NEWS Bacterioplankton are lead players in the Baltic Sea ecosystem. Climate changes will lead to a promotion of bacteria, especially in the northern parts. This will have a major impact on the Baltic Sea ecosystem. Ecochange researcher Daniela Figueroa has looked deep into the role of bacterioplankton in the changing Baltic Sea, and has now successfully defended her thesis.
Climate changes are expected to increase the precipitation with as much as thirty percent in higher latitudes during the next century. The runoff from land to lakes, rivers and seas will increase, which means that the Baltic Sea will receive higher river discharges. The discharges will contain large amounts of dissolved organic matter originally produced on land, so called dissolved allochthonous matter. This organic matter contains both carbon and nutrients. In her thesis, Daniela Figueroa shows that the increased discharge of allochtonous matter to the Baltic Sea will play a significant role, altering important features at the base of the food web.
Phytoplankton and bacteria form together the base of the food web, and play leading parts in aquatic ecosystems, catalysing crucial processes and recycling essential substances. Approximately half of the global flux of important elements is processed by these microorganisms. Phytoplankton are primary producers, and use solar energy and inorganic carbon to build up their biomass. Bacterioplankton are secondary producers, which means that they consume dissolved organic carbon to obtain energy for growth and reproduction. Bacterioplankton are very small, normally less than 2 um.
While phytoplankton are an important food resource for zooplankton, the bacteria are often too small for zooplankton to eat. Instead, they are eaten by flagellates and ciliates, which in turn are eaten by the zooplankton. Therefore, when the carbon enters the aquatic food web via the bacteria, it takes additional steps before it has reached higher trophic levels compared to phytoplankton. In her thesis, Daniela proves this to be an important feature in the Baltic Sea ecosystem in times of climate changes.
The organic carbon utilized by the bacterioplankton is either landbased (allochtonous) or originating from organisms in the surrounding water (autochtonous). The landbased carbon originates from leaf and root exudates, litter leakage and metabolites released by different soil organisms on land. It is composed by large and complex molecules of high molecular weight. Therefore it is difficult for bacteria to break down, and is generally considered as a poor source of energy for microorganisms.
The autochtonous organic carbon, on the other hand, is produced and released by for example phytoplankton. Larger organisms also excrete autochtonous carbon, for example due to sloppy feeding by zooplankton. The autochthonous carbon is composed of simple organic compounds with low molecular weight, which makes it easily available for bacterioplankton.
Daniela studied the source of the carbon utilized by the bacterioplankton. The water is a mixture of landbased dissolved carbon and dissolved carbon produced by organisms in the surrounding water. The bacterioplankton consume both types of dissolved carbon, the ratio depending on the concentrations in the water and the availability of specific compounds of the carbon mixture. The thesis shows that different types of bacteria are favoured by the different carbon compounds.
The base of the aquatic ecosystem is always composed by both phytoplankton and bacteria, and the ratio between them differs. In the north part of the Baltic Sea the river discharge is of a large magnitude, and results from the thesis shows the great importance of landbased carbon to the bacterioplankton in this area. Bacterioplankton and phytoplankton utilize and compete for the same nutrient sources, but use different sources for carbon.
It might seem unimportant whether the carbon enters the aquatic food web via phytoplankton or via the bacterioplankton. However, due to the additional foodweb steps in the bacteria case, small changes in the ratio can lead to impacts throughout the whole food web. In each step of the foodweb a large part of the consumed carbon is lost, and therefore more steps means less efficient transfer of energy to higher levels. In other words, more bacteria means less fish.
The bacterioplankton communities are not homogeneous, and Daniela shows in her thesis that they contain a large amount of different groups which respond in various ways to environmental conditions. Both salinity and the composition of the landbased organic matter was proved to affect the bacterial communities and the bacteria’s enzymatic activity. The results indicate that a large river inflow promotes bacteria production, and shifts the bacteria community towards more generalist bacteria. Modifications of the bacterial community are significant, as they may change the function of the whole ecosystem.
The bacterial communities are to a high degree shaped by the capacity of different bacteria to use various carbon sources. The results from the thesis show that the amount of the landbased organic matter is so dominating in the northernmost part of the Baltic Sea that it structures the bacterial community.
One part of the thesis describes a transplantation experiment, where bacteria from the north were placed in waters from the south, and vice versa. For bacteria moved from the north to southern waters, the composition of the bacteria community changed, followed by changes in metabolism and enzymatic activity. When the transplanted bacteria were moved back to the original conditions of the northern Baltic Sea, some of the originally rare bacteria were promoted by the disturbance and became abundant. Bacteria moved from the southern Baltic Sea to conditions corresponding to the northern parts increased their metabolic activity. The move from the southern parts to the north correspond to what is expected in the future in the southern parts of the Baltic as a result of climate changes. The transplantation experiment shows that climate changes generally will promote a shift towards more bacteria production compared to phytoplankton production. This is expected to have a significant impact on the ecosystem.
The Baltic countries have agreed on joint goals for the management of the Baltic Sea environment, and compliance monitoring is used to classify the ecological state of the marine waters. However, the monitoring has not yet focused on the climate change, even though it is expected to induce large alterations on the Baltic Sea ecosystem. Increases in temperature and precipitation will have direct and indirect impacts on all levels of the food web. Organisms at the base of the food web play a fundamental role for the whole ecosystem. In times of climate changes, the phytoplankton and bacterioplankton are the first to be affected. The thesis points out the importance of a holistic approach to management and monitoring of the Baltic Sea environment, including both phytoplankton and bacterial processes.
Editor: Kristina Viklund