Mono Lake Microbial Observatory Progress Report
Or: What you can see in this figure (if you know where to look!)
This diagram shows the distribution of physical and biological properties at a location near the deepest part of the lake, southeast of Paoha Island, on May 17, 2000. The data in the panel on the left were collected by lowering an instrument package that records, at 0.1 sec. intervals, water temperature (shown in black); light intensity (shown in red); chlorophyll fluorescence, a measure of the biomass of single-celled green plants living in the water (shown in green); and the depth below the lake surface at which the other measurements were taken. The data in the panel on the right were generated by analyzing samples of water collected at the depths indicated by the orange arrows. Lets focus on the left half of the figure first, the side that describes the physical structure of the lake at the time the samples were collected.
Light intensity decreased rapidly due to light absorption by the relatively high concentrations of plant biomass present in the lakes surface waters at this time. Light intensity decreased to values approaching 0 by a depth of 12 m, (line offset to keep it from coinciding with the depth axis). Most plants cannot photosynthesize at light levels below about 10 units, but one of the dominant plants in Mono Lake is a tiny cell that thrives at light intensities near 1 unit. This plant, named Picocystis by a scientist (Dr. Ralph Lewin) at the Scripps Institute of Oceanography (we call it Mickey Mouse because of its trilobate structure it looks like Mickey Mouses silhouette when you look at it with a microscope), is responsible for the jump in plant biomass indicated by the increase in chlorophyll fluorescence (green line) at a depth of 15 m.
This depth is also where oxygen disappears from the lakes water (the zone labeled OXYCLINE in the figure) due to a combination of restricted replenishment from oxygenated surface waters in contact with the atmosphere and oxygen consumption by the populations of bacteria living in the lake. The change of water temperature (black line) with depth exerts a strong influence on the replenishment process. Where temperature changes little with depth (upper 12 m), replenishment is easy the wind blowing over the lakes surface is enough to stir or mix oxygenated water at the lakes surface into the deeper layers of the lake. But where the temperature decreases rapidly with depth the thermocline it more difficult to stir shallower water into deeper water. The thermocline acts as a barrier to the penetration of oxygen into the deeper layers of the lake, thus the thermocline and the oxycline tend to coincide. There was no oxygen present (termed anoxic; from an = no and oxic = oxygen, literally: no oxygen) at all depths below 16 m when we sampled in May.
Mickey Mouse uses anoxia in Mono Lake to escape being eaten by brine shrimp. In addition to being able to photosynthesize at very low light levels, it is also able to tolerate high concentrations of toxic substances like sulfide that accumulate in the near-bottom waters of the lake. These substances diffuse up through the thermocline, repelling or possibly even poisoning brine shrimp living in the surface layer of the lake. That is one of the main reasons that Mickey Mouse increases to such abundance in the oxycline it is not grazed by the Mono Lake equivalent of range cattle, the brine shrimp. The oxycline and the substances diffusing up through it act kind of like a fence to exclude the brine shrimp.
Deeper in the lake, at a depth of about 22 m, lies the chemocline. This is the zone where the amount of salt dissolved in Mono Lake water increases rapidly with depth. In the middle of the lake away from the creeks, the salt content of the water is relatively constant with depth, both above and below the chemocline. The chemocline has an effect similar to that of the thermocline on mixing between two layers of the lake it acts as a barrier to mixing and exchange of water. This shows in the figure in both the temperature line and in the chlorophyll fluorescence line as follows. Below the chemocline, water temperature actually increases a degree or two. This is because the water below the chemocline has been trapped there since 1995 and is gradually being warmed from below by heat flowing up through the lake bottom from deeper in the Earth. The higher salt content of this water increases its density and keeps it from floating up off the bottom as it warms and becomes less dense. The cold water between the chemocline and the oxy- or thermocline is a kind of historical artifact it cooled to that temperature during the coldest part of last winter (Feb 2000) when the surface of the lake was so cold that there was no thermocline and surface water was mixed all the way down to the chemocline. Mixing was stopped at the chemocline by the increased salt content of the water. Since then, the sun has warmed the surface of the lake by approximately 6 degrees, leaving the relict cold winter water trapped between the chemocline and the ever-intensifying thermocline.
By the way, in February when surface water was mixing all of the way down to the chemocline, there was oxygen throughout the whole layer above the chemocline. This oxygenated water did not penetrate the chemocline, so Mono Lake remained anoxic below the chemocline with the result that during the middle of the winter the oxycline and the chemocline coincided, just as the oxycline and the thermocline coincided when we sampled in May. What happened to the oxygen that was trapped in the layer between the chemocline and the thermocline? It was used up by oxygen-respiring bacteria and by spontaneous oxidation of chemicals like sulfide that diffused across the chemocline.
The story is similar for the chlorophyll fluorescence profile: the maximum in chlorophyll fluorescence at 24 m is the remnant of a much larger biomass peak that had accumulated in the layer between the thermocline and the chemocline as a result of the growth of Mickey Mouse (primarily) during the summer of 1999! The rest of this biomass was distributed throughout the lake by winter mixing, seeding the lake with Mickey Mouse plants in time for the spring. We are not really sure what is responsible for the high fluorescence of the water below the chemocline. It appears to be caused by intact Mickey Mouse cells that have slowly rained down from above, either singly or as part of brine shrimp excrement (the Mono Lake equivalent of the cow patty). But how these cells are surviving in the environmental conditions below the chemocline is a mystery yet to be solved.
Let us now turn our attention to the right half of the figure. The horizontal streaks (theyre called lanes in the molecular biology literature) are a series of genetic fingerprints of the bacteria that were in the sample of water collected at the depth indicated by the orange arrow. These fingerprints were generated using the polymerase chain reaction (PCR) and a technique called denaturing gradient gel electrophoresis (DGGE). Within each lane are darker vertical bands. Each band, more or less, contains the DNA from a particular region of the gene for the small subunit of the ribosome from a particular type of bacteria that was present in that sample. If you follow the bands from lane to lane, you will see that they are typically found in a number of adjacent lanes, indicating that they were present throughout the depth range covered by that group of samples. The blue arrows superimposed on the image are to show you examples of places where bands do not continue between adjacent lanes. These are places where there is a shift in the composition of the microbial ecosystem in Mono Lake. If you look at the figure for a while, you will see that most shifts coincide with the chemocline and the oxycline. This means that you would find more or less the same bacteria in all the samples you collected above the oxycline, or between the oxycline and the chemocline, or below the chemocline. A very dark band in the middle of the lanes that continues across all of the lanes, from the top to the bottom of the figure, is from the chloroplast of our friend, Mickey Mouse. This band has been indicated in the middle of the panel by pointer arrows from its proper name, Picocystis.
This technique (PCR/DGGE) gives us a tantalizing look at the distribution of bacteria in the lake. With it we can examine vertical and horizontal distributions and how this changes seasonally. But it wont really give us the key information we are after in the Microbial Observatory: who are these bacteria and what are they doing? To find this out we have to go back to the lab (thats where we are now) and do more analyses. We can use the bands in the DGGE lanes to get a sort of abbreviated name for the bacterium that donated DNA for the analysis; we can then compare this to a sort of telephone book of full names created by another approach to get a full name and, if were lucky and the organism has been studied before, a sort of resume of what it does for a living. If the bacterium, like Picocystis, has never been discovered before (most of them havent), then we also have to figure out what it does for a living so we can understand its role in the ecology and geochemistry of Mono Lake. We are actively working on both of these fronts, and on other aspects of the microbiology and geochemistry of Mono Lake. We will report on our progress with periodic updates of this web page and with material posted on the SNARL web page.
2 November 2000