Question

scientists studied a species of phytoplankton. these phytoplankton are primary producers and are an important part of marine food webs. as all organisms are regulated by their environment, so are the phytoplankton. the scientists looked at the relationship between the phytoplankton population and a virus population found in the same environment. they created three environments in a laboratory setting to collect data on the numbers of phytoplankton and viruses over a period of time and graphed the results once they were averaged, as shown below. the scientists claimed that the growth and stability of the phytoplankton populations were affected by the viruses. based on the information given, which two arguments support this claim? as the phytoplankton population reached capacity, the number of viruses began to increase because the phytoplankton were the hosts to the viruses and replicated the viruses genome. the phytoplankton population was affected by the viruses because the viruses were competitors for the food sources in the environment. the phytoplankton population was unable to absorb the light necessary for growth because the viruses covered the surface of the water. the phytoplankton population decreased as the number of viruses increased because the cells of the phytoplankton were destroyed as the viruses used them to increase the number of viruses in the environment. as the phytoplankton population increased, the number of viruses began to increase because the phytoplankton were consumed by the viruses.

Explanation:

• First Argument: Viruses are obligate intracellular parasites. They need a host (here phytoplankton) to replicate. When phytoplankton population reaches a certain capacity (more hosts available), viruses can increase as they use the host cells (phytoplankton) to replicate their genomes. So, the statement “As the phytoplankton population reached capacity, the number of viruses began to increase because the phytoplankton were the hosts to the viruses and replicated the viruses’ genome” is valid.
• Second Argument: Viruses do not consume food in the traditional sense like competitors for food sources (so the argument about competition for food is wrong). Viruses also do not cover the water surface to block light (so that argument is wrong). When viruses infect phytoplankton cells, they use the cellular machinery of phytoplankton. As the virus particles are produced within the phytoplankton cell, the host (phytoplankton) cell is destroyed (lysis). So, as the number of viruses increases (by using phytoplankton cells to replicate), the phytoplankton population decreases. The statement “The phytoplankton population decreased as the number of viruses increased because the cells of the phytoplankton were destroyed as the viruses used them to increase the number of viruses in the environment” is valid.

Answer:

• As the phytoplankton population reached capacity, the number of viruses began to increase because the phytoplankton were the hosts to the viruses and replicated the viruses’ genome.
• The phytoplankton population decreased as the number of viruses increased because the cells of the phytoplankton were destroyed as the viruses used them to increase the number of viruses in the environment.