Abstract
Abstract Controlling the life cycle of the green macroalga Ulva (Chlorophyta) is essential to maintain its efficient aquaculture. A fundamental shift in cultivation occurs by transforming the thallus cells into gametangia and sporangia (sporulation), with the subsequent release of gametes and zoids. Sporulation occurrence depends on algal age and abiotic stimuli and is controlled by sporulation inhibitors. Thus, quantification of sporulation intensity is critical for identifying the biotic and abiotic factors that influence the transition to reproductive growth. Here, we propose to determine the sporulation index by measuring the number of released gametes using flow cytometry, in proportion to the total number of thallus cells present before the occurrence of the sporulation event. The flow cytometric measurements were validated by manually counting the number of released gametes. We observed a variation in the autofluorescence levels of the gametes which were released from the gametangia. High autofluorescence level correlated to phototactically active behaviour of the gametes. As autofluorescence levels varied between different groups of gametes related to their mobility, flow cytometry can also determine the physiological status of the gametes used as feedstock in seaweed cultivation.
Highlights
The green macroalgal genus Ulva (Linnaeus, 1753; Chlorophyta) comprises around 130 recognised species, most of which are distributed worldwide
= 10, 0002 N ⋅ 2 ⋅ Wsample N ⋅ 2 ⋅ Wsample where S is the area of the examined thallus, N is the number of thallus cells counted under the microscope in the area S multiplied by two for both layers of the thallus, Wsample is the weight of the sample used in the experiment, and Wsquare is the weight (g) of the 1-cm2 thallus, which was cut from the same thallus as the sample
Our research is of interest and use to those who seek to enhance their understanding of the life cycle and sporulation patterns of Ulva species
Summary
The green macroalgal genus Ulva (Linnaeus, 1753; Chlorophyta) comprises around 130 recognised species, most of which are distributed worldwide. Ulva species are edible and are often used as a raw material in various industries (Meghanath et al 2019; Nikolaisen et al 2011). Ulva undergoes alternate sexual and asexual reproduction throughout generations, forming isomorphic sporophytes and gametophytes (Hiraoka and Yoshida 2010). In both reproduction stages during sporulation, Ulva releases haploid swarmers (i.e., zoids and gametes), which differ by the number of flagella present and their phototactic behaviour (Hiraoka et al 2003; Kuwano et al 2012; Løvlie et al 1964). Specific intra- and extra-cellular sporulation inhibitors control the transformation of thallus cells into either a gametangium or sporangium
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