Research paperIntrinsic and extrinsic control of reproduction in the green tide-forming alga, Ulva rigida

•The reproductive rate of blade tissues increased with the degree of fragmentation.•Reproduction sensitivity of blade tissues to extrinsic factors increased with age.•Basal tissues remained in the vegatative state throughout the whole life span.•The presence of basal tissues suppressed reproduction of blade tissues.

The green seaweed Ulva is the causative genus behind nuisance green tides, but also has uses in the food and feed industries. Growing interest in Ulva cultivation has highlighted knowledge gaps in the mechanisms that regulate maturation and reproduction, particularly interacting intrinsic and extrinsic factors. In this study, the effects of temperature shock, dehydration, culture temperature, nitrate concentration, and thallus fragmentation were investigated on blade and basal tissue from U. rigida thalli of varying ages. A 20-min temperature shock induced a mean reproductive response of 94.7% in blade tissue by day five. The reproductive rate of blade tissues increased with the degree of fragmentation and with growth media renewal. Combining temperature shock with fragmentation triggered 97.3% of blade tissues to reproduce by day three. In contrast, dehydration reduced reproduction. A temperature of 18 °C in combination with a nitrate concentration of 100 μmol L−1 halved the maturation period (28.4 days) compared to cultivation under the lower temperature and nitrate condition (62.1 days). Reproduction in blade tissues increased with plant age but basal tissues remained in the vegetative state even after temperature shock and fragmentation. Furthermore, the presence of basal tissues suppressed reproduction of blade tissues. These findings indicate that extrinsic factors such as temperature shock and fragmentation induce reproduction in blade but not basal tissues, which appears to be under the control of intrinsic factors such as sporulation inhibitors. The differentiation of Ulva cells could support the rapid growth of Ulva when environmental conditions are favourable and also facilitate survival under unfavourable conditions.