UK scientists discover oceanic plants feast on bacteria

11th August 2008

 

The world’s oceans are teaming with tiny planktonic algae that like most plants use sunlight to make the food they need. But a new study published in the scientific journal Nature show that these microscopic plants, astonishingly, also consume large numbers of bacteria, acting in effect as mini-predators.

 

The discovery of just how significant algal feeding on bacteria is in the ocean was made by Dr Mike Zubkov from the National Oceanography Centre, Southampton and Dr Glen Tarran from PML during a scientific study in the North Atlantic Ocean last summer aboard the RRS Discovery.

 

Feeding on bacteria provides the algae with neatly packaged, concentrated, readily available nutrients, giving them a competitive edge in the ocean. This is significant because, although the algae may be tiny they are at the basis of the marine food chain and their numbers are astronomical. Consequently, their combined activities affect the ocean’s life and world’s climate.

 

This finding challenges the perception that algae use only sunlight and dissolved mineral nutrients to grow. Also, it was previously believed that most of marine bacteria were consumed by specialized predators – one-celled animals called protozoa.

 

Mike Zubkov explained, “We were amazed to find that the algae we studied were eating bacteria. What is more, algae were doing a lot of it. The combined bacterial consumption by the algae equalled or exceeded the bacterial consumption by protozoa."

 

“In all of the experiments protozoa ate between three to five bacteria every hour. This would be like us eating about our own body weight in whole chicken-sized bites every day! Similar sized algae only ate about one bacterium each every hour, but, because there were more of them, the algae were collectively eating at least as many bacteria as the protozoa.”

 

The experiments were conducted over three weeks in an area 250 miles to the south of Iceland in August 2007. Although algae are able to use the sunlight energy to make all of the food they require, eating bacteria provided 25 per cent of their overall needs. This shows how difficult it is to put these tiny organisms into clearly defined ‘boxes’ like ‘plant’ or ‘animal’ in terms of their feeding habits.

 

Dr Tarran said: “25 years ago very little was known about the really small algae or bacteria in the ocean. Now, every year we are making important discoveries about which ones are present, what they do, how they interact and how globally important they are.”

 

The current discovery was made possible through the development of new sensitive techniques involving the labelling of native oceanic bacteria with tiny amounts of radioactivity. Algae and protozoa that ate the labelled bacteria took on some of the radioactivity, allowing the scientists to track who was eating what.

 

This October, Dr Zubkov will lead a team of six scientists from the National Oceanography Centre, Southampton, PML and the University of Warwick, sailing on board the RRS James Clarke Ross to continue this work and to evaluate the ocean-scale significance of the observed phenomenon. They plan to extend this study into the Antarctic sector of the Atlantic Ocean, coming back at Christmas.

Further exploring how protozoa and marine algae control marine bacteria through predation will help explain the functioning of the nutrient-poor oceans, the Earth’s largest ecosystems, which profoundly affect global nutrient cycles and climate.