Hidden beach microbes help protect coastal waters

Beneath the sands of beaches exists a concealed world brimming with life. Researchers at Stanford University have recently analyzed how microbial communities within coastal groundwater react to the intrusion of seawater. 

Published in the journal Environmental Microbiology, the study highlights the remarkable diversity of these vital ecosystems and explores their potential responses to the challenges posed by rising sea levels.

These findings highlight the essential role of microbes thriving in coastal groundwater just beneath sandy beaches. 

Although largely unnoticed, these microorganisms help regulate the flow of nutrients and chemicals between land and sea, contributing significantly to the maintenance of clean coastal waters. 

As sea levels rise, a deeper understanding of how these ecosystems function will be crucial for their protection.

Vital role of microbial communities

“Beaches can act as a filter between land and sea, processing groundwater and associated chemicals before they reach the ocean,” said study co-first author Jessica Bullington, a Ph.D. student in Earth system science at Stanford University.

“Understanding how these ecosystems function is key to safeguarding their services in the face of sea level rise.”

Microbial communities in these underground settings are responsible for breaking down excess nutrients – such as nitrogen from natural plant decay or from agricultural and wastewater sources – that would otherwise flow unfiltered into the ocean. 

By decomposing these materials, the beach microbes effectively clean the water and play a pivotal part in balancing coastal water chemistry.

Microbial community composition and resilience

Researchers conducted their investigation at Stinson Beach, a site north of San Francisco. The beach exemplifies a “high-energy” environment, where the surf is vigorous, yet few detailed scientific examinations of its microbiome exist. 

Over two weeks, the team collected around-the-clock samples during both wet and dry seasons to capture varying tidal stages. By analyzing the microbial DNA with high-resolution gene sequencing, they obtained unparalleled insights into microbial community composition and resilience.

The results showed that microbial populations were generally stable amid changing tidal conditions and seasonal fluctuations. However, an event known as wave overtopping – when ocean water surged onto the beach’s aquifer – significantly altered the microbial makeup. 

These overtopping incidents, expected to become more common with rising seas, can disrupt microbial filtering processes.

Beach microbes in complex communities

“These microbes live in complex communities, many with specialized roles that include processing nutrients and even producing or consuming greenhouse gases,” said co-senior author Christopher Francis, a professor of Earth system science at the Stanford Doerr School of Sustainability. 

“The microbial community’s resilience under typical conditions is encouraging, but disturbances like wave overtopping highlight their vulnerability to climate change,” added co-first author Katie Langenfeld, who is now a postdoctoral fellow at the University of Michigan.

By identifying the ways in which these microbes respond to sudden inflows of seawater, the research underscores how delicate the system can be. 

Frequent disturbances make it more difficult for microbial communities to carry out tasks like nutrient processing, potentially leading to cascading effects in nearby ocean waters.

Implications for coastal management

Stinson Beach and similar coastal areas face a future shaped by rising seas. As water levels increase, beach sands are likely to shift, altering groundwater flow and threatening to undermine the natural services that subterranean microbial communities provide. This research, therefore, offers a timely contribution to debates on coastal resilience.

Study co-senior author Alexandria Boehm is a professor of environmental studies in the Stanford Doerr School of Sustainability and the Stanford School of Engineering.

“We rely on these microbial communities for essential biogeochemical cycling at the land-sea interface,” said Professor Boehm. “If their capacity diminishes due to climate impacts, we could see cascading effects on coastal water quality and marine life.”

A framework for future studies

With these findings, scientists now have a more comprehensive foundation for understanding how beach groundwater ecosystems function and cope with environmental changes. 

The findings at Stinson Beach can help guide coastal policymakers who must balance the needs of development, conservation, and climate adaptation.

By recognizing the importance of microbes as natural water purifiers, strategies can be put in place to preserve or restore beach habitats in ways that bolster microbial stability.

The ongoing research at Stanford – led by Bullington and her collaborators – adds to the growing body of work on coastal health. These communities beneath the sand remain largely out of sight, yet their ecological significance is increasingly clear. 

As sea levels rise and human impacts intensify, the tiny inhabitants of beach groundwater may become crucial allies in preserving the balance and cleanliness of coastal zones worldwide.

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