Ocean Ecosystem: Classical Model And Microbial Circuit

‭OCEAN ECOSYSTEM: CLASSICAL MODEL AND MICROBIAL CIRCUIT‬ ‭In the classic representation of the ocean ecosystem, phytoplankton is placed at the base of the‬ ‭pelagic food web, phytoplankton is consumed by zooplankton herbivores and in particular by‬ ‭copepods which constitute the most abundant component. In terms of abundance they can‬ ‭represent from 70 to 90% of the zooplankton biomass. So their grazing is definitely relevant. It is‬ ‭possible to analyze the effects of grazing on phytoplankton biomass by referring to copepods.‬ ‭Furthermore, in most temperate waters there are only a few numerically dominant copepod‬ ‭species, usually 1 to 3 species. The trophic relationship that links phytoplankton and copepods‬ ‭can explain the mismatched coupling of the phyto and zooplankton biomass curves that we have‬ ‭seen characterize temperate waters. Optimal conditions lead to an increase in primary production,‬ ‭which then feeds growing populations of copepods on primary production. It is precisely the‬ ‭grazing of copepods that is responsible for regulating phytoplankton abundance. Among other‬ ‭things, copepods have proven to be particularly efficient in consuming phytoplankton. In‬ ‭temperate waters, the decrease in primary production as we saw after the spring peak could be‬ ‭more a consequence than the decline in nutrients. In this case therefore it would not be so much‬ ‭the production that decreases, what decreases is the phytoplankton biomass. In practice,‬ ‭phytoplankton continues to maintain high reproduction rates as it reproduces, but is consumed‬ ‭even more quickly and converted directly into zooplankton biomass. Even in polar waters we have‬ ‭a similar situation, after the summer phytoplankton peak there is a decline corresponding to the‬ ‭zooplankton peak, therefore in certain conditions in temperate seas and polar seas there can be a‬ ‭decrease in algal biomass due to grazing even if the photosynthetic production rate remains high‬ ‭or even increases. The fixed carbon becomes part not so much of the phytoplankton standing‬ ‭crop but of the zooplankton one. So in situations of this type biomass measurements‬ ‭turn out to be bad indicators of primary productivity. The proof that grazing and not nutrient scarcity may be‬ ‭the reductions observed in phytoplankton populations is given by findings in polar seas. For example, in‬ ‭Antarctica, where nutrients are never limited due to the presence of upwelling, phytoplankton collapse after‬ ‭the spring bloom. Further evidence can then be obtained from the observation of the sea surface where one‬ ‭can see alternating dense and dark patches of phytoplankton and less dense and light patches where there‬ ‭are only copepods which have evidently already used the vast majority of the phytoplankton. Some studies‬ ‭have then highlighted how the grazing and therefore the breaking of the phytoplanktonic cells during the‬ ‭passage through the digestive sac of the copepods leads to a continuous release of nutrients which are then‬ ‭excreted and thus return to the water column. Given these conditions, nutrients would be constantly‬ ‭regenerated in the surface layers and productivity would never stop. From this perspective, we can therefore‬ ‭rework and review what was said previously regarding the latitudinal variations of phytoplankton primary‬ ‭production and reconsider the weight of the role played by grazing in limiting the presence of phytoplankton.‬ ‭How can you trigger a phytoplankton bloom in the presence of copepods? The answer lies in the‬ ‭differences between the life cycles of algae and those of copepods. Compared to copepods, algae‬ ‭have shorter generations, the reproduction rate is higher and the population can grow much faster.‬ ‭In the case of copepods there are weeks between one generation and the next, in the case of‬ ‭algae we are talking about days. All this means that when environmental conditions are favorable‬ ‭the primary function is engaged, the abundance of phytoplankton increases with a certain rapidity‬ ‭but for a corresponding increase in herbivores to occur it will take a certain period of time given‬ ‭precisely‬ ‭by the different generation interval of copepods compared to that of algae. When the copepods begin to‬ ‭grow the algae will begin to decrease but as soon as grazing brings the algae resource below a certain‬ ‭threshold, then the copepods also decrease, not only because they have less food available and can also‬ ‭increase predation, but also because a reduced food availability translates into a reduction in the number of‬ ‭eggs produced by copepods.