GROWTH RATE, ELEMENTAL AND BIOCHEMICAL COMPOSITION, AND SINKING DYNAMICS OF SMALL NANOPLANKTONIC DIATOMS: IMPLICATIONS FOR SEASONAL BIOGEOCHEMICAL CYCLING IN MARINE ECOSYSTEMS

Abstract

Diatoms are a highly diverse group of phytoplankton that contribute approximately half of global marine primary production and span a wide size range from less than 3 μm to more than 1000 μm in diameter. To date, most studies have focused on medium-sized diatoms between 4 to several 10s of µm in diameter. There is accumulating evidence that small nano-sized diatoms (> 2 and <4 µm in diameter) are common members of phytoplankton communities. Laboratory experiments were conducted to quantify the growth rate, elemental (C: N: Si: P), and macromolecular (protein, carbohydrate, lipids, RNA, and DNA) composition of two nanoplanktonic diatoms, Minidiscus trioculatus (CCMP 501) and Minutocellus polymorphus (CCMP 496) in comparison to the slightly larger (~5 μm) Thalassiosira pseudonana (CCMP 1335), across a gradient of irradiance (5–800 μmol photons m⁻² s⁻¹) and temperature (5–25 °C). Results showed that carbon to nitrogen (C: N) ratios were not size-dependent, but C: P, N: P, and Si: N were consistently lower in the small nanoplanktonic diatoms. The low C: P and N: P in the small nanoplanktonic diatoms are correlated with elevated of DNA: C and DNA: P. Complementary year-round field sampling (2023-2024) of surface particulate organic matter (POM) from the Bedford Basin (44.69°N, 63.64°W) supported these findings, with the lower values of C: P and N: P and higher DNA: C in the smaller size fraction (0.4 - 3 μm) relative to a larger size fraction (3 -300 μm). There were pronounced seasonal variation in C: N: Si: P in both size fractions, with spring characterized by low C: N: P and high Si: C, suggesting diatom dominance in both size fractions in spring. Elevated RNA: DNA and RNA: protein ratios during spring further supported rapid diatom growth in Spring. To evaluate whether small nanoplanktonic diatoms can sink as rapidly as larger species, a set of sinking experiments were conducted comparing the rates of M. trioculatus and M. polymorphus with that of the ~13 μm diatom Thalassiosira weissflogii (CCMP1010). Sinking experiments revealed that small nanoplanktonic diatoms, especially when forming chains or aggregates, can sink at rates comparable to or exceeding those of larger diatoms. Sinking rates increased during stationary phase, likely due to physiological and morphological changes, though species-specific responses to light and aggregate structure moderated this effect. Integrated laboratory and field data reveal that small nanoplanktonic diatoms, characterized by distinct elemental stoichiometry (low C: P, N: P), elevated DNA content, and enhanced sinking potential during aggregation significantly influence marine nutrient cycling, carbon export efficiency, and food web dynamics over different seasons.