LAUSR

Host-associated microbial communities play a fundamental role in the life of eukaryotic hosts. It is increasingly argued that hosts and their microbiota must be studied together as 'holobionts' to better understand the effects of environmental stressors on host functioning. Disruptions of host–microbiota interactions by environmental stressors can negatively affect host performance and survival. Substantial ecological impacts are likely when the affected hosts are habitat-forming species (e.g., trees, kelps) that underpin local biodiversity. In marine systems, coastal urbanisation via the addition of artificial structures is a major source of stress to habitat formers, but its effect on their associated microbial communities is unknown. We characterised kelp-associated microbial communities in two of the most common and abundant artificial structures in Sydney Harbour—pier-pilings and seawalls—and in neighbouring natural rocky reefs. The kelp Ecklonia radiata is the dominant habitat-forming species along 8000 km of the temperate Australian coast. Kelp-associated microbial communities on pilings differed significantly from those on seawalls and natural rocky reefs, possibly due to differences in abiotic (e.g., shade) and biotic (e.g., grazing) factors between habitats. Many bacteria that were more abundant on kelp on pilings belonged to taxa often associated with macroalgal diseases, including tissue bleaching in Ecklonia. There were, however, no differences in kelp photosynthetic capacity between habitats. The observed differences in microbial communities may have negative effects on the host by promoting fouling by macroorganisms or by causing and spreading disease over time. This study demonstrates that urbanisation can alter the microbiota of key habitat-forming species with potential ecological consequences.Microbial–host interaction: Holobionts and harboursArtificial structures in the marine coastal environment, such as piers and seawalls can alter microbial interactions with seaweed hosts. Researchers in Australia, led by Ezequiel Marzinelli at the University of New South Wales, studied the microbial communities growing on kelps in Sydney Harbour. They compared this altered environment with neighbouring natural rocky reefs. The work illustrates the “holobiont” concept, which treats communities of microbes and hosts as holistic entities. Significant findings included an increase in microbes associated with algal diseases in pier piling structures. This study shows that urbanisation of the coastal environment can have ecological consequences on major habitat-forming species such as kelp. The researchers argue that the combination of abiotic factors, such as shade, and biotic factors, such as altered grazing patterns, might be involved.