‬ ‭Everything we have detected so far explains what happens in terms of the seasonal evolution of‬ ‭phyto and zooplankton biomass in temperate seas and polar seas. But how can we explain the‬ ‭strange trends of tropical seas? In tropical waters, environmental conditions are almost constant‬ ‭so there is no change that determines the triggering of primary production and a moment in the‬ ‭production rates of phytoplankton. Primary production essentially always fluctuates around slightly‬ ‭different values and so does the phytoplankton mass. Furthermore, no specific relationship‬ ‭between phyto and zooplankton is highlighted and it seems that copepods do not play an effective‬ ‭role in controlling phytoplankton through grazing. In reality, the control of copepods on‬ ‭phytoplankton, as we have described it, is observable in the waters of the temperate Atlantic and‬ ‭the polar South Atlantic but not in the temperate waters of the North Pacific. Here the situation is‬ ‭very different and we even observe a peak in zooplankton biomass which only slightly precedes a‬ ‭slight increase in phytoplankton biomass.‬ ‭The two situations compared: that of the temperate North Atlantic that we already know and that of the‬ ‭temperate North Pacific, as you can see, very different with the only significant peak in zooplankton between‬ ‭June and July and just a slight increase in phytoplankton in October. The difference can be explained on the‬ ‭basis of the different characteristics of the life cycle of the dominant copepods in the temperate areas of the‬ ‭Atlantic and the Pacific.‬ ‭Among the most widespread copepod species in temperate and South polar Atlantic waters‬ ‭is‬‭Calanus finmarchicus‬‭and its biological cycle can‬‭be taken as a general scheme and the nauplius‬ ‭originates from the fertilized egg, then through 6 naupliar stages the copepodite is reached, then‬ ‭through 5 copepodite stages the adult stage is reached. The egg-adult cycle lasts about two‬ ‭months and during spring-summer there is a succession of generations. The last summer‬ ‭generation, however, does not complete the final metamorphosis, but at the last copepodite stage‬ ‭it migrates into depth and hibernates without feeding using its own reserves which are mainly‬ ‭represented by oils and fats. The fifth instar copepodites then complete the final molt at the end of‬ ‭winter/early spring. After this they migrate to the surface and begin to feed, their life cycle (unlike‬ ‭that of the summer generations) lasts 5/7 months. Once on the surface, these copepods do not‬ ‭reproduce until the phytoplankton begins to increase. In this period of time the copepod‬ ‭population reaches its minimum value both in density and biomass. The delay in reproduction‬ ‭instead allows phytoplankton, which has shorter life cycles, to reproduce at high rates and give‬ ‭rise to blooms. The abundance of phytoplankton will then be reduced only later, at the beginning‬ ‭of grazing, by the first new generation of copepods and then by the subsequent ones.‬ ‭Furthermore, the number of generations decreases with increasing latitude since the duration of‬ ‭the season favorable for phytoplankton growth and consequently also for herbivorous‬ ‭zooplankton progressively shortens.‬ ‭Everything that has been described determines a shift over time in the zooplankton abundance‬ ‭curves compared to the phytoplankton abundance curves, even if the trend of both is completely‬ ‭similar.‬ ‭In the temperate area of the North Pacific the situation is different, here the dominant species are‬‭Neocalanus‬ ‭plumchrus‬‭And‬‭Neocalanus cristatus‬‭. They have some differences compared to Calanus‬‭finmarchicus‬‭, first of‬ ‭all they are larger in size, then they feed on both phyto and zooplankton, while‬‭Calanus finmarchicus‬‭it is‬ ‭exclusively herbivorous. The two‬‭Neocalanus‬‭they are‬‭also less efficient in grazing activity, the greatest‬ ‭differences are at the biological cycle level. These Pacific copepods have only one generation per year, they‬ ‭migrate deep as adults and overwinter here, conserving enough energy to mate and produce eggs.‬ ‭Therefore they do not regulate the beginning of the reproductive period on the basis of the presence and‬ ‭of the abundance of phytoplankton, but they reproduce in depth and it will be the larvae that‬ ‭rise to the surface at the beginning of spring and therefore as soon as they rise up they‬ ‭immediately carry out a strong grazing activity. The result of all this is the lack of spring bloom‬ ‭of phytoplankton which is instead contrasted with a peak in zooplankton which precedes only‬ ‭a slight increase in phytoplankton production.‬ ‭Can we find explanations in the biological cycles of copepods also for what we have seen to be the trend for‬ ‭the phyto and zooplankton biomasses of tropical waters? In the zooplankton of tropical waters, the copepod‬ ‭component is made up of small-sized species, species that belong to the genera‬‭Acartia‬‭,‬ ‭Microcalanus‬‭,‬‭Pseudocalanus‬‭, their biological cycles‬‭practically reflect the constant thermal aspect of the‬ ‭waters, the generations are short-lived and follow one after the other all year round without winter‬ ‭interruptions. As a result of all this the phyto and zooplankton biomass curves oscillate around quite similar‬ ‭values in one situation almost balanced.