Quick Dips

Creature-Devouring Plants in Coastal Waters!

Dr Julie Hall, NIWA

Recent studies in the ocean off the east coast of the South Island have revealed that many algae -- chlorophyll-containing, single-celled aquatic plants -- are also voracious grazers, consuming both bacteria and other small algae as well as being able to photosynthesise like ordinary plants. The impact of these organisms on other micro-organisms is very significant, with the algae eating at least 50% of the bacteria and small algae that were being consumed by grazers in general. For organisms that aren't supposed to eat, this is a substantial amount of grazing.

Why would these algae want -- or need -- to graze? There are several hypotheses. The first is that these algae are unable to get all the vitamins they need for growth from the surrounding water, so they eat other organisms to obtain these essential vitamins. The second is that the cells are unable to gain enough energy from sunlight alone to support cell growth, so they graze to gain an energy source. The third is that when nutrients like nitrogen and phosphorus are in short supply in the water, cells graze to gain a second source of nutrients.

We believe this last explanation is the most likely. Our study was conducted in the middle of summer, when the water column tends to form layers which do not mix easily, due to density differences caused by summer heating. In summer, the surface waters are warm and tend to be very shallow, only 20 to 30 metres deep.

The growth of algae in this layer was unlikely to be limited by light but was likely to be limited by nutrients such as nitrogen and phosphorous whose concentrations were very low in the upper water column. This means that those algae that were capable of grazing on bacteria and small algae would be able to grow more rapidly than their competitors.

We are not alone in finding algae that eat -- similar results have been shown in other regions of the ocean and also in lakes. This shows we need to rethink the role of algae in the aquatic environment. What we are seeing is a variety of algal types, ranging from those that get all their energy from sunlight to single-celled animals that get all their energy from grazing. Between these two extremes we are finding a wide range of organisms that can do both and are able to choose their energy source, depending on environmental conditions at the time.

Dr Julie Hall works for NIWA