Algae stole gene to unlock ice

By John Upton

Polar bears are spectacular inhabitants of sea ice, which is an inhospitable habitat that grows and retreats around the poles with the seasons. But something else lives on the harsh terrain that gets much less attention: Algae.

Algae combines ocean nutrients with energy from the sun to provide whales and other herbivores with a critical source of food, particularly in the early spring.

But how did algae come to survive such challenging conditions? Scientists looking for an answer to this question discovered that some evolutionary larceny was necessary before algae could move into the icy digs.

Effect of algal ice-binding proteins on ice / PLoS ONE

Ice-dwelling algae has a special tool that allows it to live in the uninviting climate. These algae ooze gelatinous ice-binding proteins known as extracellular polymeric substances. The proteins manipulate the salt content and pore structure of the ice as it grows around the algal bloom, making the teeth-chattering microclimate all the more accommodating.

Scientists analyzed the ice-binding proteins produced by three species of sea ice-dwelling algae and reported last month in the journal PLoS ONE that the structure of the proteins was “completely incongruent” with the evolution of algae. That strongly suggested that the genes for producing the proteins came from somewhere else.

Species can hijack genes from other wildlife through a number of processes that scientists call horizontal or lateral gene transfer. Viruses are sometimes involved, but not always.

The ice-binding proteins closely resembled proteins produced by bacteria, leading the researchers to conclude that the algae initially stole the gene from bacterial neighbors.

The scientists sampled similar proteins produced by bacterium at the bottom layer of Antarctic sea ice and discovered that nearly half of its amino acids matched those of the special proteins produced by nearby algae.

“Our results strongly suggest that the [ice-binding protein] genes of sea ice diatoms were acquired from bacteria, possibly in separate events,” concluded the researchers. “The acquisition of these genes was an essential factor in allowing the diatoms to expand their range to polar sea ice.”

Sea ice and some icebergs off eastern Greenland / NASA