From the beginning of the industrial revolution to the present day, human activities have released a large amount of greenhouse gases into the atmosphere, the most notable of which is carbon dioxide, CO2. Global warming is one of the consequences of the increase in the concentration of CO2 in the atmosphere, but it is not the only one. The oceans, through their most superficial layer, function as sinks for this atmospheric CO2, absorbing a large part of this excess gas. When CO2 dissolves in the oceans, changes occur in the chemical balances of seawater that result in a decrease in pH. This process, which is already occurring today, is known as Ocean Acidification. It is estimated that by the end of this century the decrease in pH, acidification, could range from 0.3 to 0.7 units, potentially having serious effects on marine ecosystems, their organisms and the relationships that are established between them and with the environment. Heterotrophic bacteria in the oceans are a very important part of these marine ecosystems since they are responsible for recycling all the organic matter produced by other organisms and thus allowing these nutrients to be reused in the ocean food chain.
Doctoral student Antonio Fuentes Lema, directed by Dr. Cristina Sobrino García, both belonging to the Biological Oceanography Group of the University of Vigo and the Océano-ECIMAT group, with the collaboration of researchers from the Spanish Institute of Oceanography, the Marine Research Institute-CSIC and the Institute of Marine Sciences of Andalusia-CSIC, carried out a study with the aim of knowing the responses of the bacterial communities of the Ría de Vigo and their relationship with the phytoplankton communities through the organic matter that they produce in the face of the future scenario of ocean acidification.
The experiments took place at ECIMAT and consisted of incubations of seawater samples from the Ría de Vigo in the open-air mesocosms owned by ECIMAT. In half of the tanks, the ocean acidification conditions expected for the end of this century were simulated by aerating the cultures with a mixture of atmospheric air and commercial CO2 gas. In the other half of the cultures, only atmospheric air was aerated in order to simulate current ocean conditions. In addition, incubations of heterotrophic bacteria were carried out in the isothermal facilities of ECIMAT, thus maintaining conditions of darkness and constant and controlled temperature. As in the case of phytoplankton, half of the experimental units were aerated with the mixture of air and CO2 and the other half only with atmospheric air. Finally and very importantly, the inocula of acidified and non-acidified organic matter obtained in the phytoplankton incubations described previously were added to all bacterial cultures. During the experiment, the evolution of bacterial communities and organic matter produced by phytoplankton was monitored.
As a result, it was observed that the bacteria of the Ría de Vigo did not respond directly to the effects of ocean acidification, because in the experiment carried out it was found that the bacteria were indifferent to the different pH conditions in which they were incubated. However, the bacteria did respond differently to the addition of organic matter that was produced by phytoplankton in pH conditions similar to the expected scenario of ocean acidification. It was observed that in the middle of the incubation period, the bacteria grew in greater numbers when they used organic matter produced in acidic environments as sustenance. On the contrary, in the last moments of the experiment, the production of these bacteria was greater when incubations were carried out where organic matter produced under normal pH levels was added. Therefore, in a future scenario of acidification, the bacterial population would be lower as time goes by.
The researchers highlight with these results that the bacteria of the Ría de Vigo would not respond directly to the effects of ocean acidification that could occur in the Ría de Vigo at the end of the century, however there are other ways of interaction at the ecosystem level between different organisms and their relationships that seem to produce effects in an indirect way in the bacterial communities. This study is presented as the first of many since it is important to know more in depth the response of these interactions to the new threats that are to come.
This study is contextualized within the thesis of the student Antonio Fuentes Lema, who tries to answer how the increase in ultraviolet radiation that reaches the sea surface and the increase in the concentration of CO2 in the atmosphere play a very important role in the planktonic communities of the open oceans and coastal environments.
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