Brugnano, C., Granata, A., Guglielmo, L., Minutoli, R., Zagami, G., & Ianora, A. (2016). The deleterious effect of diatoms on the biomass and growth of early stages of their copepod grazers. Journal of Experimental Marine Biology and Ecology, 476, 41-49. DOI: 10.1016/j.jembe.2015.11.015
Plankton are microscopic organisms that drift in the ocean’s currents, absorbing sunlight and CO2, while producing oxygen we depend on. Diatoms are one the most dominant groups of plankton in the ocean and can form enormous blooms when nutrients and sunlight are available. Though diatoms are motionless, they have developed incredibly beautiful and intricate cell structures that help them stay afloat in the sunny surface waters (Fig. 1). Being so abundant in the ocean and incapable of fleeing from predators, diatoms are an easy target for many larger animals like copepods, shrimp and krill. Some diatoms have evolved the ability to produce toxic chemicals when damage to their cell wall occurs. However, it is unclear how severe these toxins are to potential predators.
What if the food you ate could literally kill you? This scary prospect is a reality for copepods who eat specific kinds of diatoms. To better assess the toxic effects of diatoms on copepods, researchers set out to measure the reproductive success of copepods fed on a toxic-diatom diet. Responses of copepods towards toxic diatoms was compared to that of copepods fed a known, non-toxic source of prey. Furthermore, researchers tested how long offspring could survive depending on the type of food their mother’s ate.
To test reproductive success, female and male copepods were paired together and fed either the toxic diatom, Skeletonema marinoi, or the non-toxic dinoflagellate, Prorocentrum minimum. Copepod couples fed on the toxic diatom spawned for a shorter period of time and had lower egg production than those fed the non-toxic meal (Fig. 2). In addition, the hatching success of females fed with the toxic diatom were only 33%, compared to 88% fed the non-toxic prey. Clearly, couples feeding on toxic prey do not reproduce as well and do not hatch as many eggs.
The effects of a toxic diet go beyond poor reproduction and egg hatching. The amount of fecal matter produced from copepod couples was also much lower when they were offered the toxic diatom. This indicates toxic chemicals may be disrupting the digestive system of the copepod. When couples did produce nauplii (copepod offspring), apoptosis was recorded with a ratio of 2:1 nauplii. Apoptosis occurs when cells die in the body and can lead to early stage mortality of newborn copepods. In comparison, apoptosis was not observed in any of the nauplii produced from healthy couples (Fig. 3).
Lastly, researchers wanted to find out if the type of food maternal copepods ate had any impact on the development of their offspring. It turns out, the repercussions of an unhealthy diet can be passed down to the next generation. Nauplii fed on a non-toxic diet with a healthy mother were able to complete development in the shortest time (10 days) Conversely, nauplii fed on toxic diatoms and hatched from unhealthy mothers, only reached day 4 of development and could not grow any further. Even if nauplii were switched off the toxic diet after hatching, they still experienced slower growth.
Discussion and Significance:
Female copepods that ate toxic diatoms did not fare well in this study, nor did their offspring. Both the reproductive and digestive systems of the copepods were negatively impacted after eating diatoms. Offspring of unhealthy females were visibly malnourished, unable to fully develop and had much higher incidents of cell death (apoptosis). What do these findings mean for actual populations of copepods and diatoms in the ocean? Through the release of toxins, some diatoms can be dangerous to eat and cause significant reductions in predator populations such as copepods. Increased mortality of copepods may even impact the larger food web in some areas, as copepods are important links between phytoplankton and larger predators.
Massive blooms of diatoms are common under the right environmental conditions, meaning they might be the only food option for copepods at certain times of the year. Moreover, copepods did not appear to recognize toxins once they were released by diatoms and continued to feed. Though, there is evidence that copepods may be able to form a resistance to certain toxic diatoms after a longer exposure time. Further studies will need to be performed to better assess the effects of diatom toxins on copepods and how copepods may evolve to deal with these toxins.