1. Introduction
The transition from biofuels to bio-product based economy, demands a
suitable feedstock with capability of producing multiple high-value
renewables (HVRs). Microalgae are by far the most abundant primary
producers responsible for photosynthetic conversion of light energy and
carbon dioxide (CO2) into sustainable renewables[1]. Recent advances
in bioprocess technology supports the development of microalgal cell
factories for establishing environmentally sustainable manufacturing of
HVRs. In this regard Phaeodactylum tricornutum , a unicellular,
marine pennate diatom, is considered as a potential feedstock for the
production of biofuel and HVRs such as[2,
3], eicosapentaenoic acid (EPA),
docosahexaenoic acid (DHA)[4], chrysolaminarin[5], fucoxanthin[6,
7], etc., and can be considered as a
suitable microalgal cell factory for sustainable biorefinery processes[8].
One of the major fatty acid in P. tricornutum is the ω-3 fatty
acid EPA (C20:5), having a significant commercial importance in
pharmaceutical and nutraceutical industries[9,
10]. These long chain polyunsaturated
fatty acids (LC-PUFAs) have a bioactive role against a variety of
disorders, including coronary heart disease, thrombosis, and recently in
prospective adjuvant therapy in COVID-19-related cardiovascular problems[11-14]. EPA
content in P. tricornutum is reported upto 3-5 % of DCW (dry
cell weight) with an average productivity of 56 mg L-1D-1, highlighting a potential alternate vegan source
of ω-3 fatty acid production[15]. Commercially
EPA from P. tricornutum is available in global market viabrand name SIMRIS® ALGAE OMEGA-3, containing 50 mg of EPA per capsule
(www.simris.com/pages/ingredients).
Additionally, fucoxanthin (1% - 6% DCW) an important HVR, is the
primary carotenoid produced in P.tricornutum[16][6]. During
photosynthesis, xanthophylls acts as a light harvesting pigment
connected to fucoxanthin-chlorophyll a/c-proteins (FCP), which are an
integral part of the thylakoids[17]. Furthermore,
due to its unique structure, fucoxanthin has various major bioactivities
such as anti-oxidant, anti-obesity, anti-cancer properties, and it has
been found to be an effective treatment for chronic disorders such as
Alzheimer’s[18-21]. Commercial
source of fucoxanthin is primarily brown seaweeds, which are difficult
to meet market demands due to low productivity, low quality, and high
cost. The amount of fucoxanthin generated by P. tricornutum is
substantially higher than brown seaweeds, making it a potential choice
for commercial production[22].
Media engineering strategies for microalgae cultivation have recently
acquired appeal as feasible techniques for achieving high HVR output[7,
23]. Though the high costs of
substrates, is a major impediment for generating commercially viable
product [24].
Mixotrophy, on the other hand, represents an innovative methodology for
HVRs production in P. tricornutum[25,
26]. It appears challenging to attain
high biomass, EPA, and fucoxanthin content simultaneously; hence,
designing an appropriate strategy for the mixotrophic cultivation ofP. tricornutum is critical for the commercial co-production of
HVRs.
P. tricornutum grows mixotrophically and has been reported to
grow efficiently on glucose, fructose, mannose, lactose and glycerol[27]. Furthermore,
various nitrogen sources such as nitrate, nitrite, ammonia, and urea has
been employed for its cultivation[28]. It has been
reported that cultivation of P. tricornutum on glycerol
supplemented medium yields 0.4 g L-1D-1 of biomass and 8.5 mg L-1D-1 of EPA[29].Furthermore, utilizing urea as sole nitrogen source resulted in a
considerable enhancement in EPA content (26 mg g-1)[28-30]. Compared
to the conventional nitrogen source i.e. sodium nitrate
(NaNO3), urea is the cost effective and environmental
friendly substrate for the cultivation of microalgae and thus the
production of HVRs[28,
31]. The nutrient costs for the large
scale cultivation of P. tricornutum on F/2 medium (with
NaNO3) approximately to be USD 0.15
kg-1 biomass, whereas its cost decreased to half i.e.
USD 0.07, kg-1 biomass when grown on a modified medium
(with urea) (Cui et al., 2021). Whereas
crude glycerol, a by-product of biodiesel processing, reported for the
production of β-carotene and DHA from Schizochytrium limanicumand Blakeslea trispora[32]. As a result,
using these substrates might be a viable method for sustainable
production of biomass, and HVRs from P. tricornutum .
The aim of this study is to examine the effect of glycerol, urea,
NaNO3 and their various combinations for the production
of HVRs in P. tricornutum , with a primary focus on EPA along with
fucoxanthin. Our preliminary screening revealed that glycerol (0.1M) and
urea (441 M) could be used as low-cost substrate for the generation of
biomass, lipids, carbohydrates and EPA. Moreover, feeding additional
urea to the culture supplemented with glycerol led to significant
enhancement of biomass, EPA, and fucoxanthin production. In this
context, we highlight an alternate strategy, beneficial for the
sustainable co-production of various HVRs from P. tricornutum .