Hannah Eva Blossom:
Mixotrophy and toxicity in harmful microalgae

Date: 10-04-2018    Supervisor: Per Juel Hansen

This thesis has studied the role of toxin production and mixed nutrition (mixotrophy) in species of the dinoflagellate genus Alexandrium and the haptophyte Prymnesium parvum. Alexandrium spp. are known to produce paralytic shellfish poisoning toxins (PSTs), which accumulate in shellfish and can lead to intoxication in humans upon consumption. These dinoflagellates also produce some as yet chemically uncharacterized toxins (lytic toxins/ichthyotoxins) which are released to the water and affect a wide range of organisms in the food web, including competitors, grazers, and fish. P. parvum also produces and releases toxins with lytic and ichthyotoxic effects and has been responsible for massive fish kills worldwide. One of the challenges of studying ichthyotoxicity is finding a proper bioassay in the search for ichthyotoxins. We showed that in the search of unknown ichthyotoxins, an algal bioassay may not be appropriate, despite the assumption that lytic toxins (lysing algal cells) and ichthyotoxins are the same compounds. Ichthyotoxicity was not correlated withto lytic effects on the cryptophyte Teleaulax acuta in 5 strains of P. parvum. Furthermore, the previously identified ichthyotoxins of P. parvum, oleamide and the “golden algae toxins” (GATs), were proven not to be the causative agents of toxicity to either fish or microalgae.   

An important part of this thesis was to investigate if toxic compounds are produced at a measurable cost. We used 16 strains of species in the former Alexandrium tamarense species complex, and grew them under non-limiting conditions as well as light limitation. We found a significant cost to production of lytic compounds, as there was a trade-off between lytic toxicity and growth. But this trade-off was only seen in the PST producing strains, and was more pronounced at low light. On the other hand, the trade-off of PST production and growth was only seen at high light, suggesting a cost, but not under light limitation. This study provides the first trade-off curves of toxicity for both lytic and PST toxicity and thereby contributes to a better understanding of the ecological costs of toxin production among microalgae.    

This thesis also investigated aspects of mixotrophy in Alexandrium species. Often these organisms are treated as phototrophs, whereas in nature they likely consume protist prey. Prey uptake in these organisms has proved difficult to induce under lab conditions in Alexandrium species despite screening over 40 strains of 8 different species. The only strains which showed evidence of phagotrophy were 4 out of 5 of the A. pseudogonyaulax strains. Details of the mucus trap feeding strategy of A. pseudogonyaulax were quantified with individual cell behaviors. Over 90% of cells formed a trap within 1 hour of prey addition, with an average of 45 prey cells per trap. The trap could catch prey for up to 48 hours after abandonment by A. pseudogonyaulax. The results show that the impact on prey species may extend beyond the removal of prey through ingestion. 

Three years prior to the initiation of the experiments in Paper III, a strain of A. pseudogonyaulax was separated into two sub-cultures; one was grown mixotrophically with prey, and the other was kept as a phototroph, with no prey. In less than 3 years, the sub-culture grown phototrophically almost completely lost the ability to feed. This gave us a perfect opportunity to see if the trade-off of phagotrophy involves mucus trap production or toxicity, and to measure trade-offs in a competitive setting when mixed with other algae. We showed that mixotrophy is extremely important for this species, as the non-feeding sub-strain could not compete with other microalgae, despite still being able to produce mucus traps, being more lytic than the feeding sub-strain, and producing mucus. Phagotrophy, compared to lytic toxicity or mucus trap production appears to give A. pseudogonyaulax the most benefits.