In another chapter of the present book, volatile compounds from algae and microalgae are studied as an energy production source. Biogeneous hydrocarbons of the marine system, alkenes (mono, di, and cyclic) were originated from algae. One characteris­tic of crude oils that distinguishes them from biogeneous hydrocarbons is their con­tent in cyclo alkenes and aromatic compounds [32]. But hydrocarbons are not the only volatile compounds that can be found in algae and microalgae. In fact, there is a huge number of secondary metabolites with proved antimicrobial and therapeutic activities while some of these volatile compounds have been also related to climate modifications.

When attacked by herbivores, land plants can produce a variety of volatile com­pounds that attract carnivorous mutualists. Plants and carnivores can benefit from this symbiotic relationship, because the induced defensive interaction increases for­aging success of the carnivores, while reducing the grazing pressure exerted by the herbivores on the plants. Steinke et al. [185] reviewed whether aquatic plant use volatile chemical cues in analogous tritrophic interactions.

In general, naturally produced volatile and semivolatile compounds play an essential role in the survival of organisms for chemical defense and food gathering, but high amounts of volatile compounds could produce tremendous environmental actions. Marine algae produce several classes of biogenic gases, such as nonmethane hydrocarbons, organohalogens, ammonia and methylamines, and dimethylsulfide. These gases can transfer to the air, affect atmospheric chemistry, and are climati­cally important. Grazing increases dimethylsulfide and ammonia concentrations, and it is possible that other environmentally relevant volatiles are also produced during this process.

Other compounds produced by seaweeds with high importance for environment are halogenated hydrocarbons. Stratospheric ozone depletion and volatile-haloge — nated compounds are strongly connected with each other since the discovery that a massive loss of ozone in the polar stratosphere is catalyzed by halogen radicals derived from chlorocarbons and chlorofluorocarbons. Furthermore, so far unknown natural sources of volatile organohalogens may also contribute to a further destruc­tion of the ozone layer. Marine macroalgae species from the polar regions were investigated [90I for their importance as natural sources of volatile halogenated compounds released into the biosphere. Several different halogenated Ci to C hydrocarbons were identified and their release rates determined. Although, at present, marine macroalgae are apparently not the major source on a global scale, they may become more important in the future due to the influence of changing abiotic fac­tors, such as photon fluence rate, nutrient concentration, temperature, and salinity on the formation of volatile organohalogens.

The release of volatile compounds with defensive functions has been studied in many algae, for example the brown alga Dictyota menstrualis [21]. Although the amphipod Ashinaga longimana preferentially consumes the alga D. menstrualis, its feeding rates can be reduced significantly by high concentrations of diterpe­noid dictyols (dictyol E, pachydictyol A, and dictyodial) produced by the alga. The pattern of variation in the chemical defenses of some seaweed species sug­gests herbivore-induced increases of chemical defenses may be responsible for intraspecific variation in chemical defenses. For example, seaweeds from areas of coral reefs where herbivory is intense often produce more potent and higher con­centrations of chemical defenses than plants from habitats where herbivory is less intense. Their findings suggested that seaweeds are not passive participants in seaweed-herbivore interactions, but can actively alter their susceptibility to herbi­vores in ecological time. Induced responses to herbivory help explain both spatial (i. e., within-thallus, within-site, and among-site) and temporal variation in the chemical defenses of the algae.

As seen above, macroalgae produce volatiles with defensive functions against herbivores, but microalgae also produce defensive volatile compounds. In this sense, it is common in many microalgae to share the ecological niche with bacteria and other microorganism. Therefore, the defensive compounds secreted by microalgae possess antibacterial, antifungal or antiprotozoal activity. The nature of these com­pounds is highly varied. Microalgae have been screened for potential antimicrobial activity, which have been attributed to different compounds belonging to a range of chemical classes, including indoles, terpenes, acetogenins, phenols, fatty acids, and volatile-halogenated hydrocarbons [ 105] . For example, pressurized ethanol and supercritical CO2 extracts of microalgae D. salina were studied for their antibacte­rial activity against Escherichia coli and S. aureus and for their antifungal activity against Candida albicans and Aspergillus niger [56, 111]. Inthe broth microdilu­tion assay, a high antimicrobial activity against C. albicans, E. coli, and S. aureus was observed but not against A. niger. In this work, a GC-MS analysis was per­formed to associate the antimicrobial activity found, it was concluded that antimi­crobial activity of D. salina extracts could be linked to the presence of terpenic (b-cyclocitral and a and b-ionone) and indolic (methyl-1H-indole derivative) com­pounds, Fig. 2.

Terpenoids from algae have also been associated with antiviral activity, for example the above mentioned D. menstrualis produces a terpenoid able to inhibit HIV-1 reverse transcriptase as demonstrated by Souza et al. [181]; or terpenoid derived from plastoquinone that produces Sargassum sp., which acts in the lipid oxidation chain and inhibit cytomegalovirus growing [64].

Short chain fatty acids from microalgae are also volatile compounds associated with antibacterial activity. Santoyo et al. [165] tested, using the broth microdilution assay, extracts obtained from the red hematocysts without flagella (red phase) of

H. pluvialis microalga. In this work, it was concluded that the presence of short chain fatty acid (butanoic, hexanoic) highly inhibited the growing of gram positive and negative bacteria.