# Laminar _Ulva_ Species: A Multi-Tool for Humankind?
## 2 The Biology of _Ulva_ sp.
### Mutualistic Symbiotic Relationships
[39], matrix-assisted laser desorption/ionization mass spectrometric imaging (MALDI-MSI) directly enabled the spatial distribution of chemical molecules during cell differentiation processes. Surface metabolomic snapshots were used to monitor _Ulva compressa_ (as _U. mutabilis_) gametogenesis, which occurs when lamina cells develop into gametagia and produce gametes at the same time. Molecular ion imaging of metabolites revealed dynamic changes in surface-associated chemicals. A vast collection of geographically and temporally resolved chemicals (DMSP and 55 unique molecular biomarkers) helped to separate four discrete time periods in the gametogenesis process. Although the identification of effective biomarkers is the first step, the isolation and elucidation of the structure of these compounds still require further work [13,39]. In a separate investigation focusing on the cellular metabolic responses of a temperate species and a cold-adapted species, cultures of _Ulva compressa_ (formerly known as _U. mutabilis_) from Ria Formosa, Portugal, and _U. bulbosa_ from the polar region of Antarctica were exposed to a simultaneous temperature shift from 18 to 5 \({}^{\circ}\)C in the initial experiment. Following a 14-day incubation period, Ulva biomass was harvested, extracted, and analyzed using HILIC-HR-ESI-MS. The results revealed significant differences in 18 out of 30 metabolites. The researchers discovered that ectoine was the sole component not detected in axenic cultures of _Ulva compressa_ (as _U. mutublis_) at 18 \({}^{\circ}\)C or 5 \({}^{\circ}\)C, indicating that it is generated only by bacteria associated with the holobiont. There was an increase in the production of taurine (a sulfur-containing \(\beta\)-amino acid, potentially an important carbon source for heterotrophic bacterial growth) in _U. bulbosa_ but not in _Ulva compressa_ (as _U. mutublis_), emphasizing the difference in the responses of these two species. _Ulva_ to heat stress; cysteinolic acid (a member of the marine sulfonate class, it has physiological antioxidant activity and osmoregulatory effects) was elevated in _Ulva compressa_ (as _U. mutublis_) (9.5 times) after the temperature change to cold, but the concentration was below the limit of quantification. The proline concentration increased sharply by 24.9% after the temperature change, while the methionine concentration did not change. Similarly, [PERSON] and [PERSON] 1999 [75] noted that proline accumulated (to 6.48 mg g\({}^{-1}\) DW, a 9.8-fold increase) in _U. lactuca_ (as _U. fascita_) exposed to high salinity. [PERSON] et al. (2020) [76] observed a significant increase in proline levels, reaching a four-fold elevation, in response to decreasing temperatures (\(-1\)\({}^{\circ}\)C compared to 4 \({}^{\circ}\)C) in the polar diatom, _Nitzschia lecointei_ (Bacilariophyceae) [76]. Similar to the 4.5-fold increase observed in proline levels in _Fragiliaropsis cylindrus_ (Bacilariophyceae), a prevalent psychrophilic diatom found encased in freshly formed sea ice and subjected to heightened external salinities and lower temperatures, this change reflects alterations seen under cold stress conditions. According to [PERSON] et al., 2019 [77] and [PERSON] et al., 2020 [78], the concentration of glutamine, a proxy for its precursor glutamate, increased by 20.6-fold in _Ulva compressa_ (formerly _U. mutabilis_) under similar conditions. Glutamate serves as a precursor to various metabolites, including proline, arginine, and histidine, and plays a role in numerous biochemical reactions [77; 78], including those to pathogen resistance and abiotic stress (such as cold, heat, and drought). Glycine betaine, which rose 13.4-fold following cold stress, is commonly regarded as a potent protectant against the harmful effects of stressful situations on plants. Finally, the level of production of dimethylsulfoniopropionate (DMSP) was higher in _Ulva compressa_ (as _U. mutublis_), while [PERSON] and [PERSON] 2007 [79] found a level lower than expected for species from cold regions.