Sea Lettuce Case Study

Cross-kingdom interactions between the Ulva spp. (sea lettuce) and its associated bacteria have been studied by several groups within the last 60 years. Table 1 illustrates the wide interest and versatile research carried out. The role of biotic interactions, such as alleopathic and chemotactic effects, which shape the assemblages of species in the algal environment is complex to study and made well-defined laboratory experiments along with intensive field studies necessary to prove the ecological relevance. A few bacterial infochemicals have been already suggested as being involved in the cross-kingdom cross-talk of Ulvales and its associated bacteria, including N-(3-oxododecanoyl)-homoserine lactone (AHL) for the settlement-modulation of

lactuca, (ii) U. linza (formerly Enteromorpha linza), (iii) U. pertusa, (iv) U. fasciata, and (v) U. mutabilis. In addition to the distromatic sheets (“sea lettuces”) and monostromatic tubules (“enteromorphes”) of Ulva, the monostromatic Gayralia oxyspermum (previously Monostroma oxyspermum) was also well-investigated (Tab. 1). Tatewaki et al. (1983) prepared morphogenesis-inducing extracts from various isolated bacteria of M. oxyspermum belonging to the genera Cytophaga, Flavobacterium, Caulobacter, and Pseudomonas. Matsuo et al. (2003, 2005) identified Cytophaga sp. (YM2-23) of the Cytophaga–Flavobacterium–Bacteroides complex, which excretes the morphogenesis-inducing substance thallusin and restores the foliaceous morphology of M. oxyspermum. The authors reported that the same factor also partially promotes the formation of distromatic thalli of U. pertusa and other Ulva species, highlighting the potentially important role of thallusin for the normal development of green macroalgae. Pure thallusin strongly induced the differentiation of M. oxyspermum, even at very low effective concentrations between 1 fg mL-1 and 1 ag mL-1 (Matsuo et al., 2005; Gao et al., 2006). Although thallusin can be obtained from bacterial cultivations, Nishizawa et al. (2007) undertook the total syntheses of (±)-thallusin and its analogues to allow a detailed examination of thallusin’s biological activity. Whereas the compound

Hereby, bacteria can posses various complementary functions for Ulva development (Marshall et al., 2006; Spoerner et al., 2012). The research on the genus Ulva started in 1950 with the fundamental experiments of Provasoli (1958), which have subsequently influenced generations of scientists. Provasoli was motivated to test the hypothesis: “if Ulva, when bacteria-free, would grow in mineral media or if it would require organic factors.” The phycologists B. Føyn, A. Kylin and T. Levring worked with bacterized cultures and developed synthetic seawater media (“Erdschreiber”) enriched with soil extracts, nitrates, phosphates, and trace metals to allow normal germination and development until at least the 20-cell stadium of Ulva lactuca (Føyn, 1934; Kylin, 1941; Levering, 1946). Provasoli (1958) failed to obtain a typical foliaceous thallus in media similar to “Erdschreiber” without bacteria, but, in trying to obtain it (Fig. 3A), he found that Ulva germlings respond to plant phytohormones. For the first time, an attempt was made to culture Ulva axenically starting with thalli treated with antiobiotics. Upon sporogenesis, the zoospores were collected and further cultivated in sterile artificial seawater media (ASW8): However, the young germlings observed had never developed in a complete thallus. Provasoli (1958) concluded, thus, that