Secondary thickening in pteridophytes.-The known cases of secondary thickening in recent Pteridophyta have been brought together by HILL23 in a useful resume. After stating the criteria for secondary growth, Botrychium, which has a distinct cambium, and Ophioglossum, which lacks a definite layer, are described, followed by Angiopteris and Marattia, in which a cambium forms a few xylem elements. CORMACK'S observations on the secondary wood in the nodes of Equisetum are cited, though no reference is made to the cambium in the young cone as reported by J]FFREY.24 The other cases of secondary growth include Psilotum, Selaginella spinulosa, and several species of Isoetes, especially I. hystrix, which may show a cambium outside the vascular cylinder.-M. A.

The Cell Wall

Microalgae biomass as a sustainable source for biofuel, biochemical and biobased value-added products: An integrated biorefinery concept

Hydrothermal pretreatment technologies for lignocellulosic biomass: A review of steam explosion and subcritical water hydrolysis.

Biodiesel production from algal biomass is widely considered a sustainable alternative to petroleum fuels, especially in the transportation sector. However, the high energy consumption associated with the harvesting phase poses a significant impediment to the substantial commercialization of algal biodiesel. Centrifugation is among those harvesting methods that have a high efficiency close to 100% but can consume more energy than it produces. This review aims to address possible solutions for the issue of the high energy cost of centrifugation as an efficient harvesting technique. In addition, a detailed overview of the potential types of centrifugation that can be used to harvest microalgae with their pros and cons is provided. The most prevalent centrifugal devices that have been identified to be effectively useful to harvest algal biomass include disc-stack centrifuge, solid-bowl centrifuge, hydrocyclone, tubular centrifuge, and multi-chamber centrifuge. Overall, the analysis showed that centrifugation alone is not suitable for the harvesting of microalgae for biodiesel production. Nevertheless, coupling centrifugation with other harvesting technologies, e.g. with flocculation, as a primary pre-concentration step, can increase the energy output significantly thus, reducing the production cost.

Harvesting of microalgae by centrifugation for biodiesel production: A review

Converting nitrogen and phosphorus wastewater into bioenergy using microalgae-bacteria consortia: A critical review.

Microalgae for biotechnological applications: Cultivation, harvesting and biomass processing

Several microalgae species have been exploited due to their great biotechnological potential for the production of a range of biomolecules that can be applied in a large variety of industrial sectors. However, the major challenge of biotechnological processes is to make them economically viable, through the production of commercially valuable compounds. Most of these compounds are accumulated inside the cells, requiring efficient technologies for their extraction, recovery and purification. Recent improvements approaching physicochemical treatments (e.g., supercritical fluid extraction, ultrasound-assisted extraction, pulsed electric fields, among others) and processes without solvents are seeking to establish sustainable and scalable technologies to obtain target products from microalgae with high efficiency and purity. This article reviews the currently available approaches reported in literature, highlighting some examples covering recent granted patents for the microalgae’s components extraction, recovery and purification, at small and large scales, in accordance with the worldwide trend of transition to bio-based products.

Microalgae Biomolecules: Extraction, Separation and Purification Methods

Acid pretreatment of sugarcane biomass to obtain hemicellulosic hydrolisate rich in fermentable sugar

Chromochloris zofingiensis is known to be able to produce large amounts of astaxanthin and also to coproduce other molecules with antioxidant properties. Outdoor cultivation is the cheapest way for large-scale production; however, the unstable weather conditions can hinder the productivity of the biomass and the target product. The final biomass (0.92 g·L−1) and total carotenoids (0.55 mg·g−1) concentration achieved in outdoor cultivation (i.e., during autumn in Porto, Portugal) had no statistically significant difference compared to control cultivation (i.e., constant temperature, 18 °C, and light intensity, 4000 lx) (1.36 g·L−1 and 0.56 mg·g−1, respectively), however the biomass productivity was about threefold lower. Regarding the antioxidant potential, methanolic extracts from outdoor cultivation presented one of the highest values for radical scavenging ability (44.2 %) and ferrous-ion chelating ability (59.1 %), similarly to the results obtained by ethanolic extracts from indoor cultivation under nitrogen limitation. Highest total antioxidant capacities were observed in ethanolic extracts varying from 120.0 to 185.2 mg GAE·g−1. 

Antioxidant potential of extracts of Chromochloris zofingiensis cultivated in pilot-scale outdoor tubular photobioreactors under nitrogen limitation

Microalgae are among the most promising cell factories of the near future. Their renewable nature, and ability to rely on photosynthesis to use CO2 or nutrients from wastewater to grow and multiply, make them an emergent source of valuable natural compounds. Although in the past, a few of these microalgae have been known for their value as a source of proteins, carbohydrates, exopolysaccharides, polyunsaturated fatty acids, omega 3, and omega 6 fatty acids, recently they have also been found increasingly important sources of more valuable compounds, such as carotenoids – of which astaxanthin, lutein, and β-carotene are of extreme importance in food, feed, and cosmeceutical industries – phycobiliproteins, and chlorophylls, which are finding their place in the commercial market. There are still various challenges to be addressed to make sustainable the production of some of these valuable bioproducts. However, the circular economy and the biorefinery approach are at the center of the whole process to make the microalgae-based industry one of the most dynamic, modern, and profitable industries. This chapter will present the potential microalgae sources of these valuable compounds, existing industrial applications, as well as the major ongoing research projects, and their contribution driving the blue bioeconomy. 

Priscila S. Corrêa

Papers

Microalgae for biotechnological applications: Cultivation, harvesting and biomass processing

Microalgae Biomolecules: Extraction, Separation and Purification Methods

Acid pretreatment of sugarcane biomass to obtain hemicellulosic hydrolisate rich in fermentable sugar

Antioxidant potential of extracts of Chromochloris zofingiensis cultivated in pilot-scale outdoor tubular photobioreactors under nitrogen limitation

Microalgae for Pigments and Cosmetics