Surprising bacteria discovery links Hawaiʻi’s groundwater to the ocean

A new species of bacteria has been discovered off the coast of Oahu, shedding light on how invisible microbial life links Hawaii's terrestrial and marine ecosystems.

Researchers from the University of Hawaii at Manoa have identified Caulobacter inopinatus, a previously unknown species of bacteria found in seawater collected near a beach on the south shore of Oahu. discovery – published October 16 at International Journal of Systematic and Evolutionary Microbiology— was unexpected because all other known species of the genus Caulobacter (a scientific group that includes closely related species) originate from freshwater or soil environments, not the ocean.

The discovery, made during a marine microbiology class at the University of Manoa, began with a routine demonstration of how to grow bacteria from seawater samples. When one colony growing on a petri dish looked different from all the others, further research confirmed that it was something completely new.

From land to sea

Scientists have found that C. inopinatus cannot survive in salt concentrations typical of seawater, despite being isolated from it. This paradox prompted researchers to figure out how it ended up in the ocean. They determined that it was most likely transported from land by submarine groundwater discharge—the natural movement of fresh groundwater across the seafloor and into the sea.

These discharges are known to transport nutrients and pollutants into coastal waters; in this case they can also transport terrestrial microorganisms. Microbial metabolism is important because bacteria play a critical role in nutrient cycling, water quality and the health of coastal ecosystems.

“Understanding how microbes move between land and sea helps scientists track the flow of nutrients and pollutants that can impact coastal water quality, fisheries and the health of coral reefs—issues that directly impact Hawaii's communities and economy,” said study co-author and UH Mānoa School of Life Sciences professor Stuart Donachie. “Discoveries like C. inopinatus help us better understand how land-based activities and natural processes influence the marine environment at the microscopic level.”

The species name inopinatus comes from the Latin word for “unexpected”, reflecting both the serendipitous nature of its discovery and its surprising characteristics.

The study honors the late UH Manoa geosciences professor Craig Glenn (1954–2024), whose pioneering research about underwater groundwater discharge in Hawaii helped shed light on how fresh and seawater interact along the island's coastline, as did former UH student Justin Bukunt (1983–2011), whose early research on groundwater discharge at Kawaikui Beach Park provided the basis for this discovery. Their contributions continue to inspire new generations of scientists exploring Hawaii's unique coastal environment.