stress imposed on the seaweed and include seawater nutrient pollution, increased temperature and heatwave event. The bacterial community of C. compressa was dominated by species of the Rhodobacteraceae family: 71 out of 455 unique genera in the dataset, and the top 10 most abundant taxa, were classified as representents from this family. The stress imposed by nutrient enrichment had a significant effect on the diversity and eveness on the associated microbiome; nutrient enrichment led to a lower diversity and a higher eveness. 10 taxa contributed 75% to the shift in the bacterial community as a response to nutrient pollution. Indicative that a limited number of OTUs are key in the stress response. The bacterial community showed a similar response to the increased water temperature in terms of alpha diversity, with a higher diversity and lower eveness in warmer water, but this response tested to be not significant. The population and heatwave treatment also tested to have no significant effect on diversity or eveness of the bacterial community associated with C. compressa. The composition of the bacterial community is signficantly altered as a response to the nutrient treatment, increased water temperature and the interaction of the two stressors. The nutrient pollution is dominant in this reaction, it is capable of modulating the impact of the increased water temperature. This means in terms of conservation that through local management of nutrient pollution, the stability of the bacterial community is retained or improved which possibly contributes to the resilience of C. compressa against a rising seawater temperature. Relating the shift in abundances of the six potential pathogenic genera, due to anthropogenic stress, with the results of the study of Mancuso et al. (2016), gave ambiguous results. |
