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Scientists studying microbes in 'desert' waters of the South Pacific Ocean intrigued by how the organisms adapt

The South Pacific Gyre, which covers 10% of the ocean’s surface, is one of Earth's remotest and least-studied places. The region carries some of the cleanest waters on Earth

An expedition, stretching from South America's Chile to New Zealand, has documented the tiniest residents of one of the world's most inhospitable regions: the South Pacific Gyre. The findings that are a part of a new study also suggests that one microbial inhabitant could have adapted to survive on the surface waters.

Located in the center of the South Pacific Ocean, the South Pacific Gyre is one of Earth's remotest and least-studied places. It carries some of the cleanest waters on Earth. It covers 37 mil­lion km2, which equals 10% of the ocean’s surface. The US, on the other hand, occupies less than 10 mil­lion km2. Despite its enormous size, the stretch is a biological desert. The waters are nutrient-poor or ul­trao­l­i­go­trophic. The region also experiences dangerously high UV levels. However, the deepest part of the ocean appears to be shielded. It is home to phytoplankton, the primary producers in the aquatic food chain.

There is some mystery surrounding the inhabitants. "The primary reason for the lack of microbial data on the South Pacific Gyre is due to its vast size and remoteness. Few scientific expeditions have traversed it due to high expedition costs," researchers from Max Planck Institute for Marine Microbiology wrote in their study.

To fill the knowledge gap, the team probed the waters along a 7,000 kilo­metre stretch from Chile to New Zea­l­and. They hoped to uncover more on the microbes, including its diversity and abundance. Helping them was a sophisticated instrument designed to scan and sequence microbial DNA in the makeshift lab onboard. "We de­veloped a novel on-board ana­lysis pipeline, which de­liv­ers in­form­a­tion on bac­terial iden­tity only 35 hours after sampling," Greta Re­intjes, one of the authors of the study, said in a statement.

During the cruise, they collected 147 samples from multiple depths --from the surface to the seafloor-- at 11 stations and profiled them. The surface waters hosted fewer microbes than other parts of the oceans. “It was prob­ably the low­est cell num­bers ever meas­ured in oceanic sur­face wa­ters," Bernhard Fuchs, one of the study's authors, said. Further, the identified microbial inhabitants were familiar to the scientists, as the same resided in other nu­tri­ent-poor ocean re­gions. Some of them include, Prochlorococcus, SAR11, SAR86 and SAR116, Fuchs added. But one organism in the surface waters took the researchers by surprise:  AE­GEAN-169, which, until then, occupied only deeper reaches of the ocean.

Like phytoplankton, various bacteria showed a propensity for the deepest part of the oceans, according to Fuchs and his colleagues.“The com­munity com­pos­i­tion changed strongly with depth, which was dir­ectly linked to the avail­ab­il­ity of light”, Re­intjes explained. For instance, Prochlorococcus was sparse on the surface but more prevalent at 100 to 150 meters below.

The exception was AE­GEAN-169, which appeared to have developed an affinity for the surface. “This in­dic­ates an in­ter­est­ing po­ten­tial ad­apt­a­tion to ul­trao­l­i­go­trophic wa­ters and high solar ir­ra­di­ance," Re­intjes added.  However, it is also likely that mul­tiple spe­cies exist within this group. The team will carry out further stud­ies to ex­am­ine their im­port­ance in the nutrient-deficient wa­ters of the South Pacific Gyre, she added.