By: Erin McParland

Hello! My name is Erin and I am a PhD candidate in the Marine and Environmental Biology Department at USC. I am currently finishing my final thesis experiments at the Wrigley Marine Science Center (WMSC), thanks to the Wrigley Institute’s Bakus Graduate Research Fellowship!

The last chapter of my thesis aims to understand how interactions between microscopic organisms in the surface ocean may influence organic carbon cycling. Even though we can’t see it happening, phytoplankton (microscopic plants) and bacteria are constantly ‘talking’ to each other in the ocean. Sometimes this involves sharing nutrients, and other times competing for nutrients. These nutrients are part of the organic carbon pool, which stores the same amount of carbon as atmospheric carbon dioxide, and therefore plays a very important role in controlling Earth’s climate!

What controls the production and consumption of the organic carbon pool in the surface ocean is still not fully understood, and is an active area of research in oceanography. My research combines lab and field experiments to ask if these microbes are trading a particular carbon compound: DMSP (dimethylsulfoniopropionate). DMSP is produced by phytoplankton, and quickly assimilated by microbes because it is both a carbon and sulfur compound, making it a particularly yummy component of the organic carbon pool.

Microscope pictures of diatoms that make DMSP. On the left is a single cell, on the right are multiple cells connected in a chain.

To answer my research question, I have spent a lot of time in the lab trying to grow phytoplankton cultures without bacteria. To accomplish this I add antibiotics to seawater, just as humans take antibiotics when we have a bacterial infection. This might seem like an easy task, but for the phytoplankton these bacteria are not infectious, but rather symbiotic. Some of the phytoplankton just can’t grow without their bacterial partners. Now that I have finally successfully grown phytoplankton bacteria-free, I can conduct my experiments.

Diatoms (big circles) growing with bacterial partners in the surrounding water (small white dots in the black background) (left) and without bacteria (right).

I’m going to WMSC to collect natural seawater at Catalina, which has roughly 1 million bacteria cells per milliliter of seawater! I will filter the water either to be clean seawater (with no bacteria) or bacteria-filled seawater, and will measure the production of DMSP as I add the two types of seawater to my phytoplankton. I hypothesize that the phytoplankton will make more DMSP in the bacteria-filled seawater to trade this yummy compound for other nutrients that only bacteria can make, such as vitamin B12.

These experiments will be the first evidence for differences in phytoplankton DMSP production, based on the presence or absence of bacterial partners. The results will improve our understanding of the role of microscopic interactions in organic carbon cycling in the ocean.

By: Kenny Bolster

I’m Kenny, one of this year’s Wrigley Summer fellows at Catalina. I’m a PhD candidate in the Earth Sciences department at USC, working for the Moffett lab. This is going to be my third summer on the island. I spent the last couple of years studying the effect of UV rays from the sun on the chemistry of the ocean surface, and how that might affect living organisms. This year, I’m moving onto a different project, one which I haven’t gotten to work at Wrigley for so far.

This project is focused on oxygen deficient zones (ODZs), a particular type of marine environment where the oxygen in the water completely runs out a few hundred feet beneath the surface. There are three big places in the world where this happens, two in the Pacific off of Mexico and Peru, and one in the Arabian Sea off of India. In those places, the surface environment is nutrient-rich and full of plankton, creating food for organisms that live deeper in the water and causing those deep organisms to consume the oxygen. The other thing that has to happen in order to form an ODZ is that the surface water needs to be warm and less dense than the cold, deep waters, so that waves and currents can’t mix oxygenated surface water into the deeper waters of the ODZ.

ODZs also tend to have really beautiful sunsets. That’s not particularly important to the science, but it is a nice perk for the scientists

When the oxygen runs out, it changes the chemistry of the water in some very important ways. Bacteria, which don’t have any oxygen to grow off of, begin to use nitrogen instead causing the ocean to lose some of this very important nutrient. The oxygen-free water creates a path which iron can move along in order to get to offshore environments without forming rust particles and sinking to the bottom of the ocean. ODZs are also important places to study the history of life on Earth, and what Earth would have been like when there was less oxygen on the planet.

ODZs are very important, but they’re not easy to study. Going out to one requires an oceanography research vessel, which are generally not cheap. It usually means spending several days, at least, sailing to the places you want to study. You also have to get special visas from whatever country owns the ODZ area that you’re interested in, all of which is a hassle. In May, I got back from a research expedition to the Mexican ODZ. We were mapping the boundaries of the oxygen-free region on the R/V Revelle, one of the biggest American research ships. The Revelle, shown in the picture, is 277 feet long, and has 6 decks. It can hold up to 22 crew and 37 scientists. We went out to deploy sediment traps, which are devices which capture stuff sinking through the water, which is mostly dead organisms from the surface. We were trying to measure changes in the chemistry of the sinking stuff, as well as how the sediment changes the chemistry of the water around it.

Photo of the R/V Revelle taken from a small boat. On the left is a buoy which holds one of the sediment traps we deployed. The tiny black spot above the stern of the ship is a Magnificent Frigatebird (Fregata magnificens), a seabird with a 7-8 foot wingspan.

Going out to sea is a lot of fun, but it would be nice if we could study ODZs in a simulated environment, or mesocosm, at a marine lab. I’ve applied to the Wrigley Institute for a fellowship to build an ODZ mesocosm out on Catalina. In essence, we’ll be taking water from the Wrigley Institute’s seawater system and changing the chemistry in order to get the same types of bacteria growing that are found in a real ODZ. Then, we can measure how those microbes change the chemistry of the water, and use the mesocosm to conduct controlled experiments to better understand the processes in real ODZs.