Since 2015, through funding from the NSF?s Dimensions of Biodiversity program, we have been studying a fascinating group of microalgae known as cryptophytes. As the name implies, cryptophytes are ?cryptic?. They are relatively understudied compared to other phytoplankton taxa, yet they are ubiquitous and can be found in environments ranging from tea-colored ponds to the blue-water open ocean. Our Dimensions grant explored the genetic, phylogenetic, and functional diversity of cryptophytes as related to their diversification into differently-colored light environments. We were particularly interested in the role of specialized pigments contained in cryptophytes, called phycobiliproteins (PBPs).

The first step of our investigation was to produce a molecular phylogeny (a family tree) using 99 strains of cryptophytes, and a database of phenotypic characteristics (cell size, pigment composition, light absorption characteristics, etc.). These allowed us to reclassify one species to a different genus, to update PBP diversity within the genus Hemiselmis, to classify a previously unidentified strain CCMP 2293 into the genus Falcomonas, and a previously unidentified strain CCMP 3175 into a clade with Chroomonas species. We also produced a statistical model using PBP type, cellular concentration, and cryptophyte habitat; these factors together correctly predicted 70.6% of clade composition. We found that the non-PBP pigments (chl-a, chl-c₂, a-carotene and alloxanthin) did not contribute significantly to clade classification (i.e., they played no role in determining which species were related to one another). Further work showed that the evolutionary path of different cryptophyte species was determined by which PBP pigments they contained. In particular, we found that part of the PBP molecule, the beta subunit, has been modified over evolutionary time through mechanisms like gene loss and gene duplication, that results in the variety of PBP pigments we see in cryptophytes today.

Our study of functional diversity in the cryptophytes explored phenotypic plasticity (flexibility) in cryptophytes, specifically their potential for ?complementary chromatic adaptation?. The theory posits that pigments in a photosynthetic organism should be optimally tuned to absorbing the available wavelengths of light and, if moved to a new environment, the organism will alter its pigmentation to match its new surroundings. When shifted to a new spectral environment of equal light intensity, we found significant changes in pigment composition of 8 cryptophytes we studied, but not always in ways that would be considered ?complementary?. When Storeatula sp. was shifted from a full spectrum to a green light environment it significantly increased its concentration of (green-light-absorbing) Cr-PE 545, which would be considered a ?complementary? chromatic acclimation response.  However, both Storeatula sp. and Rhodomonas salina significantly reduced their cellular concentrations of Cr-PE 545, chl-a, and chl-c2 when shifted from full spectrum to blue light -presumably because there was sufficient energy to fuel photosynthesis and growth when all available photons were blue. Results of this study also showed that cryptophytes may have the ability to tune their PBPs to the available wavelengths of light, which would give them a great ecological advantage if their habitat changed color due to deforestation, urbanization, or other anthropogenic disturbance.

Our most recent work has focused on gene expression in a cryptophyte called Rhodomonas salina, and how expression changed when cultures were exposed to different colors of light. We found that pigment-related gene expression did not vary with light color, suggesting that any regulation may occur post-transcriptionally. We did find that the expression of non-photosynthetic physiological processes, like glycolysis (respiration) and sexual reproduction, may be in some way controlled by the color of light in the cryptophyte environment. 

Finally, we examined the potential for evolutionary tradeoffs in the photosynthetic physiology of cryptophytes. Elucidating potential trade-offs among photosynthetic traits provides information on different ecological and physiological strategies for light capture. We investigated potential trade-offs by measuring photosynthetic traits for 15 species of cryptophytes. We constructed photosynthesis vs. irradiance (P-E) curves and rapid light curves (RLC) to estimate  traits that characterize photosynthetic performance and electron transport rate. Testing the gleaner-opportunist framework for resource acquisition  tradeoffs, we found no evidence for trade-offs between maximum rate of photosynthesis and the sensitivity of photosynthesis to light limitation, nor between maximum relative electron transport rate and the sensitivity of electron transport to light limitation. We also found no evidence of a power-efficiency tradeoff among photosynthetic parameters in cryptophytes. Contrary to predicted negative relationships, we observed a positive correlation between the  maximum photosynthetic rate and photosynthetic sensitivity to light intensity. We propose that  trade-offs may exist between photosynthetic traits and other resource acquisition traits, or that  the emergence of photosynthetic trade-offs may be context-dependent. 

This project has contributed to the training and professional development of the next generation of scientists. In sum, we have trained 2 postdocs (both of whom have permanent employment now), 3 graduate students, and at least 16 undergraduate students in the general field of phytoplankton ecology and evolutionary biology.

Last Modified: 01/13/2022
Modified by: Tammi L Richardson