THALASSIOSIROID DIATOM RESPONSES TO SILICON STRESS AND OCEAN ACIDIFICATION

1 Atmospheric CO2 has risen dramatically since the industrial revolution. This 2 rise in atmospheric and oceanic pCO2 has perturbed ocean carbonate chemistry and 3 led to ocean acidification. Diatoms are phytoplankton that account for 40% of oceanic 4 primary production through photosynthetic carbon fixation, which is aided by their 5 carbon concentrating mechanism (CCM). The CCM uses the bicarbonate transporters 6 (BCTs) and carbonic anhydrases (CAs). Our current understanding of how diatoms 7 might respond to ocean acidification is based on experiments using model diatoms or 8 assessing the response of the bulk diatom community, rather than assessing a diversity 9 of diatoms in a complex environment. This dissertation aims to expand our knowledge 10 regarding diatom response to CO2 in ecologically important, non-model diatoms and 11 their response in laboratory experiments and field mesocosms to alterations in CO2 12 concentration. 13 Diatoms’ primary production is a function of their growth, which is 14 constrained by the availability of nutrients in the surface ocean. Silicon is a nutrient 15 that is particularly important for diatoms, as they are unique in their requirement for 16 silicon to build their cell walls. Silicon limitation has been observed in low iron high 17 nutrient low chlorophyll (HNLC) regions and the North Atlantic Ocean, although 18 these studies have focused on the whole diatom community rather than specific diatom 19 groups that may not uniformly experience silicon limitation. Genetic markers have 20 been used to probe species-specific iron status in the field, and similar molecular 21 markers of silicon status could be powerful tools to probe the silicon status of different 22 co-existing diatom species. However, current studies of silicon limitation have relied 23 on model diatoms rather than species that are likely to be found in HNLC regions or 24 the North Atlantic Ocean, limiting the ability to develop appropriate molecular 25 markers. This dissertation aimed to fill in these knowledge gaps using transcriptomic 26 studies of Thalassiosiroid diatom isolate cultures as well as incubations of mixed 27 diatom assemblages. 28 Chapter 1 of this thesis examined the transcriptomes of two closely related 29 Thalassiosira diatoms to better understand the silicon limitation gene expression 30 response of diatoms found in ecosystems where their growth may be constrained by 31 silicon availability, toward the goal of developing appropriate molecular markers of 32 silicon status. This study found a gene family encoding putative ATP-grasp domain 33 proteins that were upregulated in silicon-limited T. oceanica and T. weissflogii. The 34 upregulation of these genes is unique to silicon limitation and its expression is not 35 induced in nitrate or iron limitation conditions. The members of this gene family were 36 also upregulated in response to silicon limitation in previously published T. 37 pseudonana transcriptome studies and homologs were found across a diversity of 38 diatoms, suggesting that it might be useful as a silicon limitation marker in many 39 diatoms, in addition to Thalassiosira spp. diatoms. 40 Chapter 2 of this thesis adds to our knowledge of the diversity of CCM and 41 high CO2 response in different species of the Thalassiosira genus of diatoms. CO2 42 manipulation experiments were conducted with four Thalassiosira species – T. 43 pseudonana, T. rotula, T. weissflogii, and T. oceanica, and transcriptomes were 44 sequenced from the latter 3 species. These species displayed a range of growth rate 45 changes across CO2 conditions, from no change across conditions (T. pseudonana), 46 slower growth in lower CO2 (T. weissflogii), slower growth in higher CO2 (T. 47 oceanica), and faster growth in higher CO2 (T. rotula). The accompanying 48 transcriptome evidence for T. rotula, T. weissflogii, and T. oceanica indicate that these 49 differences in CO2 responses boil down to the ability to dynamically regulate the 50 carbon concentrating mechanism while maintaining internal redox and ion 51 homeostasis, as well as trade-offs between funneling excess CO2 into internal storage 52 pools versus a higher growth rate. 53 Chapter 3 uses a metatranscriptome approach to generalize about how the CCM 54 of Thalassiosira spp. differs from that of Skeletonema spp. diatoms. This study 55 involved CO2 manipulation experiments of a plankton assemblage collected from 56 Massachusetts Bay during March of 2014. We show evidence of distinct enzymatic 57 strategies for N and P uptake and scavenging in these two diatom genera. There is also 58 evidence of distinct strategies in their CCMs – while Thalassiosira spp. diatoms seem 59 to use the α, δ, and ζ carbonic anhydrases and the substrate-specific SLC4 and non60 specific SLC26 bicarbonate transporters, Skeletonema spp. diatoms rely on the γ and ζ 61 carbonic anhydrases and the SLC26 bicarbonate transporters or diffuse CO2 uptake. 62 This suggests that the co-existence of Skeletonema spp. and Thalassiosira spp. might 63 be partially due to a CCM that uses distinct forms of inorganic carbon (CO2 or HCO3) 64 and CA isozymes that use different metal cofactors for their enzymatic activity (γ can 65 substitute iron, δ can substitute cobalt). Furthermore, if Skeletonema does rely on 66 diffusive CO2 uptake rather than active transport of HCO3 using SLC4 or SLC26 67 transporters, Thalassiosira may have a competitive advantage in high CO2 as it 68 downregulates these active transporters. 69