May/June 2026 Paper 41

Algae are aquatic photosynthetic protoctists. The alga Alexandrium is red in colour. Alexandrium causes harmful algal blooms (HABs) that turn seawater red when Alexandrium numbers rise to high levels. The abundance of Alexandrium depends mainly on abiotic factors.

Alexandrium releases a toxin that accumulates in shellfish such as oysters and mussels. Humans who eat shellfish contaminated with Alexandrium toxin show signs of poisoning, which in some cases may cause death.

Figure 7.1 shows the global number of cases of poisoning linked to Alexandrium HABs between 1986 and 2018. The line shows the long-term trend in cases of poisoning due to Alexandrium HABs.

Use the long-term trend line shown in Figure 7.1 to calculate the percentage change in cases of poisoning due to Alexandrium HABs between the start of 1986 and the end of 2018. Give your answer to three significant figures.

The number of cases of poisoning due to Alexandrium HABs in Figure 7.1 is an indirect measure of the abundance of Alexandrium. Changes in the abundance of Alexandrium can be related to changes in abiotic factors that affect the metabolism, growth and reproduction of the alga. Identify, with reasons, two abiotic factors that will affect the metabolism of the photosynthetic alga Alexandrium.

The toxin released by Alexandrium is described as a neurotoxin. It binds to, and blocks, voltage-gated sodium ion channels in cell surface membranes of neurones, preventing them from functioning. This results in muscle paralysis because calcium ions are not released from the sarcoplasmic reticulum of muscle cells. Explain how the action of Alexandrium toxin on a neurone prevents the release of calcium ions from the sarcoplasmic reticulum of muscle cells of a person who has consumed the toxin.