‬ ‭Again for comparison, the trends of phytoplankton biomass (always in blue)‬ ‭and zooplankton biomass (dashed black) are shown in the four situations‬ ‭described and therefore in temperate waters of the North Atlantic and in‬ ‭temperate waters of the North Pacific and below we have the Arctic waters‬ ‭and tropical waters. Tropical waters are the only case in which no clear‬ ‭trace of a relationship between phyto and zooplankton is found.‬ I‭n tropical waters, the lack of a relationship between the dominant copepod cycle and the‬ ‭phytoplankton cycle suggests that the primary consumers of phytoplankton may be other than‬ ‭copepods. In tropical waters the trophic chain is probably of the type: phytoplankton‬‭→‬ ‭protozooplankton‬‭→‬‭copepods, i.e. the protozoa play‬‭the role of primary consumers, they are the‬ ‭real consumers of the phytoplankton while the copepods are mainly secondary consumers who‬ ‭draw on the protozooplankton as a food source. Especially in tropical waters, many species of‬ ‭copepods can switch to using different food sources, even switching from a condition of‬ ‭herbivory to a condition of using other zooplanktons, depending on the availability of resources.‬ ‭Until 30 years ago, the classic model of the ocean ecosystem, the one we talked about, was the‬ ‭only one universally recognized and reference was made to that model to explain the organization‬ ‭and functioning of the marine planktonic system with a trophic chain based essentially on diatoms‬ ‭and dinoflagellates as primary producers and copepods as grazers. But the development of new‬ ‭technologies and new study methodologies has made it possible to detect, characterize and‬ ‭quantify the smallest components such as penkton (??), pico and nanoplankton. This knowledge‬ ‭has led to the delineation of a new and more complex vision of planktonic ecosystems; the roles‬ ‭considered in the classic model for microalgae and copepods have been integrated with other‬ ‭attendance. The classical model remains the predominant one in some areas, particularly coastal‬ ‭ones, upwelling areas and in polar seas. But the classical model has certainly become less‬ ‭important in oceanic areas both at tropical latitudes and in warm temperate seas where microbial‬ ‭components have proven to play a prevalent role. The new model that we can define as‬ ‭"microbial model" because it involves all those small components that are identified as microbial‬ ‭components and first of all puts the role of bacterioplankton in the foreground. So let's take up‬ ‭and point out some aspects that we have already seen in part when we talked about this‬ ‭planktonic component and the consumption of dissolved organic substances by the bacteria.‬ ‭Dissolved organic matter (DOC) constitutes the main carbon reserve in the sea and it is estimated‬ ‭that approximately ¼ of the carbon fixed through photosynthesis then returns to the sea as DOC.‬ ‭The quantity of DOC is also constant and this constancy is guaranteed by the consumption of the‬ ‭batteries.‬ ‭Approximately 50% of the DOC resulting from photosynthetic processes is consumed by‬ ‭heterotrophic bacteria. Then the bacterioplankton (mostly heterotrophic and very efficient in‬ ‭respiration processes) uses the DOC and transforms it into particulate organic substance (POC), in‬ ‭the form of bacterial biomass. In practice, thanks to the bacteria, the DOC is recovered and‬ ‭re-enters the pelagic food chain in the form of bacteria. This role of bacteria, essential in the‬ ‭economy of the oceans, is defined as a microbial loop. But alongside the role of bacteria, the‬ ‭place of smaller producers and their contribution to primary production has also been‬ ‭reconsidered. In tropical and subtropical ocean waters, nanoplankton can represent up to 80% or‬ ‭more of the photosynthetic activity and phytoplankton biomass. Smaller phytoplankton‬ ‭components also show less seasonal biomass variability than microphytoplankton. Moreover, in‬ ‭tropical and warm temperate waters the scarcity of nutrients can favor the smaller components of‬ ‭the photosynthetic plankton which exploit a high surface-to-volume ratio, therefore the‬ ‭nanoplankton absorbs those nutrients more easily and in greater quantities which due to its small‬ ‭size would have a relatively lower need. Another advantage of smaller components in warmer‬ ‭waters is the slowing down of the sinking speed in the column and this is also a winning aspect‬ ‭that should not be overlooked‬ ‭If autotrophic pico and nanoplankton are so abundant, who is consuming them at this point?