Bacterioplankton Lecture Notes

‭Bacterioplankton‬‭:‬ ‭Bacteria in the sea are ubiquitous, but even more abundant in sediments. Due to their size they fall‬ ‭within the picoplankton size range. Free bacteria in the column are smaller than those associated‬ ‭with particulate organic matter, which is common in the sea. Bacteria also have variable‬ ‭concentrations depending on the environment, more abundant in coastal waters than oceans, but‬ ‭even more so in sediments (billions of cells per ml). In the sea, prokaryotes include bacteria and‬ ‭archaea: until not long ago, the latter were considered typical of extreme environments, with high‬ ‭salinity, temperature and anoxia. More recently they have been found to make up a significant‬ ‭fraction of the picoplankton in most marine environments, so they are also ubiquitous. For‬ ‭example, in temperate waters between 200 and 600 meters they can represent between 30 and‬ ‭60% of the bacterioplankton. In terms of abundance, bacteria and archaea are just 10% of‬ ‭viruses, but if we consider the biomass they represent 90% of all the microbial components‬ ‭present in the pelagic domain. We underline that microbial components include viruses, bacteria,‬ ‭archaea, unicellular algae and protozoa. These indications apply to‬ ‭the entire pelagic domain, but if we have to distinguish based on depth things change: in‬ ‭surface waters the protists can represent more than half of the microbial biomass, but if‬ ‭we go further down to the mesopelagic and bathypelagic level, the bacteria contribute to‬ ‭the total biomass with a very small percentage, lower in percentage than that of viruses. In‬ ‭the water column, bacteria can be free or attached to organic or inorganic suspended‬ ‭particles. In this case we talk about eubacteria. Epi Bacteria are more abundant in‬ ‭particulate-rich coastal waters while free bacteria are prevalent in the open ocean. The‬ ‭presence of bacteria varies in the column, from the surface to almost 4000 m depth: these‬ ‭variations are linked to variations in density where the density increases, because the‬ ‭thermohaline characteristics of the water mass change because they change, we can find‬ ‭an accumulation of organic substance in sedimentation from above and therefore a‬ ‭concomitant accumulation of bacteria. The majority of bacteria in the sea are represented‬ ‭by heterotrophic bacteria, especially gram -, the numerically most abundant component.‬ ‭Marine bacteria can then be aerobic, anaerobic, facultative anaerobic, therefore they can‬ ‭develop although in different communities in all environments, with different levels of‬ ‭oxygenation. Heterotrophic bacteria play an important role in the degradation of organic‬ ‭matter and the regeneration of nutrients.‬ ‭Phototrophic bacteria are red or green bacteria that capture light through a bacteriochlorophyll and‬ ‭carry out anoxygenic photosynthesis, in which the synthesis of organic molecules is carried out using‬ ‭hydrogen sulfide rather than water as an electron donor. Due to their characteristics, these bacteria are‬ ‭typical of particular environments, such as illuminated, anoxic waters, rich in reduced compounds‬ ‭such as hydrogen sulfide or hydrogen sulfide. The figure shows an example with a peak abundance of‬ ‭phototrophic bacteria in a basin with poor water exchange. Here a strong stratification forms which‬ ‭prevents any exchange between warmer surface waters and deep waters and this leads to anoxic‬ ‭conditions. Starting from the pycnocline, with the disappearance of dissolved oxygen the presence of‬ ‭hydrogen sulfide increases. Right at‬ ‭At the pycnocline level, bacteria accumulate, favored by the still presence of light but by the lack of oxygen‬ ‭and the‬ ‭presence of hydrogen sulfide and hydrogen sulfide. There is also a third component of bacteria,‬ ‭chemosynthetic bacteria, which are still autotrophic bacteria which however obtain the energy‬ ‭necessary for the organization of carbon, starting from the oxidation of inorganic compounds‬ ‭(sulfur bacteria, ferro bacteria, nitrifying bacteria). In the sea, bacteria use three sources of organic‬ ‭matter, DOM, POM, and direct attack on larger organisms. Bacterioplankton has a high‬ ‭assimilation efficiency when compared to the assimilation efficiency of larger organisms: thanks to‬ ‭this, bacterioplankton is able to convert considerable quantities of DOM and POM into bacterial‬ ‭biomass. With these methods the bacteria recover the considerable presence of dom in the sea by‬ ‭returning them to the marine food network. At the same time, the regeneration of nutrients carried‬ ‭out by bacteria in the photic zone makes these compounds available again for phytoplankton. This‬ ‭regeneration process is essential in tropical waters where there is nutrient poverty due to stratified‬ ‭waters and therefore they are not able to receive supplies from deep waters, where nutrients are‬ ‭not lacking.‬