The nitrogen costs of photosynthesis in a diatom under current and future pCO2.

With each cellular generation, oxygenic photoautotrophs must accumulate abundant protein complexes that mediate light capture, photosynthetic electron transport and carbon fixation. In addition to this net synthesis, oxygenic photoautotrophs must counter the light-dependent photoinactivation of Photosystem II (PSII), using metabolically expensive proteolysis, disassembly, resynthesis and re-assembly of protein subunits. We used growth rates, elemental analyses and protein quantitations to estimate the nitrogen (N) metabolism costs to both accumulate the photosynthetic system and to maintain PSII function in the diatom Thalassiosira pseudonana, growing at two pCO2 levels across a range of light levels. The photosynthetic system contains c. 15-25% of total cellular N. Under low growth light, N (re)cycling through PSII repair is only c. 1% of the cellular N assimilation rate. As growth light increases to inhibitory levels, N metabolite cycling through PSII repair increases to c. 14% of the cellular N assimilation rate. Cells growing under the assumed future 750 ppmv pCO2 show higher growth rates under optimal light, coinciding with a lowered N metabolic cost to maintain photosynthesis, but then suffer greater photoinhibition of growth under excess light, coincident with rising costs to maintain photosynthesis. We predict this quantitative trait response to light will vary across taxa.

NUCLEOMORPH KARYOTYPE DIVERSITY IN THE FRESHWATER CRYPTOPHYTE GENUS CRYPTOMONAS(1).

Cryptophytes are unicellular, biflagellate algae with plastids (chloroplasts) derived from the uptake of a red algal endosymbiont. These organisms are unusual in that the nucleus of the engulfed red alga persists in a highly reduced form called a nucleomorph. Nucleomorph genomes are remarkable in their small size (<1,000 kilobase pairs [kbp]) and high degree of compaction (∼1 kbp per gene). Here, we investigated the molecular and karyotypic diversity of nucleomorph genomes in members of the genus Cryptomonas. 18S rDNA genes were amplified, sequenced, and analyzed from C. tetrapyrenoidosa Skuja CCAP979/63, C. erosa Ehrenb. emmend. Hoef-Emden CCAP979/67, Cryptomonas sp. CCAP979/52, C. lundii Hoef-Emden et Melkonian CCAP979/69, and C. lucens Skuja CCAP979/35 in the context of a large set of publicly available nucleomorph 18S rDNA sequences. Pulsed-field gel electrophoresis (PFGE) was used to examine the nucleomorph genome karyotype of each of these strains. Individual chromosomes ranged from ∼160 to 280 kbp in size, with total genome sizes estimated to be ∼600-655 kbp. Unexpectedly, the nucleomorph karyotype of Cryptomonas sp. CCAP979/52 is significantly different from that of C. tetrapyrenoidosa and C. lucens, despite the fact that their 18S rDNA genes are >99% identical to one another. These results suggest that nucleomorph karyotype similarity is not a reliable indicator of evolutionary affinity and provides a starting point for further investigation of the fine-scale dynamics of nucleomorph genome evolution within members of the genus Cryptomonas.