Grazing, which leads to losses in biomass and drastic declines in total crop production, is one of the main concerns in seaweed aquaculture. This is also thought to affect the composition of the associated bacterial communities which are believed to play a crucial role in determining the host's health and development. Apart from morphological impairment, studying changes in the prokaryotic microbiome composition and predicted functional responses to grazing will allow us to understand the underlying effects of grazing on the seaweed host. This study is the first report of the effect of grazing on the prokaryotic microbiome of two economically important agarophytes, Gracilaria firma and Gracilaria salicornia, by high-throughput sequencing targeting the V3-V4 variable region of the 16S rRNA gene. The results indicated that for G. firma, the microbiome composition of tissues grazed by marine herbivores had significantly more agarolytic bacteria Marinagarivorans sp. and Algisphaera sp. than in ungrazed tissues. The predictive functional metagenomics for this species revealed that grazing escalated the pathway activities related to nucleotide degradation, aromatic compound degradation and aerobic sugar metabolism, while pathways associated with cell wall synthesis, aerobic respiration, vitamin biosynthesis and amino acid biosynthesis were reduced. However, for G. salicornia, the bacterial communities were not significantly affected by grazing. Nevertheless, pathways relating to anaerobic respiration and amino acid, coenzyme and vitamin B-6 biosynthesis in this species were predicted to be more active in grazed tissues, whereas the microbiome of ungrazed tissues had higher activities in bacteriochlorophyll a, fatty acid, secondary metabolite and heme biosynthesis.

The Gracilariaceae are important in the biotechnology industry and microbiology research. Some Gracilaria species are difficult to identify morphologically and molecular phylogenetic data hold a key role in species level identification. In this study, the chloroplast genomes of G. changii and G. salicornia from the Peninsular Malaysia were sequenced to compare with the published taxa in order to gain a better understanding of the phylogenetic questions regarding the red algae, particularly G. changii and G. firma. Phylogenetic analysis for 66 plastid genes showed that G. changii from Malaysia formed a sister lineage with G. firma with a genetic distance of 0.43 %. However, there was a genetic distance of p = 4.32 % between the Malaysian and Philippine samples of G. changii, indicating they are genetically distinct. Five chloroplast genes (petF, rps12, rps14, rpl18 and petB) showed higher variation of genetic distance between Malaysian G. changii and G. firma (p = 1.23–2.00 %) and the intraspecific variation in G. salicornia from Peninsular Malaysia and Hawaii varies from p = 0–0.31 %. The lack of overlap between the intraspecific genetic distance of Malaysian G. changii and interspecific genetic distance of Malaysian G. changii-G. firma indicates that G. changii and G. firma are not conspecific.