Efficient CO2 fixation by surface Prochlorococcus in the Atlantic Ocean

Abstract

Nearly half of the Earth's surface is covered by the ocean populated by the most abundant photosynthetic organisms on the planet—Prochlorococcus cyanobacteria. However, in the oligotrophic open ocean, the majority of their cells in the top half of the photic layer have levels of photosynthetic pigmentation barely detectable by flow cytometry, suggesting low efficiency of CO2 fixation compared with other phytoplankton living in the same waters. To test the latter assumption, CO2 fixation rates of flow cytometrically sorted 14C-labelled phytoplankton cells were directly compared in surface waters of the open Atlantic Ocean (30°S to 30°N). CO2 fixation rates of Prochlorococcus are at least 1.5–2.0 times higher than CO2 fixation rates of the smallest plastidic protists and Synechococcus cyanobacteria when normalised to photosynthetic pigmentation assessed using cellular red autofluorescence. Therefore, our data indicate that in oligotrophic oceanic surface waters, pigment minimisation allows Prochlorococcus cells to harvest plentiful sunlight more effectively than other phytoplankton.

Small pigmented eukaryotes play a major role in carbon cycling in the P‐depleted western subtropical North Atlantic, which may be supported by mixotrophy

“…Using cellular carbon quotas measured in the same study area (60, 276, and 14,424 fg C cell −1 for Prochlorococcus , Synechococcus , and P‐Euk, respectively; Baer et al ), P‐Euk represented the large majority of the small phytoplankton (< 5 μ m) C biomass (81% ± 10%, 9.5 ± 1.3 μ g C L −1 ) and Prochlorococcus and Synechococcus contributed to a similar amount of C biomass (1.1 ± 1.1 and 1.4 ± 0.7 μ g C L −1 or 8% ± 7% and 11% ± 3% of the total small phytoplankton C biomass). Using values from Hartmann et al () for the Atlantic Ocean (24, 95, and 1978 fg C cell −1 for Prochlorococcus , Synechococcus , and P‐Euk, respectively), P‐Euk would still represent the majority of the small phytoplankton C biomass at 63% ± 15%, whereas Prochlorococcus and Synechococcus contribution would be 17% ± 12% and 20% ± 4%, respectively (1.3 ± 0.2, 0.4 ± 0.5, and 0.5 ± 0.2 μ g C L −1 , respectively).…”

“…The isometric scaling of phytoplankton photosynthesis with cell size was first demonstrated using size fractionated measurements of CO 2 fixation rates (Marañón et al ; Duhamel and Moutin ) and later verified using measurements in flow‐cytometry–sorted groups of phytoplankton similar to this study (Jardillier et al ; Grob et al ; Rii et al ). Calculated biomass‐normalized rates of CO 2 fixation, a proxy for growth rates (Marañón ; Duhamel et al ), using conversion factors in Hartmann et al (), were 0.56 ± 0.11, 0.78 ± 0.27, 0.58 ± 0.06, and 0.44 ± 0.08 d −1 for Prochlorococcus , Synechococcus , P‐Euk_A, and P‐Euk_B, respectively. Although comparable to values previously reported (Jardillier et al ; Rii et al ), these estimates should be taken with caution as they are highly dependent on volume‐specific C conversion factors and equations that are used to calculate biovolume and C content (Duhamel et al ), particularly considering the much wider size range for P‐Euk compared to picocyanobacteria.…”

Bacterial community structure of a lab‐scale anammox membrane bioreactor

“…Metabolic activity of primary producers such as Prochlorococcus spp. yields oxygen to the environment . If the abundance of Prochlorococcus inside the MBR Anammox becomes excessive the oxygen resulting from its metabolism could cause inhibition of anammox bacteria.…”

Photosynthetic parameters in the northern South China Sea in relation to phytoplankton community structure

“…However, although the surface values in our study were associated with a wide range of cell sizes, they did not always decrease when there were more picophytoplankton (Figure ). The picophytoplankton in our study also evidenced a high capacity for photosynthesis [ Hartmann et al ., ]. The results of Marañón et al .…”