My name is Jeff Bowman. I'm an assistant professor at Scripps Institution of Oceanography. Today, I'll be talking about adaptations to life in sea ice. Now what you're looking at here is an image of a particular type of ice structure called frost flowers that are growing on the surface of newly forming sea ice. Now these frosts flowers are interesting in a couple of different ways. They are very, very high in salinity. They're being impacted by light and ultraviolet light, and because they are projecting up into the atmosphere, which is very, very cold, they are very, very cold. They've also concentrated a large number of bacteria into their structure. So the bacteria that are surviving within these frost flowers are having to deal with very low temperatures, very high salinity, and other environmental factors that are stressful to them. Interestingly, the adaptations that bacteria and sea ice have evolved to deal with these types of stressors, in many cases, have larger ecological and environmental impacts. I'd like to start by talking about differences in adaptive strategies between warm bodied organisms, such as these polar bears shown here and cold bodied organisms, such as microorganisms and invertebrates, such as these Eutherians on the bottom of the Arctic Ocean. All bodied organisms rely on a process called homeostasis to survive in these harsh conditions. Now homeostasis means that they maintain an infertile environment regardless of the external conditions. There are a variety of different adaptations that they might have to do this. Examples of these adaptations include fur. They might include blubber. It could include a heightened metabolism to generate more body heat. In contrast, cold bodied organisms or microorganisms are at a thermal equilibrium with the external environment. As a result of that, they have very different adaptive strategies. Adaptations are focused on maintaining function under equilibrium. So all of the proteins and the enzymes and the internal structures that these organisms have, have to deal with the very low temperatures. They're not shielded from them. This diagram is showing some of the key challenges in adaptations that life has to the sea ice environment. The sea ice environment is an incredibly stressful environment to life. Some of the key stressors include strong diurnal temperature fluctuations. So we can have a very strong fluctuation between warm daytime conditions and very cold nighttime conditions. We can have an intense amount of UV radiation. There's a very high salinity within the internal brines of the see ice environment, and of course, we have very low temperatures in sea ice. A key part of the adaptation of microorganisms and cold bodied organisms to low temperature environments are with the structure of their enzymes. Enzymes are proteins that catalyze biochemical reactions. This is an example of an enzyme here. This is the maltase enzyme that converts maltose to glucose. In order to effect this conversion, this enzyme is actually going to change its three-dimensional structure, and that's what's shown here. Here we have an enzyme that's interacting with the substrate, and it's actually changing its structure in order to modify that substrate to facilitate some downstream biochemical process. Now enzyme reaction rates are dependent on the temperature of the environment and the concentration of the substrate that they are acting on. This is an example of a typical activity profile across temperature for a given enzyme. This enzyme is designed to function optimally at 40 degrees. As the temperature decreases, the activity of that enzyme decreases because that enzyme is becoming less flexible and less able to go through that change in conformation so that it can interact with the substrate. Reaction rate is also controlled by substrate concentration. The more substrate you give to an enzyme, the faster that reaction rate will progress until such point as the enzyme becomes saturated. Now we have two different parameters that we can use to describe the shape of this curve. Now, low temperatures reduce the flexibility of enzymes leading to a reduced efficiency. So organisms that are adapted to low temperatures, and we call these organisms psychrophiles or cold loving organisms, have enzymes with enhanced flexibility. The enhanced flexibility allows them to change conformation even as their flexibility is reduced by low temperatures. Mechanisms to increase substrate concentration can also help overcome the limitations of reduced flexibility. The more substrate that's available to an enzyme, the greater the reaction rate will proceed even if flexibility is reduced. Now one way that organisms can do this is by importing more substrate into the cell body. This is an example here of the cell membrane. We have a lipid bilayer, and then we have proteins that are spanning that lipid bilayer that are channeling substrates and nutrients that are required by enzymes from the exterior of the cell to the interior of the cell. Now simply by producing more of the protein channels that are required to transport a given substrate, they can increase the concentration of the substrate inside the cell body and enhance the reaction rate. In the sea ice biology module, we talked about how the growth of sea ice concentrates everything that was in the starting material together. That's analogous to enhancing substrate concentration to help drive these enzymatic reaction rates forward. So here we see different sea ice algae that are partitioned into a very small pore space than the sea ice. Everything that was contained in the seawater that this ice was formed from has been partitioned into those spaces, yielding a much higher concentration of nutrients and carbon and what had been found in seawater. This is shown in this diagram here. We have temperature of the ice, and we have the bulk salinity of the ice going from zero out to 100 parts per 1000. The lower the salinity of the starting material, which is equivalent to the bulk salinity of the ice, and the lower the temperature, the higher the concentration factor of that ice. So ideal ice conditions that are very, very low in salinity and very, very low in temperature can yield concentrations of solutes that are several 100 factor higher than would have been present within the starting seawater. Now this concentration effect has a negative impact on life in sea ice as well, and that comes from the high salinity of this environment. As temperature decreases, the salinity of the brines that are contained within that sea ice increases. The adaptations to this high salinity environment include the production of what are known as extracellular polymeric substances or EPS. Now this EPS forms a hydrated gel that sequesters water close to the cell and also exclude some ions that can be potentially damaging from the immediate vicinity of the cell. Interestingly, the protein fraction of some EPS produced by ice algae has been shown to actually modify the microstructure of the ice itself. What you're looking at in these two sets of micrographs are laboratory grown sea ice that was produced in the absence of EPS and then in the presence of EPS. We can see that the microstructure of these two laboratory grown ices is very distinct. Note here, particularly the very regular shape of the pore spaces, and the highly irregular fractal shape of the pore spaces when the laboratory ice has been doped with EPS. This leads to a much higher connectivity within the EPS doped ice that creates a greater connectivity between pore spaces in that ice and a greater exchange of water and gases through the ice. This has big implications for the transport of salt, heat, and gas through the ice across both the Arctic and the Antarctic oceans. Compatible solutes are molecules that can compete for water, but that don't interfere with protein functions. So they're competing against the salts, the ions in the seawater for those water molecules and helping biology hold onto those water molecules and not lose them to the external environment. This is example of how this is playing out for some key compatible solutes; proline and ectoine. Water activity is the degree to which water is able to move across the membrane. The lower the water activity, the more tightly bound that water is to a compound, and here we have the concentration of those two solutes. So as a cell stockpiles those solutes, the ability of the water to leave that cell and move across the membrane to the external environment greatly decreases. These molecules are produced in very large quantities by cells in response to salinity stress. I'm going to talk a little bit more in detail about a very specific compatible solute that's very present within sea ice, and this is the compound dimethylsulfoniopropionate, which is more commonly referred to as DMSP. This is a widely used compatible solute in the sea ice environment, and its structure is shown here. Now, an interesting thing about DMSP is that it's widely used as a source of carbon and energy by a variety of heterotrophic bacteria within sea ice and within the broader marine environment. When they do this, one pathway for using DMSP produces a compound known as dimethylsulfide or DMS. Dimethylsulfide is volatile, meaning a gaseous compound, and it can leave the seawater or sea ice environment and end up in the atmosphere. Once in the atmosphere, DMS is oxidized to dimethylsulfoxide or DMSO, and eventually to sulfuric acid. Now interestingly, sulfuric acid is a very effective cloud condensation nuclei in the atmosphere. But a cloud condensation nuclei means that it's a particle that's very effective at aggregating water molecules to form small droplets that can ultimately go on to form clouds. Through this process of biological and abiotic conversion, this compatible solute can lead to the production of compounds that can have a big impact on regional and global climate. Interestingly, the production of DMS seems to be changing within the Arctic region.
