NEWS Every year news media report on troublesome blooms of cyanobacteria. The problems are related to the eutrophication of the Baltic Sea, and with the climate change the blooms are predicted to worsen. Cyanobacteria seem to be extremely capable of adapting to changes in the environment. However, it is still not clear what exactly regulates the growth of the cyanobacteria. What makes them so adaptive? …and are the blooms of cyanobacteria as homogenic as we have presumed? A group of EcoChange researchers decided to look deeper into the subject.
Indeed, cyanobacteria are bacteria. But in contrast to other bacteria, and just like phytoplankton, they do photosynthesis. They are larger than other bacteria, and they appear either as single cells (unicellular), filaments or in colonies. They are a natural part of the ecosystem, contribute significantly to the total primary production, and they are among the most adaptive organisms on earth. The filamentous cyanobacteria of the Baltic Sea are capable of nitrogen fixation, and thereby provide new nitrogen to the aquatic ecosystem. They cause problems in aquatic environments during massive summer blooms, and in some cases the filamentous cyanobacteria develop toxins. This of course enhances the problems.
In recent decades the cyanobacteria blooms have increased, and the blooms have started earlier than before. The changes have been assumed to be connected with the eutrophication problems of the Baltic as well as the climate change. However, it is still not clear what exactly regulates the growth of the cyanobacteria. A group of EcoChange researchers decided to look deeper into the subject, and hauled off to the waters in the Kalmar strait and north of Öland. Water samples were collected along transects from the coast to the open sea, from April to October during two years.
The cyanobacteria in the Baltic Sea appear either as filaments, colonies or as single cells. The filamentous species have the capability to fix nitrogen, which means that they have access to the ubiquitous supply of atmospheric nitrogen. Some of the filamentous cyanobacteria produce toxins, and thereby cause problems in the environment. Blooms of filamentous cyanobacteria develop during the summer season in the Baltic Sea, particularly in the central and southern parts. When the weather is calm the blooming cyanobacteria float in large patches on the surface of the sea.
Among the filamentous cyanobacteria Aphanizomenon sp is the most common in the Baltic Proper. In this study it was found in the water all year around, and during summer it dominated the biomass. Nodularia spumigena, another common filamentous cyanobacteria, was just found in the water during the summer season. This is probably due to a life strategy to overwinter in the sediments of the sea bottom. Dolichospermum seems to use the same strategy as Nodularia, and spends the winter period in the bottom sediments. Both Nodularia and Dolichospermum are capable of producing toxins. The unicellular cyanobacteria were found all year round in the water.
Many of the cyanobacteria are impossible to determine to species level by using microscope, and the knowledge about the diversity of cyanobacteria has been limited. The EcoChange researchers completed the microscope classification with molecular methods, to get a better picture of the taxonomic diversity and variety of life strategies among the cyanobacteria. The molecular analysis revealed a far larger diversity among cyanobacteria than was earlier known; over five hundred genetically different cyanobacteria were found. This probably explains these organisms great ability to thrive in a changing environment.
So, what regulates the occurrence of cyanobacteria? The extensive blooms are often mentioned in a nutrient context, and the eutrophication situation and status of nitrogen and phosphorus in the waters of the Baltic Sea are known to have an impact on the extensive cyanobacteria blooms. However, in this study salinity and temperature explained most of the variations in the cyanobacteria communities. Both salinity and water temperature are predicted to change in the future, which makes these results important for future measures in the Baltic Sea region.
The unicellular cyanobacteria appeared in conjunction with the blooms of filamentous cyanobacteria, which implies that there is an interaction between these two groups. Maybe the unicellular cyanobacteria are able to use nutrients which have leaked from the filamentous cyanobacteria blooms.
Future scenarios for the Baltic Sea predict higher temperature and lower salinity. Surely, this will have a great impact on the cyanobacteria community, as well as on the whole marine ecosystem. In this study, the EcoChange researchers identified the predicted changes in the environment as favourable for opportunistic filamentous cyanobacteria, such as the toxic Nodularia spumigena. Further studies of the regulating factors are necessary to confirm these results, as a way of assessing climate change effects on the Baltic Sea ecosystem.
Photo: Emil Fridolfsson
Mireia Bertos-Fortis, PhD studentLinnaeus University, Kalmar+46 480-44 61 61
mireia.bertos.fortis@lnu.se
Editor: Kristina Viklund