Elemental and macromolecular composition of the marine Chloropicophyceae, a major group of oceanic photosynthetic picoeukaryotes

Abstract

Chloropicophyceae (Prasinophyte Clade VII) are small nonmotile coccoid cells with cell diameters ranging from 1 to 3 μm. Molecular surveys indicate they are relatively high in abundance in moderately oligotrophic oceanic waters and may substantively contribute to biogeochemical cycling in the sea. Here, we quantify the elemental and macromolecular composition of three subtropical Chloropicophyceae strains: Chloropicon mariensis, Chloropicon maureeniae, and Chloropicon roscoffensis under nutrient‐sufficient exponential growth and nitrate starvation. Under nutrient‐sufficient conditions the Chloropicophyceae are high in C : N and quite low in C : P and N : P relative to the canonical Redfield ratio, reflecting their relatively high nucleic acid composition compared to many other phytoplankton taxa. Nitrate starvation causes increases in C : N and C : P and decreases in N : P, primarily due to increases in carbohydrate and lipid and decreases in protein and RNA. There is genetic evidence that unlike most other green algae, Chloropicophyceae are diploid. The high nucleic acid content in the Chloropicon is consistent with the hypothesis that the nucleus, as a nonscalable component, takes up a larger and substantial proportion of cell mass in diploid picoeukaryotes. The elemental and macromolecular composition of these Chloropicophyceae, and relatively homeostatic response to N‐starvation compared to diatoms, provides some insight into their success in the moderately oligotrophic ocean.

Elemental and macromolecular plasticity of Chlamydomonas reinhardtii (Chlorophyta) in response to resource limitation and growth rate

“…This demonstrates the high variability of protein content across different algal species. Other macromolecules, such as RNA, chlorophyll a , DNA, and ATP only contributed <10% of the biomass C (File S4: Figure S4a–c), regardless of the growth rate or resource limitation, in line with other studies (Ebenezer et al., 2022, Finkel et al., 2016).…”

“…Hence, we estimate that biomass P potentially attributable to phospholipids under fast growth conditions varied between ~13% (low light), ~16% (low N:P), ~25% (balanced N:P), and ~48% (high N:P). Similar contributions of phospholipids in explaining total organismal P have been found previously in marine phytoplankton (20%–60%, Ebenezer et al., 2022). Microalgae are also capable of luxury P uptake, resulting in intracellular accumulation of P pools.…”

“…That we observed the highest contribution of protein to C (Protein‐C) was perhaps not unexpected, since faster growth likely requires higher concentrations of ribosomal proteins and enzymes (Sterner & Elser, 2002). In other algae, protein has been observed to be the largest contributor to cellular C, ranging from 31% to 43% (Ebenezer et al., 2022; Finkel et al., 2016). This demonstrates the high variability of protein content across different algal species.…”

Improving quantification of particulate phosphorus

“…Qualitative observation under a light microscope indicated samples from June 2021 were dominated by diatoms and dinoflagellates, but samples from March 2022 were dominated by smaller nanoplankton and picoplankton (consistent with Robicheau et al 2022). There are large differences in the relative proportion of DNA, RNA, ATP, phospholipids, polyphosphate, and absorbed P across species and within species exposed to different environmental conditions (Zimmerman et al 2014; Ebenezer et al 2022) that will influence the magnitude of improvement provided by the X‐HTDC method. The X‐HTDC method will not provide much improvement for samples with relatively high concentrations of inorganic P (Sañudo‐Wilhelmy et al 2004), for example, when adsorbed P content is a large proportion of total TPP, because inorganic P does not require temperatures greater than 500°C to be converted to pyrophosphate.…”

“…The lower limit of detection on this microtiter plate format is 6.13 nmol phosphorus. A detailed protocol for our method is available at protocols.io (Hu and Finkel 2022).…”