‬ ‭They are too small to be efficiently captured by copepods or other members of the‬ ‭mesozooplankton equipped with almost similar filtration systems. Attention then shifted to the‬ ‭smaller components of heterotrophic plankton and a notable abundance of flagellates and ciliates‬ ‭emerged that are capable of using autotrophic picoplankton and nanoplankton. Some studies‬ ‭have shown that the respiration rates of heterotrophic nanoplankton are 10 times greater than‬ ‭those of mesozooplankton and this confirms that most of the energy fixed through photosynthesis‬ ‭is consumed by nano zooplankton, particularly in medium-warm waters and tropical waters. .‬ ‭Therefore, summarizing the transition from the classical to the microbial model we can say that‬ ‭the lack of adequate study methodologies had not allowed us to focus on the role of the smallest‬ ‭components, the pico and nanoplankton components, the bacteria were relegated to the sole role‬ ‭of mineralization of organic substance. The trophic chain model was outlined on the basis of the‬ ‭larger pelagic components, and was therefore a considerably simplified model. A model based on‬ ‭producers represented by phytoplankton such as diatoms and dinoflagellates, by consumers‬ ‭represented by phytophagous zooplankton such as copepods, a model then represented at‬ ‭higher levels by zooplanktivorous fish and then by carnivorous consumers of the highest levels of‬ ‭the pelagic food web. With the adoption of the microbial model the food network is integrated and‬ ‭articulated. The microbial model focuses on the role of dissolved organic substances; all the‬ ‭components of the pelagic food network contribute to its formation but the essential contribution‬ ‭is still provided by the primary production of all the phytoplankton. Equally central is the role of the‬ ‭bacteria which recover the dissolved organic substance and transform it into bacterial biomass‬ ‭available to the nanoplankton heterotrophs which in turn can be a food basis for the‬ ‭microplankton heterotrophs, then used by the mesozooplankton together with the phytoplankton.‬ ‭Through the‬ ‭bacteria-flagellates-mesozooplankton circuit, the energy released as DOC can re-enter the trophic‬ ‭chain and be used by other higher trophic levels. It is a highly dynamic system that plays on the‬ ‭presence of DOC (or DOM) regulated by bacteria and on the conversion of DOC into bacterial‬ ‭biomass with high nutritional value. The bacteria therefore create an essential link in returning‬ ‭DOC to higher levels. The trophic chain based on the smallest components of plankton, the‬ ‭microbial components, plays a very important role in the oceans even if it does not completely‬ ‭exclude the classical chain,but overall it scales it down and gives it less weight. The trend that‬ ‭could emerge from a perspective of climate change and water warming is that of an‬ ‭ever-increasing importance of microbial components even in temperate and south-polar waters.‬ ‭The discovery of abundant viral populations, therefore also viruses are the last dimensional‬ ‭category of planktonic "living forms" that has received particular attention, which we have noted‬ ‭when talking about virioplankton has also highlighted the important role of viruses in the pelagic‬ ‭ecosystem . The study of the effects produced by viral infections on both prokaryotic and‬ ‭eukaryotic cells led to the formulation of the viral circuit concert.‬ ‭This is a process within the microbial loop where viruses, through infection and lysis of cells,‬ ‭determine the transfer of living organic matter into the pool of dissolved and particulate organic‬ ‭matter. Viruses divert the flow of fixed carbon as bacterial and phytoplankton biomass from higher‬ ‭trophic levels into the water. The movement of organic matter from the living compartment to the‬ ‭pool of non-living organic matter present in the column in particulate and dissolved form is defined‬ ‭as a viral circuit or viral shunt. The viral circuit exerts a negative feedback control on the microbial‬ ‭circuit; if the microbial loop brings organic matter towards the higher trophic levels, the viral shunt‬ ‭makes dissolved organic matter available which can then be used by the bacteria. Therefore a‬ ‭profound synergistic action of all these microbial components.‬ ‭The viral circuit moves organic matter from heterotrophs and photoautotrophs and the movement is‬ ‭represented in the figure by the red arrows for the movement relating to the lysis carried out by the‬ ‭heterotrophs and by the green arrows for the movement determined by the viral action on the cells‬ ‭phytoplanktons. This movement involves the formation of doc, dissolved organic matter, and poc. The DOC‬ ‭is more labile and tends to be recycled more easily in the photic zone while the more recalcitrant materials‬ ‭that constitute the POC (or POM), for example the cell walls, tend to persist longer in the column and to be‬ ‭exported to depth. The composition of the Dom and the pom does not correspond to that of the wind‬ ‭components from which they derive. The numbers indicated in brackets express the ratios‬ ‭atomic values between carbon, nitrogen and phosphorus and were estimated in the four‬ ‭heterotrophic