NEWS DNA-based analyses detect more species than traditional microscopy in marine environments. At the same time, they are not yet sufficient to replace the methods currently used in environmental monitoring. This is shown by a new study from the Baltic Sea, the Kattegat and the Skagerrak.
Water sampling with a hand net for environmental monitoring.
Image Martina JeutheThe study compares DNA metabarcoding with classical light microscopy, which is currently the standard method in phytoplankton monitoring. The material comprises 232 water samples from 17 stations within the Swedish national marine monitoring programme.
The results point in two directions. The DNA method identifies more groups and more species, while also capturing organisms that are difficult to detect under the microscope. However, when it comes to estimating how much plankton is present in the water, the methods provide different pictures, and the relationship between them is often weak.
This means that the two approaches in practice answer different questions. Microscopy continues to provide a basis for assessments of quantity and biomass, while DNA analysis broadens the picture of which organisms are present.
Agneta Andersson, Professor at Department of Ecology, Environment and Geoscience, talking at IceLab Lunch Pitch.
Image[Simon Jönsson]–It is important to combine these two methods because phytoplankton constitute a key group for assessing the state of the marine environment, both in terms of biomass and biodiversity, says Agneta Andersson, Professor at Umeå University and scientific coordinator for EcoChange.
The study also shows that the differences between the methods vary. For some organism groups, the results are more consistent, for others less so. The patterns also shift between different marine areas.
One clear result is that the DNA method more frequently detects small and hard-to-identify species. This contributes to a more detailed description of biodiversity, particularly in complex environments where many species resemble each other morphologically. At the same time, there are limitations linked to reference databases and to how DNA signals are interpreted.
The challenge of linking DNA data to actual quantities of organisms in the water remains. Despite various attempts to adjust the analyses, it is currently not possible to reliably convert DNA signals into biomass or cell numbers.
This means that DNA metabarcoding currently functions best as a complement, rather than a replacement.
–Development must take place on several fronts and follow a well-considered strategy and process, where a cross-section of species in different size classes is captured across different marine areas. This will contribute to increased knowledge of biodiversity and to improving reference databases.
There is also a need to develop quantitative DNA methods, possibly by gaining a better understanding of gene copy numbers in different phytoplankton and by further developing spiking methods using synthetic DNA, Agneta says.
With that said, the results clearly point forward. The method can help detect changes in species composition and provide information that is currently missed in routine monitoring. At the same time, further development is required, both in reference databases and in the interpretation of results, for the technique to play a larger practical role.