Going beyond counts: the physiological ecology of zooplankton

Life in the plankton is hard! Plankton, which means wanderers or drifters, includes a large and diverse collection organisms that are in constant motion as they become advected by oceanic currents, often over large distances. How do these organisms thrive in such a dynamic environment?

Sep 02, 2019

In high latitude marine systems, peaks in primary production are short and sporadic, yet these waters are also known for their valuable commercial fisheries. The key trophic link is a nutritious planktonic food source: large lipid-rich copepods that store fat and regulate their metabolism. These copepods synchronize their life histories to the seasonal cycle: growing and accumulating fat during the phytoplankton bloom then migrating to deep waters and going dormant for several months.

Diagram of a high-latitude foodweb with large lipid-rich copepods. Copepods like Neocalanus flemingeri serve as a key link between large phytoplankton and microzooplankton and upper trophic levels including fishes.

In May 2015, we were invited to come on an oceanographic cruise of the Seward Line Long-term Observation Program in the Gulf of Alaska (now northern Gulf of Alaska LTER). Our goal was to collect specimens of the copepod Neocalanus flemingeri in the pre-adult stage.  

Live-sorting copepods under the microscope on a rocking ship was a new experience – the first batch ended up on the floor! As we got better the copepods ended up in microcentrifuge tubes as we had planned. We collected from different locations and discovered that N. flemingeri were abundant at all stations, but varied in size, fat stores and coloration.  

Tiglax Lab
Top left: Our research vessel R/V Tiglax US US Fish and Wildlife Service. Top right: Lab and microscope on Tiglax. Bottom: Not so calm seas on the Gulf of Alaska - Photo credit D. Hartline.

From our colleagues we learned that phytoplankton densities differed by an order of magnitude – an opportunity to investigate how a copepod responds to its environment in the real world. We had used transcriptomics to study zooplankton responses to experimental treatments in the past. Using RNA-Seq on field-collected copepods was a new research direction for us. 

We found large differences in gene expression among N. flemingeri from different regions in the Gulf of Alaska. Lipid biosynthesis genes were up-regulated in copepods collected in high food areas, while lipid and protein degradation genes were up-regulated in copepods from low phytoplankton areas. Interestingly, copepods collected in a transition area between two major current systems showed up-regulation of genes involved in the cellular stress response.

As passive drifters, copepods are limited in their ability to locate and exploit food patches.  Differences in gene expression indicate that the copepod regulates its transcriptome in response to ambient conditions. Physiological acclimatization to variable and unpredictable food resources may be key to their success and resilience.  Healthy N. flemingeri were found throughout the region, in spite of order of magnitude differences in food resources. 

Summary of the study showing how genes are differentially regulated in N. flemingeri collected from different regions in the Gulf of Alaska. N. flemingeri in high food regions accumulate more lipids, which increases their fecundity. Individuals growing in low productivity areas tend to be lipid poor and produce fewer eggs as adults.

    Vittoria Roncalli

    Postdoctoral reseracher, University of Barcelona

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