[MUSIC] My name is Jeff Bowman, I'm Assistant Professor at Scripps Institution of Oceanography. I'm a biological oceanographer. My lab studies marine ecological processes, and the Arctic and the Antarctic. Today I'll be talking about life in sea-ice. In any environment, the amount of life that can be found in its level of activity is a balance between the metabolic challenges and opportunities presented by that environment. So for example, in sea-ice, this is a typical sea-ice core. And we see this dense band of pigmentation at the base of that sea-ice core. That pigmentation is there because of the growth of ice algae. These ice algae are roughly located at this narrow band within the sea-ice core, because it gives them optimal access to both sunlight coming from above. And nutrients coming from below, but diminishes their exposure to different stressors in the sea-ice environment. So other opportunities that present in the sea-ice environment include sunlight to support photosynthesis. Because the photosynthesis there's organic carbon that can support heterotrophic growth by bacteria, and other microorganisms. There's typically abundant oxygen within sea-ice, because it's close to the atmosphere. And if conditions are right, they're going to be abundant nutrients within sea-ice as well. Challenges that are present include intensive sunlight and ultraviolet light. Too much sunlight, although it supports photosynthesis, can be detrimental to cells. Oxygen, well, it can be beneficial for some metabolisms, also creates oxidative stress that can be very challenging for microorganisms. There are, of course, low temperatures to deal with within the ice environment, and there is also high salinity. If we look at this picture of sea-ice cores taken from McMurdo sound in Antarctica, and these are representative of sea-ice cores. Springtime sea-ice cores from both the Arctic and the Antarctic, we can the range of temperatures that we might experience here at the top of the ice core. The temperature is in near equilibrium with the atmosphere, which might be as cold as -20 degrees Celsius during the Spring. In contrast, the ocean is much warmer, about -1.8 degrees Celsius. So the ice at the bottom of the ice core is close to equilibrium, with the ocean, about -1.8 degrees Celsius. We can see that dense growth of ice algae on the bottom of these ice cores, indicating their preferential growth in that environment. Now, microorganisms such as E.coli, and other organisms that we think of that are associated with the human body. And the environments that we most commonly come in contact with, grow best at about 40 degrees Celsius. That's the temperature that's much, much warmer than the sea-ice environment. So our typical microorganisms that we would encounter in our daily lives cannot perform well in this environment. Another way to look at this is to look at different temperature classes of microorganisms. E.coli and similar organisms are known as mesophiles. They have an optimal growth of about 40 degrees Celsius. Organisms that grow warmer than them include thermophiles and hyperthermophiles. These are organisms that come from hydrothermal vents in the deep ocean, as well as hot springs and other specialized environments. Below the mesophiles are the psychrophiles. Psychrophiles stands for cold loving microorganism. And these are the types of organisms that are going to perform well in the sea-ice environment. They might have a growth optimum at about 10 degrees Celsius, but they're going to be unable to grow above about 20 degrees Celsius. And they could continue to grow well below their growth optimum, well below the freezing point of seawater. An interesting question is at what temperature do psychrophiles stop growing? Bacterial activity has been observed down below -30 degrees Celsius, which is much colder than we would typically find, within even very cold sea-ice. This micrograph here illustrates microbial activity at about -15 degrees Celsius within sea-ice. There's a bacterium that's been partitioned into this pore spaces within the sea-ice here. The sea-ice was then incubated with a fluorescent dye that fluoresces in the microbial activity. So here, we see that fluorescent dye excited by UV light, and fluorescing back, indicating that there was microbial activity from this bacterium. Bacterial growth, however, is distinct from bacterial activity. Bacterial growth implies an increase in biomass, that eventually leads to a reproduction of the cell to form two daughter cells. And this is observed in much warmer temperatures than bacterial activity. Bacterial growth down to -15 degrees Celsius in permafrost environments. However, -15 degrees Celsius still gives us broad access to a fairly large swath of the sea-ice core, if we think about it in terms of temperature. Now, given that these values are well below the freezing point of seawater at about -1.8 degrees Celsius, how do these microbes survive? How do they survive in a ice environment? It's important to recognize that sea-ice is not a solid, unlike fresh water ice. An ice cube from your freezer would be a solid piece of ice, sea-ice freezes differently. It's composed of solid, nearly salt free ice crystals, and then also a liquid brine fraction. So the sea water starts to freeze. The ice crystal that forms is entirely fresh, and it's excluding the salt to the remaining liquid water fraction of the sea water. As the temperature continues to drop, and then ice