Effects of environmental parameters on chytrid infection prevalence of four marine diatoms: a laboratory case study

The influence of environmental factors on the infection susceptibility of four different marine diatom host species to chytrid infection was tested under laboratory conditions, using host and parasite isolates obtained from diverse coastal areas in north-west Iceland in 2015. Specifically, a total of 120 monoclonal marine diatom host cultures of Navicula, Nitzschia, Rhizosolenia and Chaetoceros were exposed to their chytrid parasites Chytridium type I and Rhizophydium type I and II in Hellendahl glass staining jars which were subdivided in two compartments by nylon filters (mesh size 5 μm). Infection densities were assessed at different temperatures (5, 15, 20°C), salinities (0, 5, 10, 20, 40), photon fluence rates (PFR; 10, 50, 100, 200 μmol photons m−2 s−1) and photoperiods (24 h dark, 8:16 h, 16:8 h light:dark and 24 h light) after 168 h exposure, using the one-factor-at-a-time method. In addition, growth rates and proline concentrations of the non-infected monoclonal host cultures were determined. In most cases, decreasing growth rates during the acclimatisation process to abiotic stressors were directly related to increases of proline in the host cells. Significant positive associations of infection densities to cell based proline concentrations were predominantly observed in the high-PFR assays and 24-h daylight treatments. At least for half of the tested host-parasite pairs, positive correlations of proline and parasite prevalence were found. In addition, chytrid abundance was also positively correlated with host densities of Navicula sp., Rhizosolenia sp. and Chaetoceros sp. Only in Nitzschia sp., was parasite density negatively associated with proline and showed no significant relationship to host densities, suggesting that other physiological/biochemical factors related to stress might have an impact on the susceptibility of this peculiar host diatom species.