Filtering the Gulf of Mexico

Author: Gabrielle Corradino

“Why would you spend 35 days on a boat just to filter seawater?”

This was the most common question (second most common was: “Don’t you get seasick?”) that I received as I explained what I would be doing during the GOMECC trip to my friends and family. The biology component of the GOMECC trip does include lots of filtering of water onto specialty glass fiber filters, but the research does not stop there!

Pic 2
Preserved copepods from CTD 47. The copepods were isolated and placed into filtered seawater and formalin. This will allow observations to be made about the individual organisms in each sample.

Team Plankton (Mrun and myself) will have filtered over 236,000ml of seawater onto 550 filters to help answer questions on microbial species diversity using both molecular and pigment profiling. While invisible to the naked eye, each of the filters will have tens of thousands of tiny organisms (phytoplankton and protozoa) retained on their surface that represent the base of the food web within the GOM. The filters, which may turn a greenish color, if phytoplankton are present (Fig 1), are frozen on ship and will be brought back to North Carolina State University or University of Louisiana for further analyses.

Each filter will be used to collect a snapshot look at microbial assemblages, the presence/absence of certain taxa (DNA signal) and their activities (RNA signal). In unison, we also use several preservation methods to obtain intact plankton for microscopy analyses (Fig 2) from the CTD, a bucket (Fig 3) or with a plankton net.

This trip is intensive, but with the guidance from our rockstar chief scientists (Leticia and Denis), we will be able to gain unique insight into the microbial biogeography, biodiversity and functionality. We believe this data will serve as an important baseline as we study the impact of ocean acidification on the Gulf of Mexico.

Pic 3
Whole water surface samples being filtered through a 200µm mesh and into a carboy. This water will be used for filtering and for the on-deck grazing experiments.
Pic 1
Example DNA filter from a surface water sample. The filter will be frozen and brought back to North Carolina State University to have the DNA extracted for processing.


Plankton communities and incubation experiments on GOMECC-3

Author: Mrunmayee Pathare

image 1
The microscopic phytoplankton that we study.

Our lab comprises some of the biological sampling being conducted aboard the Ronald Brown on the GOMECC-3 cruise. We study tiny ocean organisms called plankton which range in size from microscopic phytoplankton that use photosynthesis to produce energy, to millimeter sized copepods that can be seen by the naked eye “jumping” to catch their prey.

Phytoplankton form the base of ocean food webs, they are the tiny plants of the ocean, floating in the water column turning carbon dioxide into energy. Phytoplankton fix organic carbon found in the atmosphere and dissolved in the water into energy that is transferred through the food web by bigger organisms eating the smaller organisms. Most of these tiny organisms are eaten, but those that are not eaten fall to the ocean floor, drifting thousands of meters down the water column to be decomposed by bacteria. Phytoplankton fix 45 gigatons of inorganic carbon per year, and are an integral part of the mechanism removing CO2 from the ocean (fixing it), and turning it into food that gets passed up through the food chain, or falls to the sea floor as marine snow.

image 2
A copepod predator that eats prey plankton.

On this cruise, we will be looking at the plankton communities in the top 5 meters of the Gulf of Mexico and who is eating whom. We are conducting a 24-hour incubation on a series of light and dark bottles containing seawater sampled by the CTD. Some of these bottles will contain only phytoplankton and small grazers, and some of them will contain phytoplankton and copepods. This set up will give us a snapshot of predator-prey dynamics at the base of the food chain (who is eating whom), how carbon moves through the base of the food chain in different conditions within the Gulf of Mexico (how much is being eaten and how it changes in different parts of the Gulf of Mexico). We also have some oxygen optodes fixed inside these bottles that will let us measure the amount of respiration taking place in the bottles during their incubation.

image 3
Gluing the optodes (tiny orange circle on the forceps) inside the bottles took a surprising amount of contortion and skill!

To simulate the environment that we are taking these little critters from, we rigged up an incubation tank on the back deck of the ship. We had to get creative with the materials and the location, and then strap it down securely so it won’t move when the Gulf decides to throw bad weather at us.

The tank simulates the natural environment of the ocean and there is sea water constantly trickling through a hose to keep up the circulation and make sure the water inside the bottles doesn’t turn into plankton soup or get the photosynthesizing plankton fried by the sun.

We are conducting a total of 8 of these incubations over the course of the cruise, and although the results will be analyzed after we return from the cruise we are very excited to study the plankton communities of the Gulf of Mexico and contribute to the better understanding of carbon fate and transport.