crystal continues to grow, the remaining liquid water fraction gets saltier and saltier. This has profound consequences for sea-ice biota. We can see this process playing out here in these x-ray tomography images of laboratory-grown sea-ice. This is sea-ice at -15 degrees Celsius, -6 degrees Celsius, and -3 degrees Celsius. The gold is indicating the liquid water fraction of that sea-ice. In -15 degrees Celsius, we can see that there is a significant amount of liquid water still within that ice matrix. But it is much less reduced than it is at -6 degrees Celsius, or even at -3 degrees Celsius. Now, an interesting thing happens right around -6 degrees Celsius, which is that the connectivity between these pore spaces shuts off. This is described in what we call the rule of fives. For sea-ice of bulk salinity of 5 part per thousand, and by bulk salinity, I mean the salinity of the ice. If you took the whole piece of ice, and you melted it down, and measured the salinity. A 5% porosity is achieved at about -5 degrees Celsius. And a 5% porosity is a point at which the connectivity between those pore spaces shuts off. That means for microorganisms that are trapped in that ice, there can be no further exchange of nutrients, or no access to new carbon, other that's what's contained in your immediate neighborhood. Now, brine salinity is solely a function of temperature. It is independent of the salinity of the starting material, and this is described here by this plot. We have brine salinity on the y-axis, temperature on the x-axis. And as we go down in temperature from a starting temperature of -1.8 degrees Celsius, approximately when sea water starts to freeze. We quickly accumulate a higher salinity of the Interior brines. By the time we're down towards cold spring time or wintertime sea-ice, we have a brine salinity that's in excess of 200 parts per thousand. That's five to six times the salinity of the starting seawater, and it exerts a significant stress on the microorganisms that are contained in the ice. Now the fraction of the space which the brines contained within, however, is a function of both bulk salinity and temperature. The more saline, the starting material, the lower its freezing point, and the less ice crystal formation will happen at a given temperature. The concentration factor of all the particles that are contained in that starting material for the ice is the inverse of that brine volume fraction. So the more saline the starting material, the less concentrated those solutes will be when the ice is fully formed. Here we can see how this plays out for different microorganisms that are trapped in the sea-ice environment. Here's our image of a bacterium partitioned in the brine spaces within ice at -15 degrees Celsius. And all of the solids in its immediate neighborhood reflect a high concentration over what was present in the surrounding seawater. Here we see different diatoms at even a lower temperature, -25 degrees Celsius. Partitioned into that pore space, and all of the bacteria and nutrients in virus particles, and anything else, it was contained in that original starting material, has been compressed down into this very small pore space. Now, not all microbiology in the sea-ice environment is associated with the interior of the sea-ice environment. And there's a significant amount of biology that's taking place on the surface of the sea-ice, particular at the interface between the sea-ice and the sea water. These images are showing large colonies of sea-ice algae that are growing underneath the sea-ice late in the summer, as well as in open waters immediately adjacent the sea-ice. Now, the size of these colonies is very important. They allow the colonies to sink very quickly, as they start to die. And that represents a significant flux of carbon and other material from the surface, down to the deep ocean. The impact of that on the deep ocean is quite large. That's seen here by these several thousand meters deep in the Arctic Ocean, feeding on relatively fresh to trial material that originated in the sea-ice zone several thousand meters above it. So here we see, again, those ice, our little colonies underneath the sea-ice. And as those become senescent at the end of their life, they start to slough off and fall to the deep sea floor, feeling a rich benthic ecosystem. The sea-ice ecosystem can be effectively visualized by considering the food web that's contained within it. Everything starts with the solar radiation that drives the gross of autotrophs. These are the sea-ice diatoms that are growing inside the ice, and at the Ice water interface. These are going to feed the small metazoans, such as the krill and the amphipods that are in turn food for fish, birds, and upper trophic level consumers. These autotrophs, however, also go to feed other small produce that are themselves as secondary food source for the small metazoans. These include ciliates, such as the one shown here. These cilliates are also feeding on heterotrophic flagellants, that are in turn feeding on very small bacteria. The bacteria are the smallest orange dots in this microscope image of sea-ice. And then all of these groups are in turn being preyed upon, and in some cases, consuming marine viruses. And the marine viruses are performing a very critical function, by converting some of these single-celled organisms. And to dissolve organic carbon that can fuel additional bacterial growth, keeping carbon moving through the food web.