components and phytoplankton, the two sources and then dom and pom, the two‬ ‭forms of derived organic matter. It can be seen that the presence of carbon in the dom is always‬ ‭lower, we see 66 versus 69 in heterotrophs and 106 in autotrophs. But if you look at the particulate‬ ‭material that is exported the situation is reversed and the carbon actually increases significantly‬ ‭compared to the carbon in the two sources. Therefore the viral shunt has a further very important‬ ‭role, that of enhancing the transfer of carbon towards deep environments, thus increasing the‬ ‭efficiency of the biological pump. The biological pump becomes more efficient when carbon‬ ‭exported to depth increases relative to nutrients, which means a preferential transport of carbon to‬ ‭depth while nutrients remain relatively more available at surface levels where they can be used in‬ ‭photosynthetic processes.‬ ‭These images highlight the trophic relationships proposed by the microbial‬ ‭model (the different components are indicated on the left dimensions of‬ ‭the phytoplankton and the arrows indicate their prevalent use by the‬ ‭various heterotrophic components).‬ ‭The image on the right highlights the trophic relationships between‬ ‭planktonic components in even more detail, integrating the classical model‬ ‭and the microbial model.‬ ‭TROPHICATION, EUTROPHICATION, DEAD ZONES‬ ‭In this lesson we will start with some clarifications on the definition of trophy. Talking about‬ ‭trophies means talking about nutrient availability. Nutrients obviously with the very precise‬ ‭meaning that we have reiterated several times, therefore inorganic compounds of nitrogen,‬ ‭phosphorus and silicon plus a whole series of other elements that we can define as micronutrients‬ ‭sometimes capable of limiting phytoplankton production in some oceanic areas. Eutrophic waters,‬ ‭rich in nutrients, are characterized by an intense development of phytoplankton which can reach‬ ‭high biomass values.‬ ‭Conditions of this type can roughly be traced back to the subpolar regions, vice versa oligotrophic‬ ‭waters (poor in nutrients) have low values of phytoplankton biomass and this condition particularly‬ ‭concerns subtropical oceanic areas. This division into eutrophic and oligotrophic areas is more‬ ‭than anything indicative because particular conditions at different latitudes can lead to results that‬ ‭do not fit into this general scheme, for example the Mediterranean is an oligotrophic sea poor in‬ ‭nutrients and with poor development of phytoplankton biomass, it is therefore for this reason a‬ ‭particularly transparent sea.‬ ‭Can some characteristic traits of planktonic communities be delineated on the basis of water‬ ‭trophy? In eutrophic waters, phytoplankton biomass is higher than zooplankton biomass, larger‬ ‭species generally prevail, diatoms predominate in phytoplankton and zooplankton is particularly‬ ‭monotonous, represented in large part by a few species of mainly herbivorous copepods. The food‬ ‭chains are overall short, less complex than in oligotrophic waters. In oligotrophic waters, however,‬ ‭primary production is low, the zooplankton biomass is generally greater than the phytoplankton‬ ‭one, the phytoplankton is dominated by nano plankton forms, the zooplankton is equally rich in‬ ‭small-sized species which are, among other things, particularly efficient in the intake of food with‬ ‭the possibility of changing food base depending on availability, there are more carnivores than in‬ ‭eutrophic waters but at the same time many defense mechanisms against predation have also‬ ‭developed, for example through a form of symbiosis. Copepods tend to always be the most‬ ‭abundant taxon but with a very high number of species. Food chains in oligotrophic waters are‬ ‭longer and more complex with various trophic levels and many connections between each level‬ ‭and the others. The definition of trophy then leads us to also introduce the concept of‬ ‭eutrophication. Eutrophication is a process that involves an increase in trophic, i.e. an increase in‬ ‭the availability of nutrients, this evolution can have natural causes as for example occurs in many‬ ‭lake and lagoon environments, but often anthropic causes that affect many coastal areas and‬ ‭sometimes entire basins , in this case we also talk about cultural eutrophication. The term‬ ‭eutrophication, however, is commonly used to describe not only the event represented by the‬ ‭increase in nutrients but also the consequences that this increase entails. When an excess of‬ ‭nutrients is created, the first consequence is represented by an increase in primary production and‬ ‭the appearance of algal blooms. Depending on the taxon that produced the bloom, the waters can‬ ‭take on different colors, for example diatom blooms generally lead to a dark green color of the‬ ‭water. The blooms of such a dinoflagellate‬‭Fecistis‬‭they‬‭are a characteristic aspect of the waters of‬