As one moves north, Maine's kelp forests are being overtaken by dense mats of carpet-like turf algae. The steady decline of kelp, and transition into this new state, is altering the ecosystem all the way down to the microbial level, according to a new study in PNAS from Bigelow Laboratory for Ocean Sciences.
The research shows that microbes play an important role in kelp forests — a role that is different on turf-dominated reefs, where the types of microbes present, and the chemical compounds they use and produce, are fundamentally different. That finding highlights how the loss of foundation species due to environmental change cascades through all levels of the ecosystem, altering basic biochemical processes and species relationships.
"We've shown in the past that the loss of kelp leads to changes in the ecology and habitat value of our coastal reefs; we now show that this loss ripples out, all the way down to microbial scales," said Senior Research Scientist Douglas Rasher, the study's senior author. "The relationships we're unraveling between these underwater forests, their microbial members, and the chemical landscapes they collectively create are invisible to the naked eye but may be critical to kelp forest functioning and resilience in this warming world."
Rasher and his team have previously shown how the steady decline of kelp forests driven by human-induced warming impacts ecosystem services , biodiversity , nutrient cycling , and food web dynamics .
It remained unknown, however, what impact this shift had on the resident microbial community and how it functions. This is one of the first studies to grapple with that question in the context of cold-water kelp forests. The new research also provides a unique perspective by focusing on the microbial community living on the reef, rather than in the water column or on individual kelp fronds.
"We know from other ecosystems that when these foundational habitats collapse, that has widespread impacts on the microbial community, which is extremely important to the health and functioning of reefs," said the study's lead author, Shane Farrell, a former University of Maine PhD student in Rasher's lab. "We also know that there's distinct microbial communities in the water, on kelp surfaces, and even within the algae; we asked ourselves what would be the broadest and most ecologically important scale to consider, and that's the reef itself."
That kind of ecosystem-level approach required a novel synthesis of traditional ecological methods and cutting-edge molecular techniques. Focusing on six sites — three dominated by turf algae and three by kelp — during both spring and summer, the team combined dive surveys to characterize the algae community; metagenomics to understand what microbial species are present and what biochemical processes their DNA codes for; and metabolomics to get a snapshot of the chemical compounds present at different times.
The findings reveal that kelp forests and turf-dominated sites are characterized by taxonomically and functionally different microbial species, all producing and using different suites of chemicals. This aligns with what other scientists have seen in response to similar state shifts on coral reefs and in terrestrial forests.
The study does not provide measurements of the precise rates of metabolic processes underway on the reef; instead, it provides a snapshot of what microbes and compounds are present, which can be used to infer how the ecosystem is likely functioning.
For example, the loss of kelp, and the associated loss of canopy cover, appears to increase the abundance of photosynthetically-active microbial communities. Synechococcus, a type of cyanobacteria that tends to be more common and active in warmer, well-lit waters, was rare at the kelp forest sites but abundant on turf-dominated reefs, suggesting a possible shift to photosynthesis as a primary energy source. These kinds of microbial-scale changes, the authors say, could affect the beneficial services these ecosystems provide to humans and alter fundamental processes like nutrient retention and carbon storage, which are rich areas for future research.
"Every habitat is an integrated system of all the organisms it contains, even those that cannot be seen," said Senior Research Associate and co-author Tim D'Angelo. "These results reinforce that microbial life is altered by environmental changes and, in turn, is an engineer of its own environment, so a full understanding of ecosystem change requires investigation of its microbial component."
This study was supported by the NSF Established Program to Stimulate Competitive Research (Grant #OIA-1849227), the Louise H. & David S. Ingalls Foundation, the PADI Foundation, the Essex Avenue Foundation, and the German Research Foundation
